Saturday, July 9, 2011

Pluto, formal designation 134340 Pluto

Pluto, formal designation 134340 Pluto, is the second most massive known dwarf planet in the Solar System (after Eris) and the tenth most massive body observed directly orbiting the Sun. Originally classified as the ninth planet from the Sun, HP 586006-321 Battery

Pluto was recategorized due to recent discoveries and is now considered the largest dwarf planet within the newly charted Kuiper belt.

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moonand a third its volume. HP 586006-361 Battery

It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune.

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, HP 586028-341 Battery

following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto.HP h588178-141 Battery

On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres.HP 593553-001 Battery

After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.

Pluto and its largest moon, Charon, are sometimes treated as a binary system because the barycentre of their orbits does not lie within either body.HP 593554-001 Battery

The IAU has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a moon of Pluto.[19] Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.HP HSTNN-CB0W Battery


In the 1840s, using Newtonian mechanics, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune.HP HSTNN-CB0X Battery

In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X".[22] By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet.HP HSTNN-I79C Battery

Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but did not recognise them for what they were.

Due to a ten-year legal battle with Constance Lowell, Percival's widow, HP HSTNN-I81C Battery

who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,[25] when its director, Vesto Melvin Slipher, summarily handed the job of locating Planet X toClyde Tombaugh, HP HSTNN-IB0N Battery

a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.

Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position.HP HSTNN-IB0X Battery

Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates, to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, HP HSTNN-LB0W Battery

Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to theHarvard College Observatory on March 13, 1930.HP HSTNN-OB0X Battery


The discovery made headlines across the globe. The Lowell Observatory, who had the right to name the new object, received over 1000 suggestions from all over the world, ranging from Atlas to Zymal. Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.HP HSTNN-OB0Y Battery

Constance Lowell proposed Zeus, then Percival and finallyConstance – her own first name. These suggestions were disregarded.

The name Pluto was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England.HP HSTNN-Q61C Battery

Venetia was interested in classical mythology as well as astronomy, and considered the name, a name for the god of the underworld, appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfatherFalconer Madan, a former librarian at the University of Oxford's Bodleian Library.HP HSTNN-Q62C Battery

Madan passed the name to ProfessorHerbert Hall Turner, who then cabled it to colleagues in the United States.

The object was officially named on March 24, 1930. Each member of the Lowell Observatory was allowed to vote on a short-list of three: HP HSTNN-YB0X Battery

Minerva (which was already the name for an asteroid), Cronus (which had lost reputation through being proposed by the unpopular astronomer Thomas Jefferson Jackson See), and Pluto. Pluto received every vote. The name was announced on May 1, 1930. Upon the announcement, Madan gave Venetia five pounds (£5) (£234 as of 2011), as a reward.HP NBP6A174B1 Battery

It has been noted that the first two letters of Pluto are the initials of Percival Lowell, and Pluto's astronomical symbol (?) is a monogram constructed from the letters 'PL'. Pluto's astrological symbol resembles that of Neptune (Neptune symbol.svg), but has a circle in place of the middle prong of the trident (Pluto's astrological symbol.svg).HP HSTNN-YB0X Battery

The name was soon embraced by wider culture. In 1930, Walt Disney introduced for Mickey Mouse a canine companion, named Pluto apparently in the object's honour, although Disney animator Ben Sharpsteen could not confirm why the name was given.HP NBP6A174 Battery

In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, which was named after Uranus, and neptunium, which was named after Neptune.HP NBP6A174B1 Battery

In Chinese, Japanese and Korean the name was translated as underworld king star (???), as suggested by Houei Nojiri in 1930. Many other non-European languages use a transliteration of "Pluto" as their name for the object; however, some Indian languages use a form of Yama, the Guardian of Hell in Hindu mythology, such as the Gujarati Yamdev.HP NBP6A175 Battery

Demise of Planet X

Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's Planet X. Estimates of Pluto's mass were revised downward throughout the 20th century. In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time. HP NBP6A175B1 Battery

Its mass, roughly 0.2% that of the Earth, was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternate Planet X, notably by Robert Sutton Harrington, failed. In 1992, Myles Standish used data fromVoyager 2's 1989 flyby of Neptune, HP WD548AA Battery

which had revised the planet's total mass downward by 0.5%, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished. Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.HP WD549AA Battery

Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's actual position at that time; however, Ernest W. Brown concluded almost immediately that this was a coincidence, a view still held today.HP Envy 17 Notebook PC Battery

Orbit and rotation

Pluto's orbital period is 248 Earth years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). HP Envy 17T Battery

This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than Neptune's. The Pluto–Charon barycentre came to perihelion on September 5, 1989, and was last closer to the Sun than Neptune between February 7, 1979 and February 11, 1999.HP HSTNN-Q61C Battery

Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years. The durations of the relationship vary because of the varying speed of motion of Neptune along its orbit;HP GSTNN-Q62C Battery

the relationship between the orbits themselves varies much more slowly, as Neptune's orbit precesses and otherwise evolves.

Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. HP HSTNN-IB1E Battery

While computer simulations can be used to predict its position for several million years (both forward and backwardin time), after intervals longer than the Lyapunov time of 10–20 million years, calculations become speculative: Pluto's tiny size makes it sensitive to unmeasurably small details of the solar system, hard-to-predict factors that will gradually disrupt its orbit.HP HSTNN-IB0X Battery

Millions of years from now, Pluto may well be at aphelion, at perihelion or anywhere in between, with no way for us to predict which. This does not mean Pluto's orbit itself is unstable, but its position onthat orbit is impossible to determine so far ahead. Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.HP HSTNN-OB0X Battery

Relationship with Neptune

Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. There are several reasons why.

At the simplest level, one can examine the two orbits and see that they do not intersect.HP HSTNN-YB0X Battery

When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 AU above that of Neptune, preventing a collision. Pluto's ascending and descending nodes, the points at which its orbit crosses the ecliptic, are currently separated from Neptune's by over 21°.HP 586006-361 Battery

However, this alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter aspects of Pluto's orbit (such as its orbital precession) over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work.HP NBP6A174 Battery

The most significant of these is that Pluto lies in the 3:2 mean motion resonance with Neptune: for every three of Neptune's orbits around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. HP NBP6A174B1 Battery

This pattern is configured so that, in each 500-year cycle, the first time Pluto is near perihelion Neptune is over 50° behind Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ahead of Pluto. HP 588178-141 Battery

Pluto and Neptune's minimum separation is over 17 AU. Pluto comes closer toUranus (11 AU) than it does to Neptune.

The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years. This prevents their orbits from changing relative to one another; HP Pavilion dv5 Adapter

the cycle always repeats in the same way, and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide.

Other factors

Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune's orbits does not change.Sony NP-F330 Battery

However, there are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 3:2 mean motion resonance).

First, Pluto's argument of perihelion, the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, librates around 90°.Asus Eee PC 904HA Adapter

This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the Kozai mechanism,[54] which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case Neptune. Nikon D80 Battery

Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.

Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are in near-resonance with the above libration.Sony PCGA-AC19V9 Adapter

When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. Dell T555C Battery

This is known as the 1:1 superresonance, and is controlled by all the Jovian planets.

To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit counter-clockwise. HP Pavilion DV6-3013SL Battery

After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes angular momentum to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to Kepler's third law. HP Pavilion DV7-2085EL Battery

As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). HP Pavilion DV9700 Battery

The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node. The whole process takes about 20,000 years to complete. HP Mini 702EA Battery


Pluto's rotation period, its day, is equal to 6.39 Earth days.[60] Like Uranus, Pluto rotates on its "side" on its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at itssolstices, one-fourth of its surface is in permanent daylight, while another fourth is in permanent darkness.HP Pavilion DV7-1120EO Battery

Physical characteristics

Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons spacecraft is expected to arrive there.

Appearance and surface

Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion.HP Compaq 491278-001 Battery

To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[63] It looks star-like and without a visible disk even in large telescopes, because its angular diameter is only 0.11". Distance, and current limits on telescope technology, make it impossible to directly photograph surface details on Pluto.HP Pavilion DV6-1030US Battery

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses.HP Compaq 490306-001 Battery

For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.HP Pavilion DV5-1211EM Battery

Current maps have been produced from images from the Hubble Space Telescope, which offers the highest resolution currently available, and show considerably more detail, resolving variations several hundred kilometres across, including polar regions and large bright spots.HP EliteBook 2530P Battery

The maps are produced by complex computer processing, which find the best-fit projected maps for the few pixels of the Hubble images. As the two cameras on the HST used for these maps are no longer in service, these will remain the most detailed maps of Pluto until the 2015 flyby of New Horizons.HP Pavilion DV4385EA Battery

These maps, together with Pluto's lightcurve and the periodic variations in its infrared spectra, reveal that Pluto's surface is remarkably varied, with large changes in both brightness and colour. Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as Saturn's moonIapetus.Compaq PP2100 Battery

The colour varies between charcoal black, dark orange and white: Buie et al. term it "significantly less red than Mars and much more similar to the hues seen on Io with a slightly more orange cast".

Pluto's surface has changed between 1994 and 2002-3: the northern polar region has brightened and the southern hemisphere darkened.HP Compaq KU532AA Battery

Pluto's overall redness has also increased substantially between 2000 and 2002. These rapid changes are probably related to seasonal variation, which is expected to be complex due to Pluto's extreme axial tilt and high orbital eccentricity. HP Pavilion DV6-1270EQ Battery

Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide. The face of Pluto oriented toward Charon contains more methane ice, while the opposite face contains more nitrogen and carbon monoxide ice.HP Pavilion DV4385EA Battery


Observations by the Hubble Space Telescope place Pluto's density at between 1.8 and 2.1 g/cm3, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.HP Compaq Business NC4400 Battery

Because decay of radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. The diameter of the core should be around 1,700 km, 70% of Pluto's diameter.HP Pavilion ZD8227 Battery

It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some 100 to 180 km thick at the core–mantle boundary. The DLR Institute of Planetary Research calculated that Pluto's density-to-radius ratio lies in a transition zone,Nikon D200 Battery

along with Neptune's moon Triton, between icy satellites like the mid-sized moons of Uranus and Saturn, and rocky satellites such as Jupiter's Europa.

Mass and size

Pluto's mass is 1.31×1022 kg, less than 0.24 percent that of the Earth,[77] while its diameter is 2,306 (+/- 20) km, or roughly 66% that of the Moon. Pluto's atmosphere complicates determining its true solid size within a certain margin.Acer PA-1650-02 Adapter

Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars.Sony CYBER-SHOT DSC-T10 Battery

However, in 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth.[78] Pluto's albedo is 1.3–2.0 times greater than that of Earth.HP Compaq Business 6715b Battery

The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect was measured, Pluto's true mass could be determined. Kodak KLIC-7004 Battery

Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, while the invention of adaptive optics allowed them to determine its shape more accurately.

Among the objects of the Solar System, Pluto is much less massive than the terrestrial planets, Acer Aspire 5920G Adapter

and at less than 0.2 lunar masses, it is also less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. Pluto is more than twice the diameter and a dozen times the mass of the dwarf planet Ceres, the largest object in the asteroid belt. Canon Digital IXUS 90 IS Battery

However, it is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005. Given the error bars in the different size estimates, it is currently unknown whether Eris or Pluto has the larger diameter.[81] Both Pluto and Eris are estimated to have solid-body diameters of about 2330 km.[HP 2133 Mini-Note PC Adapter

Determinations of Pluto's size are complicated by its atmosphere, and possible hydrocarbon haze.


Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, which are derived from the ices of these substances on its surface.Toshiba PA3536U-1BRS Battery

Its surface pressure ranges from 6.5 to 24 ?bar.[84] Pluto's elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. HP Compaq Business NX6110 Battery

This creates an anti-greenhouse effect; much as sweatcools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the Submillimeter Array have recently discovered that Pluto's temperature is about 43 K (?230 °C), 10 K colder than would otherwise be expected.HP Pavilion DV9000 Adapter

The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with average temperatures 36 K warmer 10 km above the surface.[86] The lower atmosphere contains a higher concentration of methane than its upper atmosphere.[Dell Studio 1537 Battery

The first evidence of Pluto's atmosphere was found by the Kuiper Airborne Observatory in 1985, from observations of the occultation of a star behind Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.HP PPP009H Adapter

From the rate of dimming, the atmospheric pressure was determined to be 0.15 pascal, roughly 1/700,000 that of Earth.[88] The conclusion was confirmed and significantly strengthened by extensive observations of another similar occultation in 1988.Sony VGP-BPS8 Battery

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the Paris Observatory,[89] James L. Elliot of MIT,[90] and Jay Pasachoff of Williams College.[91] Surprisingly, the atmospheric pressure was estimated to be 0.3 pascal,Dell PA-12 Adapter

even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. Sony Cyber-shot DSC-T7/B Battery

It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole's now permanently dark ice cap.[92] Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere.Dell LATITUDE M65 Battery

Another stellar occultation was observed by the MIT-Williams College team of James Elliot, Jay Pasachoff, and a Southwest Research Instituteteam led by Leslie Young on June 12, 2006 from sites in Australia.[93]

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ethane on Pluto's surface. HP Compaq 6720s Adapter

This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.[94]


Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.Acer AS07B41 Battery

The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius, the stable gravitational zone of Pluto's influence. For example, Psamathe orbits Neptune at 40% of the Hill radius. Dell Vostro 1310 Battery

In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty",[96] although others have pointed out the possibility of additional objects, including a small ring system.Sony VGP-BPL13 Battery


The Pluto–Charon system is noteworthy for being the largest of the Solar System's few binary systems, defined as those whose barycentre lies above the primary's surface (617 Patroclus is a smaller example).[98] This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.Sony VGP-BPS13 Battery

The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.Sony VGP-BPS13/B Battery

Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity. Just as Pluto revolves on its side relative to the orbital plane, so the Pluto–Charon system does also.Sony VGP-BPS13/S Battery

In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.[101]

Nix and Hydra

Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005,Sony VGP-BPS13A/B Battery

and received provisional designations of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially named Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.Sony VGP-BPS13A/S Battery

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon, and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.Sony VGP-BPS13AS Battery

Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity to Charon suggests a 35% albedo similar to Charon's;Sony VGP-BPS13B/B Battery

this value results in diameter estimates of 46 kilometres for Nix and 61 kilometres for the brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km, respectively. Sony VGP-BPS13B/Q Battery

At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of Pluto's.

The discovery of the two small moons suggests that Pluto may possess a variable ring system. Small body impacts can create debris that can form into planetary rings. Sony VGP-BPS13B/S Battery

Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggest that no ring system is present. If such a system exists, it is either tenuous like the rings of Jupiter or is tightly confined to less than 1,000 km in width.Sony VGP-BPS13Q Battery

Similar conclusions have been made from occultation studies.[105] In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet.Sony VGP-BPS13S Battery


Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This notion has been heavily criticised because Pluto never comes near Neptune in its orbit.Sony VGP-BPL21 Battery

Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers found a population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is believed to be the source of many short-period comets.Sony VGP-BPS21 Battery

Astronomers now believe Pluto to be the largest[note 9] member of the Kuiper belt, a somewhat stable ring of objects located between 30 and 50 AU from the Sun. Like other Kuiper belt objects (KBOs), Pluto shares features with comets; for example, the solar wind is gradually blowing Pluto's surface into space, in the manner of a comet.Sony VGP-BPS21/S Battery

If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.

Though Pluto is the largest of the Kuiper belt objects discovered so far, Neptune's moon Triton, which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object.Sony VGP-BPS21A Battery

Eris (see below) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc.

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "plutinos", after Pluto.Sony VGP-BPS21A/B Battery

Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original protoplanetary disc around theSun that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a sudden migrationundergone by Neptune early in the Solar System's formation. Sony VGP-BPS21B Battery

As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself, which became its moon Triton, locking others into resonances and knocking others into chaotic orbits. The objects in the scattered disc, a dynamically unstable region beyond the Kuiper belt,Sony VGP-BPL8 Battery

are believed to have been placed in their current positions by interactions with Neptune's migrating resonances. A 2004 computer model by Alessandro Morbidelli of the Observatoire de la Côte d'Azur in Nice suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, Sony VGP-BPL8A Battery

which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the Late Heavy Bombardment 600 million years after the Solar System's formation and the origin of Jupiter's trojan asteroids.Sony VGP-BPL8B Battery

It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration perturbed it into a resonant capture.[114] The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the bodies which became Triton and Eris.Sony VGP-BPS8 Battery


Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, resulting in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto.Sony VGP-BPS8A Battery

No serious attempt to explore Pluto by spacecraft occurred until the last decade of the 20th century. In August 1992, JPL scientist Robert Staehletelephoned Pluto's discoverer, Clyde Tombaugh, requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip."Sony VGP-BPS8B Battery

Despite this early momentum, in 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.

After Pluto's place within the Kuiper belt was determined, its official status as a planet became controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population.HP Compaq Business 6910p Battery

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System. The Hayden Planetarium reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.Panasonic LUMIX DMC-TZ4S Battery

In 2002, the KBO 50000 Quaoar was discovered, with a diameter then thought to be roughly 1280 kilometres, about half that of Pluto.In 2004, the discoverers of 90377 Sedna placed an upper limit of 1800 km on its diameter, nearer to Pluto's diameter of 2320 km, although Sedna's diameter was revised downward to less than 1600 km by 2007.Sony NP-FH100 Battery

Just as Ceres, Pallas, Juno and Vesta eventually lost their planet status after the discovery of many other asteroids, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.

On July 29, 2005, the discovery of a new Trans-Neptunian object was announced. Toshiba Equium M50-192 Battery

Named Eris, it is now known to be approximately the same size as Pluto.[81] This was the largest object discovered in the Solar System since Triton in 1846. Its discoverers and the press initially called it the tenth planet, although there was no official consensus at the time on whether to call it a planet.Apple MacBook Pro Battery

Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.

2006: IAU classification

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':Apple PowerBook G4 Battery

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared the neighbourhood around its orbit.Acer BATBL50L6 Battery

Pluto fails to meet the third condition, since its mass is only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as the prototype for the plutoid category of trans-Neptunian objects, in which it would be separately, but concurrently, classified. Canon EOS 450D Battery

On September 13, 2006, the IAU included Pluto, Eris, and the Eridian moon Dysnomia in their Minor Planet Catalogue, giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia". If Pluto had been given a minor planet name upon its discovery, the number would have been a little over a thousand rather than over 100,000.Dell 451-10617 battery

There has been some resistance within the astronomical community toward the reclassification. Alan Stern, principal investigator with NASA's New Horizons mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons."Acer Travelmate 2400 Battery

Stern's contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded.[137] His other claim is that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.Dell Inspiron 1525 Adapter

Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition.[138] Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. Compaq Presario 2100 Adapter

It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."

Researchers on both sides of the debate gathered on August 14–16, 2008, at The Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the current IAU definition of a planet.Acer Aspire 1362 Battery

Entitled "The Great Planet Debate", the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of a planet. Just before the conference, on June 11, 2008,Toshiba Equium M50-192 Battery

the IAU announced in a press release that the term "plutoid" would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital semimajor axis greater than that of Neptune and enough mass to be of near-spherical shape. Dell XPS M1330 Battery

American public's reaction

Reception to the IAU decision was mixed. While some accepted the reclassification, others seek to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy," among other crimes.Acer Travelmate 2200 Adapter

The U.S. state of New Mexico's House of Representatives passed a resolution in honor of Tombaugh, a longtime resident of that state, which declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007 was Pluto Planet Day.HP Compaq 6735b Adapter

The Illinois State Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU. Dell Vostro 1000 Battery

Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.HP Compaq nx7300 Adapter


The verb to pluto (preterite and past participle: plutoed) was coined in the aftermath of the 2006 IAU decision. In January 2007, the American Dialect Societychose plutoed as its 2006 Word of the Year, defining to pluto as "to demote or devalue someone or something,Asus f82 Battery

as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."

Society president Cleveland Evans stated the reason for the organization's selection of plutoed: Dell XPS M1330 Battery

"Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former planet."HP Compaq Business 6910p Battery

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X.Compaq PRESARIO R3000 Battery

Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune,[1] speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.HP Compaq Business NX6110 Battery

Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially considered the ninth planet until 2006. In 1978, however, Pluto was found to be too small for its gravity to affect the gas giants, resulting in a brief search for a tenth planet. Dell Latitude D610 Adapter

The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass.[3] After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet,Asus Eee PC 701 Adapter

or whether it and its neighbours should, like the asteroids, be given their own separate classification. Although a number of the larger members of this group were initially described as planets, in 2006 the International Astronomical Unionreclassified Pluto and its largest neighbours as dwarf planets, leaving only eight planets in the Solar System.HP Compaq Business NX6325 Battery

Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist. However, the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. Dell Studio XPS 16 Battery

In popular culture, and even among some astronomers,[5] Planet X has become a stand-in term for any undiscovered planet in the outer Solar System, regardless of its relationship to Lowell's hypothesis. Other trans-Neptunian planets have also been suggested, based on different evidence.Acer Aspire 5520 Battery

Early speculation

In the 1840s, the French mathematician Urbain Le Verrier used Newtonian mechanics to analyse perturbations in the orbit of Uranus, and hypothesised that they were caused by the gravitational pull of a yet-undiscovered planet. Le Verrier predicted the position of this new planet and sent his calculations to German astronomer Johann Gottfried Galle. Acer TravelMate 5720 Adapter

On 23 September 1846, the night following his receipt of the letter, Galle and his student Heinrich d'Arrest discoveredNeptune, exactly where Le Verrier had predicted.[6] There remained, however, some slight discrepancies in the gas giants' orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune.HP Compaq 6735s Adapter

Even before Neptune's discovery, some speculated that one planet alone would not be enough to explain the discrepancy. On 17 November 1834, the British amateur astronomer the Reverend Thomas John Hussey reported a conversation he had had with French astronomer Alexis Bouvard to George Biddell Airy, the British Astronomer Royal.Dell XPS M1330 Battery

Hussey reported that when he suggested to Bouvard that the unusual motion of Uranus might be due to the gravitational influence of an undiscovered planet, Bouvard replied that the idea had occurred to him, and that he had corresponded with Peter Andreas Hansen, director of the Seeberg Observatory in Gotha, about the subject. HP Compaq nc6230 Adapter

Hansen's opinion was that a single body could not adequately explain the motion of Uranus, and theorised that two planets lay beyond Uranus.[7]

In 1848, Jacques Babinet raised an objection to Le Verrier's calculations, claiming that Neptune's observed mass was smaller and its orbit larger than Le Verrier had initially predicted.Dell T117C Battery

He postulated, based largely on simple subtraction from Le Verrier's calculations, that another planet of roughly 12 Earth masses, which he named "Hyperion", must exist beyond Neptune.[7] Le Verrier denounced Babinet's hypothesis, saying, " absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part."Asus x5dij-sx039c Battery

In 1850 James Ferguson, Assistant Astronomer at the US Naval Observatory, noted that he had "lost" a star he had observed, GR1719k, which Lt. Matthew Maury, the superintendent of the Observatory, claimed was evidence that it must be a new planet.Dell Inspiron 1464 Battery

However, subsequent searches failed to recover the "planet" in a different position, and in 1878, CHF Peters, director of the Hamilton College Observatory in New York, showed that the star had not in fact vanished, and that the previous results had been due to human error.HP Compaq 6720s Adapter

In 1879, Camille Flammarion noted that the comets 1862 III and 1889 III had aphelia of 47 and 49 AU, respectively, suggesting that they might mark the orbital radius of an unknown planet that had dragged them into an elliptical orbit. Astronomer Georges Forbes concluded on the basis of this evidence that two planets must exist beyond Neptune. Dell Latitude D630 Adapter

He calculated, based on the fact that four comets possessed aphelia at around 100 AU and a further six with aphelia clustered at around 300 AU, the orbital elements of a pair of hypothetical trans–Neptunian planets. These elements concorded suggestively with those made independently by another astronomer named David Peck Todd, suggesting to many that they might be valid.HP Pavilion dm4 Battery

However, skeptics argued that the orbits of the comets involved were still too uncertain to produce meaningful results.

In 1900 and 1901, Harvard Observatory director William Henry Pickering led two searches for trans-Neptunian planets. Sony NP-FS11 Battery

The first was begun by Danish astronomer Hans Emil Lau who, after studying the data on the orbit of Uranus from 1690 to 1895, concluded that one trans-Neptunian planet alone could not account for the discrepancies in its orbit, and postulated the position of two planets he believed were responsible. Dell Inspiron 1520 Adapter

The second was launched when Gabriel Dallet suggested that a single trans-Neptunian planet lying at 47 AU could account for the motion of Uranus. Pickering agreed to examine plates for any suspected planets. In neither case was any found.Toshiba PA3399U-1BRS Battery

In 1909, Thomas Jefferson Jackson See, an astronomer with a reputation as an egocentric contrarian, opined "that there is certainly one, most likely two and possibly three planets beyond Neptune". Tentatively naming the first planet "Oceanus", he placed their respective distances at 42, 56 and 72 AU from the Sun. Acer Aspire 5920G Adapter

However, he gave no indication as to how he determined their existence, and no known searches were mounted to locate them.

In 1911, Indian astronomer Venkatesh P. Ketakar suggested the existence of two trans-Neptunian planets, which he named Brahma and Vishnu, Acer Aspire 5920 Battery

by reworking the patterns observed by Pierre-Simon Laplace in the planetary satellites of Jupiter and applying them to the outer planets.[9] The three inner Galilean moons of Jupiter, Io, Europa and Ganymede, are locked in a complicated 1:2:4 resonance called a Laplace resonance.IBM Thinkpad T40 Adapter

Ketakar suggested that Uranus, Neptune and his hypothetical trans-Neptunian planets were locked in Laplace-like resonances. His calculations predicted a mean distance for Brahma of 38.95 AU and an orbital period of 242.28 Earth years (3:4 resonance with Neptune). Nikon COOLPIX 4300 Battery

When Pluto was discovered 19 years later, its mean distance of 39.48 AU and orbital period of 248 Earth years would be quite close to Ketakar's prediction (Pluto in fact has a 2:3 resonance with Neptune). However Ketakar made no predictions for the orbital elements other than mean distance and period.Apple MB771 Battery

It is not clear how Ketakar arrived at these figures, and his second planet, Vishnu, was never located.[9]

Planet X

In 1894, with the help of William Pickering, Percival Lowell, a wealthy Bostonian aristocrat, founded the Lowell Observatory in Flagstaff, Arizona.Sony DSC-P5 Battery

In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet,[11] which he named Planet X. The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (Planet X would, at the time, have been the ninth planet). Dell Inspiron 6400 Adapter

Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had eluded him thanks to his widely derided belief that channel-like features visible on the surface of Mars were canals constructed by an intelligent civilisation.Toshiba PA3534U-1BRS Battery

Lowell's first search focused on the ecliptic, the plane encompassed by the zodiac where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, but found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey.Sony NP-FP50 Battery

After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916. In 1915, he published his Memoir of a Trans-Neptunian Planet, in which he concluded that Planet X had a mass roughly seven times that of the Earth—about half that of Neptune—and a mean distance from the Sun of 43 AU. HP Compaq nx7300 Adapter

He assumed Planet X would be a large, low-density object with a high albedo, like the gas giants. As the result it would show a disc with diameter of about one arcsecond and an apparent magnitude of between 12 and 13—bright enough to be spotted. Dell Latitude D820 Adapter

Separately, in 1908, Pickering announced that, by analysing "kinks" in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet O" (because it came after "N", i.e. Neptune),[14] possessed a mean orbital radius of 51.9 AU and an orbital period of 373.5 years.Toshiba PA3399U-1BRS Battery

However, plates taken at his observatory in Arequipa, Peru, showed no evidence for the predicted planet, and British astronomer PH Cowell showed that the "kinks" observed in Uranus's orbit virtually disappeared once the planet's displacement of longitude was taken into account.Dell Studio XPS 1640 Battery

Lowell himself, despite his close association with Pickering, dismissed Planet O out of hand, saying, "This planet is very properly designated "O", [for it] is nothing at all." Unbeknownst to Pickering, four of the photographic plates taken in the search for "Planet O" by astronomers at the Mount Wilson Observatory in 1919 would in fact capture Pluto,Dell Latitude D830 Battery

though this was only recognised years later.[16]Pickering would go on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named P, Q, R, S, T and U, but none were ever detected.[9]

Discovery of Pluto

Lowell's sudden death in 1916 temporarily halted the search for Planet X. Dell Inspiron E1505 Adapter

Failing to find the planet, according to one friend, "virtually killed him".[17] Constance Lowell, Percival Lowell's widow, subsequently embroiled the observatory in a long legal battle to secure its million-dollar portion of Lowell's legacy for herself, which meant that the search for Planet X could not resume for several years.Asus Eee PC 1201PN Battery

In 1925, the observatory obtained glass discs for a new 13-inch wide-field telescope to continue the search, constructed with funds from George Lowell, Percival's brother. In 1929 the observatory's director, Vesto Melvin Slipher, summarily handed the job of locating the planet to Clyde Tombaugh, Asus Eee PC 701 Adapter

a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.

Tombaugh's task was to systematically capture sections of the night sky in pairs of images. Dell XPS M1330 Battery

Each image in a pair was taken two weeks apart. He then placed both images of each section in a machine called a blink comparator, which by exchanging images quickly created a time lapse illusion of the movement of any planetary body. To reduce the chances that a faster-moving (and thus closer) object be mistaken for the new planet, fujifilm np95 Battery

Tombaugh imaged each region near its opposition point, 180 degrees from the Sun, where the apparent retrograde motion for objects beyond Earth's orbit is at its strongest. He also took a third image as a control to eliminate any false results caused by defects in an individual plate.Asus Eee PC 900 Adapter

Tombaugh decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.

By the beginning of 1930, Tombaugh's search had reached the constellation of Gemini. On 18 February 1930, after searching for nearly a year and examining nearly 2 million stars, Tombaugh discovered a moving object on photographic plates taken on 23 January and 29 January of that year.Sony VAIO PCG-713 Battery

A lesser-quality photograph taken on January 21 confirmed the movement.[18] Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet X."[18] The object lay just six degrees from one of two locations for Planet X Lowell had suggested; thus it seemed he had at last been vindicated.Dell Latitude D830 Adapter

After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The new object would later be found on photographs dating back to 19 March 1915.Dell Studio XPS 1640 Battery

The decision to name the object Pluto was intended in part to honour Percival Lowell, as his initials made up the word's first two letters.[20] After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds ofvariable stars and asteroids, as well as two comets, but no further planets.Panasonic LUMIX DMC-TZ4S Battery

Pluto loses Planet X title

To the observatory's disappointment and surprise, Pluto showed no visible disc; it appeared as a point, no different from a star, and, at only 15th magnitude, was six times dimmer than Lowell had predicted, which meant it was either very small, or very dark.HP Compaq Business NX6320 Battery

Since Lowell astronomers thought Pluto was massive enough to perturb planets, they assumed that it should have an albedo of 0.07 (meaning that it reflected only 7% of the light that hit it); about as dark as asphalt and similar to that of Mercury, the least reflective planet known.Canon POWERSHOT G2 Battery

This would give Pluto an assumed diameter of about 8,000 km, or about 60% that of Earth.[22] Observations also revealed that Pluto's orbit was very elliptical, far more than for any planet.[23]

Some astronomers expressed skepticism at the result. Shortly after its discovery in 1930, Armin O. Dell Latitude E6500 Battery

Leuschner disputed Pluto's status, suggesting that its dimness and high orbital eccentricity made it more similar to an asteroid or comet; "The Lowell result confirms the possible high eccentricity announced by us on April 5. Among the possibilities are a large asteroid greatly disturbed in its orbit by close approach to a major planet such as Jupiter, Dell LATITUDE D630 Battery

or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object." In 1931, Ernest W. Brown asserted, using a mathematical formula, that the observed irregularities in the orbit of Uranus could not be due to the gravitational effect of a more distant planet, and thus that Lowell's supposed prediction was "purely accidental."Asus R33030 Adapter

Throughout the mid-20th century, estimates of Pluto's mass were revised downward. In 1931, Nicholson and Mayall calculated its mass, based on its supposed effect on the gas giants, as roughly that of the Earth, while in 1949, measurements of Pluto's diameter led to the conclusion that Dell Mini 1011 Battery

it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass.[26] In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii analysed spectra from Pluto's surface and determined that it must contain methane ice, which is highly pa3332u-1brs Battery

This meant that Pluto, far from being dark, was in fact exceptionally bright, and thus was probably no more than 0.01 Earth mass.

Pluto's size was finally determined conclusively in 1978, when American astronomer James W. Christy discovered its moon Charon.HP Compaq 6710b Adapter

This enabled him, together with Robert Sutton Harrington of the US Naval Observatory, to measure the mass of the Pluto-Charon system directly by observing the moon's orbital motion around Pluto. They determined Pluto's mass to be 1.31×1022 kg; roughly one five–hundredth that of the Earth or one sixth that of the Moon,HP Compaq nx6325 Adapter

and far too small to account for the observed discrepancies in the orbits of the outer planets. Lowell's "prediction" had been a coincidence: if there was a Planet X, it was not Pluto.

Further searches for Planet(s) X

After 1978, a number of astronomers kept up the search for Lowell's Planet X, convinced that, since Pluto was no longer a viable candidate, an unseen tenth planet must have been perturbing the outer planets.Acer Aspire 5720 Adapter

In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities. He calculated that any Planet X would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly eccentric, and strongly inclined to the ecliptic—Toshiba Equium A110-252 Battery

the planet's orbit would be at roughly a 32-degree angle from the orbital plane of the other known planets. This hypothesis was met with a mixed reception. Noted Planet X sceptic Brian Marsden of Harvard University's Minor Planet Center pointed out that these discrepancies were a hundred times smaller than those noticed by Le Verrier, and could easily be due to observational error. Acer Aspire 5052ANWXMi Battery

In 1972, Joseph Brady of the Lawrence Livermore National Laboratory studied irregularities in the motion of Halley's Comet. Brady claimed that they could have been caused by a Jupiter-sized planet beyond Neptune that is in a retrograde orbit around the Sun.Dell Mini 1011v Battery

However, both Marsden and Planet X proponent P. Kenneth Seidelmann attacked the hypothesis, showing that Halley's Comet randomly and irregularly ejects jets of material, causing changes to its own orbital trajectory, and that such a massive object as Brady's Planet X would have severely affected the orbits of known outer planets.HP Compaq nx7300 Adapter

While its mission did not involve a search for Planet X, the IRAS space observatory made headlines briefly in 1983 due to an "unknown object" that was at first described as "possibly as large as the giant planet Jupiter and possibly so close to Earth that it would be part of this Solar System".Dell Vostro 1510 Battery

However, further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "interstellar cirrus"; none were found to be Solar System bodies.

In 1988, Jackson and Killen studied the stability of Pluto's resonance with Neptune by placing test "Planet X-es" with various masses and at various distances from Pluto. Toshiba PA3534U-1BRS Battery

Pluto and Neptune's orbits are in a 3:2 resonance, which prevents their collision or even any close approaches, regardless of their separation in the z-dimension. It was found that the hypothetical object's mass had to exceed 5 Earth masses to break the resonance;Asus M50 Battery

however, the parameter space is quite large and a large variety of objects could have existed beyond Pluto without disturbing the resonance. Four test orbits of a trans-Plutonian planet have been integrated forward for four million years in order to determine the effects of such a body on the stability of the Neptune-Pluto 3:2 resonance. Asus Eee PC 901 Battery

Planets beyond Pluto with masses of 0.1 M and 1.0 Earth masses in orbits at 48.3 and 75.5 AU, respectively, do not disturb the 3:2 resonance. Test planets of 5 Earth masses with semimajor axes of 52.5 and 62.5 AU disrupt the four-million-year libration of Pluto's argument of perihelion.Dell Inspiron 6400 Battery

Planet X disproved

Harrington died in January 1993, without having found Planet X.Six months before, Myles Standish had used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%—an amount comparable to the mass of Mars[37]—to recalculate its gravitational effect on Uranus.HP Compaq Business NC6220 Battery

When Neptune's newly determined mass was used in the Jet Propulsion Laboratory Developmental Ephemeris (JPL DE), the supposed discrepancies in the Uranian orbit, and with them the need for a Planet X, vanished.Sony VGP-BPS9 Battery

Moreover, there are no discrepancies in the trajectories of any space probes such as Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 that can be attributed to the gravitational pull of a large undiscovered object in the outer Solar System.[39]Today, most astronomers agree that Planet X, as Lowell defined it, does not exist.Sony VGP-BPS13 Battery

Discovery of further trans-Neptunian objects

After the discovery of Pluto and Charon, no more trans-Neptunian objects (TNOs) were found until (15760) 1992 QB1 in 1992. Since then, however, hundreds of such objects have been observed. Most are now recognised as part of the Kuiper belt, a swarm of icy bodies left over from the Solar System's formation that orbit near the ecliptic plane beyond Neptune.HP Compaq nx7300 Adapter

Though none were as large as Pluto, some of these distant trans-Neptunian objects, such as Sedna, were initially described in the media as "new planets".

In 2005, astronomer Mike Brown and his team announced the discovery of 2003 UB313 (later named Eris after the Greek goddess of discord and strife), a trans-Neptunian object just barely larger than Pluto.HP Compaq 2510p Adapter

Soon afterwards, a NASA Jet Propulsion Laboratory press release described the object as the "tenth planet".

Eris, however, was never officially classified as a planet, and the 2006 definition of planet defined both Eris and Pluto not as planets but as dwarf planetsbecause they have not cleared their neighbourhoods.Dell Studio PP39L Battery

In other words, they do not orbit the Sun alone, but as part of a population of similarly sized objects. Pluto itself is now recognized as being a member of the Kuiper belt and the second largest dwarf planet after Eris. A number of astronomers, most notably Alan Stern, the head of NASA's New Horizons mission to Pluto, Nikon COOLPIX 5700 Battery

contend that the IAU's definition is flawed, and that not only Pluto and Eris, but all large trans-Neptunian objects, such as Makemake, Sedna, Quaoar and Varuna, should be considered planets in their own right.Eris is far too small to have significant effects on the outer planets' orbits. It is not Planet X, nor does its discovery rehabilitate the theory.Dell Studio XPS 16 Battery

Subsequent proposed trans-Neptunian planets

Although most astronomers accept that Lowell's Planet X does not exist, a number have revived the idea that a large unseen planet could create observable gravitational effects in the outer Solar System. These hypothetical objects are often referred to as "Planet X", although their link to that world is purely conceptual.HP Compaq nx7400 Adapter

Kuiper cliff

The Kuiper belt terminates suddenly at a distance of 48 astronomical units (AU) from the Sun (by comparison, Neptune lies 30 AU from the Sun), and there has been some speculation that this sudden drop-off, known as the "Kuiper cliff", may be attributed to the presence of an object with a mass between that of Mars and Earth located beyond 48 AU.HP Pavilion DV6500 Adapter

However, the presence of a Mars-like planet in a circular orbit at 60 AU leads to a TNO population incompatible with observations. For instance, it would severely deplete the plutino population. Astronomers have not excluded the possibility of a more massive Earth-like planet located further than 100 AU with an eccentric and inclined orbit. Dell T117C Battery

Computer simulations by Patryk Lykawka of Kobe University have suggested that a body with a mass between 0.3 and 0.7 that of the Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 101 and 200 AU from the Sun, Acer PA-1650-02 Adapter

could explain not only the Kuiper cliff but also the peculiar detached objects such as Sedna. While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".


Another hypothesis argues that long-period comets, rather than arriving from random points across the sky as is commonly thought,Acer Aspire 5920G Adapter

are in fact clustered in a band inclined to the ecliptic. Such clustering could be explained if they were disturbed by an unseen object at least as large as Jupiter, possibly a brown dwarf. The hypothetical planet—or companion of the Sun—would be located in the outer part of the Oort cloud.Asus S1 Adapter

This hypothesis was first proposed by John Matese of the University of Louisiana at Lafayette in 1999. He also suggested that the gravitational pull of such an object might explain Sedna's peculiar orbit. In 2011, Matese and Daniel Whitmire claimed that evidence of this object, which they named Tyche, would be detectable in the archive of data that was collected by NASA's WISE telescope. HP Pavilion dv6000 Battery

The name is derived from the sister of Nemesis in Greek mythology, a reference which they selected to avoid confusion with Nemesis, a similar hypothetical object first proposed by Richard A. Muller in 1984. They believe Tyche's orbit to lie at approximately 500 times the distance of Neptune, Ibm Thinkpad T30 Battery

equivalent to 15,000 AU or roughly a fourth of a light-year, and speculate that Tyche could be from one to four times the mass of Jupiter, and have a relatively high surface temperature of approximately 200 Kelvin (-73°C), due to residual heat from its formation and Kelvin–Helmholtz heating.Dell Studio PP39L Battery


In addition, probability arguments have also been used to suggest the existence of planet-sized objects in the outer Solar System. Sedna's 12,000-year orbit is so eccentric that it spends only a small fraction of its orbital period near the Sun, where it can be easily observed. HP Pavilion dv5 Adapter

This means that unless its discovery was a freak accident, there are probably a substantial population of objects roughly Sedna's diameter yet to be observed in its orbital region. Mike Brown, the discoverer of Sedna, noted in his 2007 Lowell Lecture that, "Sedna is about three-quarters the size of Pluto. HP Compaq Business NC6400 Battery

If there are sixty objects three-quarters the size of Pluto then there are probably forty objects the size of Pluto ... If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, HP Compaq 490306-001 Battery

and the biggest of these objects ... is probably the size of Mars or the size of the Earth."[58] However, he notes that, should such an object be found, even though it might approach the Earth in size, it would still be a dwarf planet by the current definition, since it will not have cleared its neighborhood sufficiently.Compaq Presario CQ35-100 Battery

Halo of oligarch planets

The oligarch theory of planet formation suggests that there were hundreds of planet-sized objects, known as oligarchs, in the early stages of the Solar System's evolution. In 2005, astronomer Eugene Chiang speculated that while some of these oligarchs became the planets we know today, most would have been flung outward by gravitational interactions. HP Compaq HSTNN-LB51 Battery

Some may have escaped the Solar System altogether to become free-floating planets, while others would be orbiting in a halo around the Solar System, with orbital periods of millions of years. This halo would lie at between 1,000 and 10,000 AU from the Sun, or between a third and a thirtieth the distance to the Oort cloud.HP Pavilion ZE4500 Battery

The Oort cloud (play /??rt/; alternatively the Öpik–Oort cloud IPA: [?øpik]) is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the neareststar to the Sun. HP G62-100EB Battery

The Kuiper belt and scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.HP Pavilion DV6-1211AX Battery

The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. HP Pavilion DV6-1220SO Battery

Astronomers believe that the matter composing the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution.

Although no confirmed direct observations of the Oort cloud have been made,HP EliteBook 6930P Battery

astronomers believe that it is the source of all long-period andHalley-type comets entering the inner Solar System and many of the Centaurs and Jupiter-family comets as well. The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way Galaxy itself. HP Pavilion DV6-1210SA Battery

These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System. Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.Compaq Presario CQ71 Battery

Although the Kuiper belt and the farther scattered disc have been observed and mapped, only four currently known trans-Neptunian objects—90377 Sedna, 2000 CR105, 2006 SQ372, and 2008 KV42—are considered possible members of the inner Oort cloud.HP Pavilion DV7-1290EO Battery


In 1932, Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System. In 1950, the idea was independently revived by Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox:HP G6000 Battery

over the course of the Solar System's existence, the orbits of comets are unstable; eventually, dynamics dictate that a comet must either collide with the Sun or a planet, or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun,HP HDX X18-1005EA Battery

radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence.HP Pavilion DV6-3013SL Battery

There are two main classes of comet: short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie.HP Compaq HSTNN-IB62 Battery

Nearly all isotropic comets have very large orbits, on the order of thousands of AU, and appear from every corner of the sky. Oort noted that there was a peak in numbers of nearly isotropic comets with aphelia—their farthest distance from the Sun—of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.HP Pavilion DV6100 Battery

Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.[9]

Structure and composition

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly)[1] from the Sun. HP EliteBook 2730P Battery

Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).[9] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). HP Pavilion DV9000 Battery

The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[1] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.HP Pavilion DV7 Battery

Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years. HP Pavilion DV7-1290EO Battery

The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km (0.62 mi)[1] (with many billions withabsolute magnitudes[14] brighter than 11—corresponding to approximately 20 km (12 mi) diameter), with neighboring objects typically tens of millions of kilometres apart.HP Compaq Business 6735S Battery

Its total mass is not known with certainty, but, assuming that Halley's comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3×1025 kg (7×1025 lb or roughly five times the mass of the Earth) Earlier it was thought to be more massive (up to 380 Earth masses), HP 540 Battery

but improved knowledge of the size distribution of long-period comets has led to much lower estimates. The mass of the inner Oort Cloud is not currently known.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of various ices such as water, methane,ethane, carbon monoxide and hydrogen cyanide.HP Compaq Business NW9440 Battery

However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also be home to rocky objects. Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and Jupiter-family comets shows little difference between the two, despite their vastly separate regions of origin.HP Compaq Business TC4400 Battery

This suggests that both originated from the original protosolar cloud, a conclusion also supported by studies of granular size in Oort cloud comets and by the recent impact study of Jupiter-family comet Tempel 1.


The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago.HP Compaq 491278-001 Battery

The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giant planets such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits.HP HSTNN-LB31 Battery

Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.HP Pavilion DV3-4000 Battery

Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward towards the Oort cloud,HP Pavilion TX1000 Battery

while a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.[24] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.HP Pavilion DV6-1210SA Battery

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. HP 448007-001 Battery

Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.[26] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.[1]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. HP Compaq Business 6530B Battery

This explains the nearly spherical shape of the outer Oort cloud.[1] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. HP Pavilion DV5-1015EA Battery

These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.

In June 2010 Harold F. Levison and others have suggested on the basis of enhanced computer simulations that the Sun "HP Pavilion DV6-1230SB Battery

captured comets from other stars while it was in its birth cluster." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary disks of other stars." [28]


Comets are believed to have two separate points of origin in the Solar System. HP Pavilion ZD8181EA Battery

Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from the Kuiper belt or scattered disc, two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU from the Sun.HP Pavilion DV9500 Battery

Long-period comets, such as comet Hale-Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are believed to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets.HP Pavilion DV4-1125BR Battery

Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.[29] These centaurs are then sent farther inward to become the short-period comets.

There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. HP Pavilion DV5-1111EA Battery

Halley-family comets, named for their prototype, Halley's Comet, are unusual in that while they are short-period comets, it is believed that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is believed they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.HP Pavilion DV6-1234TX Battery

This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved.HP Pavilion DV3-2008TX Battery

No known dynamical process can explain this undercount of observed comets. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.HP Pavilion DV8 Battery

Dynamical studies of Oort Cloud comets have shown that their occurrence in the outer planet region is several times higher than in the inner planet region. This discrepancy may be due to the gravitational attraction ofJupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker-Levy 9 in 1994.HP Pavilion DV6-1215SA Battery

Tidal effects

Most of the comets seen close to the Sun are believed to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way galaxy. Just as the Moon's tidal force bends and deforms the Earth's oceans,HP Compaq 490306-001 Battery

causing the tides to rise and fall, the galactic tide also bends and distorts the orbits of bodies in the outer Solar System, pulling them towards the galactic centre. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun.Compaq Presario CQ50 Battery

At the outer reaches of the system, however, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides can deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes. HP Pavilion DV6-1223EO Battery

These small galactic perturbations may be enough to dislodge members of the Oort cloud from their orbits, sending them towards the Sun.[33] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.HP Compaq 490306-001 Battery

Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia—closest distances to the Sun—of planetesimals with large aphelia.[34]The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system.Compaq Presario CQ35-100 Battery

Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.[35] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.HP Compaq HSTNN-LB51 Battery

Star perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is believed to be interaction between the Sun's Oort cloud and the gravitational fields of near-by stars[1] orgiant molecular clouds.[32] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. HP Pavilion ZE4500 Battery

For example, during the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.[37] This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.HP G62-100EB Battery

In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf star, a dim red dwarf star or a gaseous giant planet, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, is hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, HP Pavilion DV6-1211AX Battery

bombarding the inner Solar System with comets. However, to date no direct evidence of Nemesis has been found.[39] One star survey using parallax measurements in order to clarify local star distances, the WISE mission, is currently underway with part of its mission being to help with either proving or disproving the Nemesis hypothesis.HP EliteBook 6930P Battery

The analysis of this survey is expected to be completed by 2013. Two other planned astronomical missions, the Pan-STARRS and the LSST astronomical surveys may also be able to aid in resolving the debate over the existence or non-existence of a Nemesis companion star to our Sun.HP Pavilion DV9500 Battery

A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the Oort Cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause is a Jupiter-mass object in a distant orbit.Compaq Evo N610C Battery

Oort cloud objects (OCOs)

Apart from long-period comets, only four known objects have orbits which suggest that they may belong to the Oort cloud: 90377 Sedna, 2000 CR105, 2006 SQ372 and 2008 KV42. The first two, unlike scattered disc objects, have perihelia outside the gravitational reach of Neptune, and thus their orbits cannot be explained by perturbations from the gas giant planets.HP Pavilion DM3-1050EO Battery

If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive.Compaq Presario 1245 Battery

Their present-day elliptical orbits can be explained by a number of hypotheses:

  1. These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster.HP G6000 Battery
  2. Their orbits could have been disrupted by an as-yet-unknown planet-sized body within the Oort cloud.
  3. They could have been scattered by Neptune during a period of particularly high eccentricity or by the gravity of a far larger primordial trans-Neptunian disc.HP Compaq 451086-661 Battery
  4. They could have been captured from around smaller passing stars.

Of these, the stellar disruption and "lift" hypothesis appears to agree most closely with observations.[4] Some astronomers prefer to refer to Sedna and 2000 CR105 as belonging to the "extended scattered disc" rather than to the inner Oort cloud.HP Pavilion DV2166EA Battery,HP Pavilion DM4 Battery,Dell Latitude E6400 Battery

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