Pluto
The largest known body in the Kuiper belt and the first dwarf planet, counted as the ninth planet from 1930 to 2006 and visited by New Horizons in July 2015.
At a glance
- Status
- IAU dwarf planet since 2006[2]Counted as the ninth planet from its discovery in 1930 until the International Astronomical Union adopted its definition of a planet in 2006
- Size
- About 2,377 km across, about one fifth the width of Earth[2]About two thirds the diameter of Earth's Moon, with about one sixth of the Moon's mass
- Orbit
- 30 to 49.3 au from the Sun, once every 248 years[2]Average distance 39 au; from 1979 to 1999 it was closer to the Sun than Neptune
- Orbit tilt
- 17.1° to the plane of the planets[3]Semi-major axis 39.6 au; a trans-Neptunian object in JPL's catalogue
- Day
- About 153 hours[2]Its axis is tilted 57° to its orbit, so it spins almost on its side, and backwards
- Moons
- Five: Charon, Nix, Hydra, Kerberos and Styx[2]Charon is about half the size of Pluto, the largest moon relative to its planet in the solar system; the other four are less than 160 km wide
- Visited
- New Horizons, 14 July 2015[7]Closest approach at 11:49 UTC, about 7,800 km above the surface
Missions to Pluto
Timeline
- January 23, 1930Discovery observation[3]Clyde Tombaugh at Flagstaff, Arizona; the date JPL's record gives for the discovery
- March 14, 1930The name Pluto is suggested[2]By Venetia Burney, aged 11, of Oxford, England, on the day the discovery was announced; her grandfather forwarded it to the Lowell Observatory, which chose it
- June 22, 1978Charon discovered[5]James Christy, US Naval Observatory
- 2005Nix and Hydra discovered[5]With the Hubble Space Telescope, by a team led by Hal Weaver
- 2006Reclassified as a dwarf planet[2]The International Astronomical Union's new definition of a planet, which Pluto fails because other objects cross its orbit
- January 19, 2006New Horizons launches[7]On an Atlas V from Cape Canaveral, at 19:00 UTC
- January 19, 2006Launch on Atlas VNew Horizons
- 2011Kerberos discovered[5]Hubble Space Telescope, a team led by Mark Showalter
- 2012Styx discovered[5]Hubble Space Telescope, a team led by Mark Showalter
- July 14, 2015New Horizons flies past[7]The first spacecraft to explore Pluto up close; the full data set took over 15 months to send home
- July 14, 2015Flyby of PlutoNew Horizons
- January 1, 2019Flyby of ArrokothNew Horizons
Pluto is the largest known object in the Kuiper belt, the ring of icy bodies beyond Neptune, and the best known of the dwarf planets.[1] Discovered in 1930, it was counted as the solar system's ninth planet until 2006, when the International Astronomical Union adopted a definition of a planet that it does not meet, because other objects cross its orbit.[2] It is about 2,377 km across, about one fifth the width of Earth and two thirds the diameter of Earth's Moon.[2] The guide What is a dwarf planet? explains the definition.
Discovery and name
JPL's record of the discovery names Clyde Tombaugh at Flagstaff, Arizona, with a discovery observation of 23 January 1930.[3] The name came from an eleven-year-old: on 14 March 1930 Venetia Burney of Oxford, England, suggested to her grandfather that the new world be named for the Roman god of the underworld, and he forwarded the idea to the Lowell Observatory, which chose it.[2] Pluto was also the first Kuiper belt object to be found, at a time when nobody expected a large population of icy worlds beyond Neptune.[1]
Orbit
Pluto's orbit is an oval that takes it from 30 to 49.3 astronomical units from the Sun over 248 years; on average it is 39 astronomical units away, so far that sunlight takes 5.5 hours to reach it.[2] From 1979 to 1999 it was closer to the Sun than Neptune.[2] The orbit is tilted 17.1 degrees to the plane of the planets.[3] It is locked in a 3:2 resonance with Neptune, circling the Sun twice for every three orbits of Neptune, and the many smaller objects on similar orbits are called plutinos after it.[1] A day on Pluto lasts about 153 hours, and its axis is tilted 57 degrees to its orbit, so it spins almost on its side.[2]
Surface and interior
Pluto probably has a rocky core surrounded by a mantle of water ice, and some researchers think there could be an ocean deep inside.[2] Its surface is among the coldest places in the solar system, between about -226 and -240 °C.[2] New Horizons found mountains 2 to 3 km high that are great blocks of water ice, sometimes coated with frozen methane, craters up to 260 km across, troughs and valleys as long as 600 km, and broad plains of frozen nitrogen with no craters at all.[2] The thin atmosphere is mostly nitrogen, with traces of methane and carbon monoxide.[2] Whether Pluto has a magnetic field is not known; its small size and slow rotation suggest little or none.[2]
Five moons
Pluto has five known moons: Charon, Nix, Hydra, Kerberos and Styx.[2][4] Charon, discovered in 1978 by James Christy, is about half the size of Pluto, which makes it the largest satellite relative to its planet in the solar system; the two are sometimes described as a double dwarf planet.[5][6] It is about 1,214 km across and orbits Pluto every 6.4 days at a distance of 19,640 km.[6] The other four moons were found with the Hubble Space Telescope between 2005 and 2012 and are each less than 160 km wide.[5][2] The whole system is believed to have formed after a collision between Pluto and another Kuiper belt object early in the history of the solar system.[5]
The visit
NASA's New Horizons launched on 19 January 2006 and flew past Pluto on 14 July 2015, passing about 7,800 km above the surface at 11:49 UTC: the first spacecraft to explore Pluto and its moons up close.[7] Sending the data home took over 15 months.[7] The spacecraft went on to fly past the small Kuiper belt object Arrokoth on 1 January 2019.[7]
Can I see it?
Not without a large telescope. At Pluto's distance the Sun at noon is about one nine-hundredth as bright as on Earth, and Pluto itself is far too faint for the eye or binoculars.[2]
Sources
- [1]NASA, Kuiper Belt: Facts — science.nasa.gov, retrieved November 5, 2026
- [2]NASA, Pluto: Facts — science.nasa.gov, retrieved November 5, 2026
- [3]NASA/JPL Small-Body Database, 134340 Pluto (1930 BM) — ssd.jpl.nasa.gov, retrieved November 5, 2026
- [4]NASA, Pluto — science.nasa.gov, retrieved November 5, 2026
- [5]NASA, Pluto Moons: Facts — science.nasa.gov, retrieved November 5, 2026
- [6]NASA, Charon — science.nasa.gov, retrieved November 5, 2026
- [7]NASA, New Horizons — science.nasa.gov, retrieved November 5, 2026
Related
- What is a dwarf planet?The definition that made Pluto a dwarf planet in 2006, the five the IAU recognises, why the list grows so slowly, and the candidates waiting to join it.
- The Kuiper belt and beyondThe ring of icy worlds past Neptune, the scattered disk and the Oort cloud beyond it, how many objects are known, and how new ones are found.
- New HorizonsNASA mission that made the first close study of Pluto in 2015 and flew past Arrokoth, the most distant object explored up close, in 2019.
- ErisThe dwarf planet about the size of Pluto whose discovery in 2005 led to Pluto's reclassification; it orbits about 68 au from the Sun once every 557 years.
- HaumeaA dwarf planet shaped like a football by its four-hour spin, with two moons and a ring found in 2017, orbiting about 43 au from the Sun once every 285 years.
- MakemakeA dwarf planet slightly smaller than Pluto and the second-brightest object in the Kuiper belt, with frozen methane on its surface and a moon found in 2016.
- OrcusA dwarf-planet candidate about 917 km across on an orbit much like Pluto's, in the same resonance with Neptune, with a large moon, Vanth, about half its size.
- 2026 ED138A dwarf-planet candidate found by the Rubin Observatory beyond Neptune and announced on 27 October 2026, an icy world that may be 600 to 1,500 km across.