Pluto is classified as a dwarf planet and is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant realm is populated with thousands of miniature icy worlds, which formed early in the history of our solar system. These icy, rocky bodies are called Kuiper Belt objects or transneptunian objects.
Pluto is about two-thirds the diameter of Earth's moon and probably has a rocky core surrounded by a mantle of water ice. More exotic ices like methane and nitrogen frost coat its surface. Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's moon. Pluto is more massive than Ceres -- the dwarf planet that resides in the asteroid belt between Mars and Jupiter -- by a factor of 14.
Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the sun. (One AU is the mean distance between Earth and the sun: about 93 million miles or 150 million kilometers.) From 1979 to 1999, Pluto was actually closer to the sun than Neptune, and in 1989, Pluto came to within 29.8 AU of the sun, providing rare opportunities to study this small, cold, distant world.
Since its orbit is so elliptical, when Pluto is close to the sun, its surface ices sublimate, changing directly from solid to a gas, and rise and temporarily form a thin atmosphere. Pluto's low gravity (about six percent of Earth's) causes the atmosphere to be much more extended in altitude than our planet's atmosphere. Pluto becomes much colder during the part of each orbit when it is traveling far away from the sun. During this time, the bulk of the planet's atmosphere is thought to freeze and fall as snow to the surface.
Pluto has a very large moon that is almost half its size named Charon, which was discovered in 1978. This moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 12,200 miles (19,640 kilometers).
The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth. These photos showed that Charon is grayer than Pluto (which is redder), indicating that they have different surface compositions and structure.
Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days. Charon neither rises nor sets, but hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto -- this is called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus. Pluto's rotation is retrograde: it rotates backwards, from east to west (Uranus and Venus also have retrograde rotations).
Because Pluto and Charon are so small and far away, they are extremely difficult to observe from Earth. In the late 1980s, Pluto and Charon passed in front of each other repeatedly for several years. Observations of these rare events allowed astronomers to make rudimentary maps of each body showing areas of relative brightness and darkness.
In 2005, scientists photographing Pluto with the Hubble Space Telescope in preparation for the New Horizons mission found two tiny moons orbiting in the same plane as Charon. These two moons, named Nix and Hydra, are two to three times farther away from Pluto than Charon.
In 2011 and 2012, scientists used Hubble to spot two more moons (originally designated P4 and P5). In 2013, the two moons were named Kerberos (P4) and Styx (P5).
Late in 2014 and early in 2015, image animations displayed the mutual orbital waltz of Pluto and Charon around their center of mass. Beginning in the spring of 2015, New Horizons started its detailed studies of Pluto including searches for additional moons and for rings. Various studies continued through its close approach on July 14 at a distance of 8507 miles (13,691 kilometers) and after. The return of New Horizons' best data began shortly after its close approach and will continue for more than a year due to the large transmission distance. Because of the speed of the flyby and Pluto's slow rotation rate only one hemisphere of this dwarf planet has been photographed and measured at high resolution, limiting generalizations about all of Pluto's surface. Still, Pluto's diameter could be measured and had to be revised upward to 1475 miles (2,374 kilometers) based on New Horizons' imagery.
Results from New Horizons, besides forcing the revision of textbooks, left planetary scientists struggling to explain this cold, distant world and its system of moons. Pluto's surface exhibits craters as large as 162 miles (260 kilometers) in diameter on the dayside, near encounter hemisphere. Craters are widely distributed there and show degradation or infill. This was a surprise to find on a dwarf planet that was anticipated to be heavily cratered and lacking activity that might affect craters. Dense cratering is seen in some areas, but so are tectonic features including scarps and troughs as long as about 370 miles (600 kilometers).
Mountains are also seen rising 6500 to 9800 feet (2 to 3 kilometers) above their surroundings. The likely materials that can hold up the mountains and maintain their shapes over millions of years in Pluto's cold is limited. The science team concludes that the mountains are made of "water ice-based 'bedrock'." Frozen gases on Pluto's surface (the temperature there is about -391 F = -235 C = 38 K) include nitrogen (N2), carbon monoxide (CO),and methane (CH4). These were detected by ground-based telescopes and are now thought to be thin layers on top of the 'bedrock' water ice.
Dark surface coloring appears to be due carbon residues called tholins. These are created by solar ultraviolet rays or charged particles falling on mixtures of nitrogen and methane.
Detailed views of a plain on this dwarf planet show no confirmed craters but it has large (tens of kilometers) polygonal or egg-shaped features defined and separated by troughs between neighbors. Features similar in look to glaciers on Earth are seen in this region. This is consistent with the strength and flow properties of the frozen gases mentioned above.
Radio transmission measurements from New Horizons measured Pluto's atmosphere having a pressure of 10 microbars (millionths of a bar). For comparison, Earth's atmospheric pressure at sea level is about 1 bar, 100,000 times greater than Pluto's surface pressure. Those measurements also showed that Pluto has a shallow tropospheric boundary layer (Earth has a troposphere too). Imagery from the cameras shows a haze layer and even some structure in it.
It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or none. Data from two of New Horizons instruments may give an indirect answer to this question.
Charon was also studied in detail. Its diameter came out slightly larger than expected, at 753 miles (1212 kilometers). Surface variations of 9800 feet (3 kilometers) seen on this moon imply that it, like Pluto, has water ice that runs deep in its structure. Also similar to Pluto, Charon exhibits cratered and smooth plains, fault scarps, and an extensive system of faults and graben. (A graben [Grah-ben] is a block of surface material that has dropped lower than its surroundings, creating a wide valley whose walls are the fault planes along which the block dropped.)
Rolling plains are moderately cratered. They even show several rille-like structures (first observed on Earth's moon). Craters with rays (both light, as on Earth's moon, and dark, as sometimes seen on Mars) indicating freshness are found on Charon. Other craters show evidence of aging.
The two largest fractures (visible during the flyby) on Charon extend at least 650 miles (1050 kilometers) across the surface and at least one other, with a depth of 3 miles (5 kilometers) is seen going over the horizon to Charon's night side. One of the dayside fractures is seen as a double-walled graben-like structure.
There is no evidence that Charon has an atmosphere.
Nix is not spherical. It has three different diameters (making it a tri-axial ellipsoid): 67 x 51 x 45 miles (108 x 82 x 72 kilometers). There are variations in composition over its surface but it reflects more light than Charon. This suggests that its ice is cleaner than Charon's.
Hydra is also not spherical with rough diameters of 53 x 41 miles (86 x 66 kilometers) with the third dimension not well determined. It also is more reflective than Charon, leaving scientists to puzzle over how the ice on these small moons could stay so bright over billions of years given the processes known to darken material over time.
More information on Kerberos and Styx awaits the return of more data from New Horizons. Data used for searches for additional moons and rings has not detected any.
Pluto is the only world named by an 11-year-old girl. In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for the Roman god of the underworld. He forwarded the name to the Lowell Observatory and it was selected. Pluto's moons are named for other mythological figures associated with the underworld. Charon is named for the river Styx boatman who ferries souls in the underworld (as well as honoring Sharon, the wife of discoverer James Christy); Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology (and called Fluffy in the Harry Potter novels); and Styx is named for the mythological river that separates the world of the living from the realm of the dead.
Pluto's place in mythology can get a little muddled, so we asked Dr. Elizabeth Vandiver, chair of the Department of Classics in Whitman College in Walla Walla, Washington, to clarify the origins of the name: "Pluto is the name of the Roman god of the Underworld, equivalent to the Greek Hades. However, the Greek name "Plouton" (from which the Romans derived their name "Pluto") was also occasionally used as an alternative name for Hades. But Pluto is definitely the Roman spelling."
