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 20.
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 150 million km or 93 million miles.) From 1979 to 1999, Pluto was actually closer to the sun than Neptune, and in 1989, Pluto came to within 29.7 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 thaw, rise and temporarily form a thin atmosphere. Pluto's low gravity (about 6 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.
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 19,640 km (12,200 miles).
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 red), 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).
It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or no magnetic field.
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).
How Pluto Got its Name
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 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; 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 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."
- 1930: Clyde Tombaugh discovers Pluto.
- 1977-1999: Pluto's lopsided orbit brings it slightly closer to the sun than Neptune. It will be at least 230 years before Pluto moves inward of Neptune's orbit for 20 years.
- 1978: American astronomers James Christy and Robert Harrington discover Pluto's unusually large moon, Charon.
- 1988: Astronomers discover that Pluto has an atmosphere.
- 2005: Scientists using the Hubble Space Telescope announce the discovery of two additional moons of Pluto. Named Nix and Hydra, the little moons may have formed at the same time as Charon did, perhaps all three splitting off from Pluto in a giant impact event.
- 2006: NASA's New Horizons mission launches on a path to explore Pluto and the Kuiper Belt region. The spacecraft is scheduled to reach Pluto in 2015.
- 2006: The International Astronomical Union classifies Pluto as a dwarf planet and recognizes similar worlds beyond the orbit of Neptune as plutoids.
- 2011: Astronomers using the Hubble Space Telescope discovered a fourth moon, named Kerberos, orbiting the icy dwarf planet.
- 2012: Astronomers using the Hubble Space Telescope to search for potential hazards to the New Horizon mission discovered a fifth moon, named Styx, orbiting the icy dwarf planet.