Pluto

   Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the 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 Moon and a third its volume.

  From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned. Later, in the early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union (IAU) defined the term "planet" for the first time. This definition excluded Pluto as a planet, and added it as a member of the new category "dwarf planet" along with Eris and Ceres. 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 together as a binary system because the barycentre of their orbits does not lie within either body. 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. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.

   Pluto's orbital period lasts for 248 Earth years. Its orbital characteristics are substantially different from those of the planets. The planets all orbit the Sun close to a flat reference plane called the ecliptic and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined relative to the ecliptic (over 17°) and highly eccentric (elliptical). This high eccentricity leads to a small region of Pluto's orbit lying closer to the Sun than Neptune's. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. 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.

  Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact chaotic. While computer simulations can be used to predict its position for several million years (both forward and backward in time), after intervals longer than the Lyapunov time of 10–20 million years, it is impossible to determine exactly where Pluto will be because its position becomes too sensitive to unmeasurably small details of the present state of the Solar System. For example, at any specific time many millions of years from now, Pluto may be at aphelion or perihelion (or anywhere in between), with no way for us to predict which. This does not mean that the orbit of Pluto itself is unstable, however, only that its position along that orbit is impossible to determine far into the future. Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.

   Pluto's rotation period, its day, is equal to 6.39 Earth days. 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 its solstices, one hemisphere is in permanent daylight, while the other is in permanent darkness.

  Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. To see it, a telescope is required; around 30 cm (12 in) aperture being desirable. 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.

  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. 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.

  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. 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.

  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 moon Iapetus. 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. 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.

   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.

  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. 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. 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, 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.

  Pluto's mass is 1.31×1022 kg, less than 0.24 percent that of the Earth, while its diameter is roughly 2,390 km, or roughly 70% that of the Moon.

 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. Its surface pressure ranges from 6.5 to 24 μbar. 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. This creates an anti-greenhouse effect; much as sweat cools 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.

  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. The lower atmosphere contains a higher concentration of methane than its upper atmosphere.

  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.

  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. Astronomers now believe Pluto to be the largest 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. 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. Eris  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.

 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.