Pluto

Pluto is a dwarf planet located in a distant region of the solar system beyond Neptune known as the Kuiper Belt. Pluto is about 5.5 times smaller than Earth.

Pluto is a complex and mysterious world with mountains, valleys, plains, craters, and glaciers.

Pluto was discovered on 18 February 1930 by Clyde W. Tombaugh at Lowell Observatory in Flagstaff, Arizona. The search for Planet X had resumed in 1929 when the work was handed to Tombaugh, then twenty-three years old, who used a blink comparator to spot a moving object on photographic plates taken on January 23 and January 29 in 1930 from the Lowell Observatory. The discovery plates were taken only six days apart, and the object Tombaugh found had moved about 3 mm on the plates between the two exposures, indicating an orbital distance of about 40-43 AU, at about the right place to be the predicted planet. He captured wide-field images using 14 x 17 inch glass photographic plates that revealed Pluto's location among distant stars, and on February 18 in 1930 he found a tiny spot of light moving slowly against the fixed pattern of stars in the constellation Gemini. Tombaugh told Vesto Slipher he had found Planet X, and the Observatory announced the finding on March 13 in 1930; the announcement date matched the anniversary of Herschel's discovery of Uranus and Percival Lowell's birthday.

The new world was christened Pluto in 1930 on a name suggested by eleven-year-old Venetia Burney of Oxford, England, favored partly because its first two letters were the initials of Percival Lowell. The name was announced on May 1 in 1930 and she received five pounds as a reward.

Sixteen pre-discovery photographs of Pluto are known, the oldest taken by Yerkes Observatory on 20 August 1909.

After the discovery Tombaugh received a scholarship to study astronomy at the University of Kansas; he began as a freshman in 1932 and worked in astronomy for many years.

Pluto's diameter is about 1,477 miles (2,377 kilometers), making it roughly two-thirds the diameter of Earth's Moon. Its mass is about one-sixth that of Earth's Moon. The body probably has a rocky core surrounded by a mantle of water ice.

Surface temperatures on Pluto range from -375 to -400 degrees Fahrenheit (-226 to -240 degrees Celsius). The dwarf planet is very bright, with an average reflectivity of 0.5 compared with only about 10% for Earth's Moon.

Pluto's tallest mountains stand 6,500 to 9,800 feet (2 to 3 kilometers) high and are big blocks of water ice, sometimes coated with frozen gases such as methane. The most prominent plains appear to be made of frozen nitrogen and show no craters, with structures suggesting convection of material. Sputnik Planitia is a vast frozen nitrogen glacier larger than the U.S. states of Texas and Oklahoma combined. An analysis of New Horizons imagery from the 2015 encounter found evidence that liquid nitrogen is rising to Pluto's surface through cracks at the northern edge of Sputnik Planitia, the first evidence of recently flowing liquid on Pluto.

Pluto travels the Sun on an orbit that is both elliptical and tilted, completing one circuit in 248 years. Its mean distance from the Sun is about 3.7 billion miles (5.9 billion kilometers), or 39 AU, but the oval-shaped path carries it as far out as 49.3 astronomical units and as close in as 30 AU. The orbit is inclined by 17.14 degrees with respect to the ecliptic, the highest inclination of any planet in the solar system. Pluto and Neptune never come very close to each other because their orbits are held in an exact resonance: Pluto orbits the Sun twice for every three orbits of Neptune.

From 1979 to 1999 Pluto was near perihelion and was actually closer to the Sun than Neptune. One day on Pluto takes about 153 hours, and its axis is tilted 57 degrees with respect to the plane of its orbit, so it spins almost on its side, with a retrograde rotation from east to west.

Pluto has five moons: Charon, Nix, Hydra, Kerberos and Styx. Charon is the largest of them, measuring 754 miles (1,214 kilometers) across. At about half the size of Pluto, it is the largest known satellite relative to its parent body in the solar system, and the two are often called a double planet. The same surfaces of Charon and Pluto always face each other in mutual tidal locking, and Charon orbits Pluto every 6.4 Earth days.

Charon was discovered in June 1978 by James Christy and Robert Harrington at the U.S. Naval Observatory in Flagstaff, Arizona, about six miles from where Pluto was discovered. Christy noticed that images of Pluto were strangely elongated, with the direction of elongation cycling back and forth over 6.39 days, Pluto's rotation period. He proposed the name Charon for the mythological ferryman of the underworld, also choosing it because its first four letters matched the name of his wife, Charlene.

Nix and Hydra were discovered in June 2005 by a large team led by New Horizons science team members Hal Weaver and Alan Stern using the Hubble Space Telescope. Kerberos was discovered on June 28 in 2011 by a large team led by Mark Showalter, while astronomers searched for rings around Pluto with the Hubble Space Telescope. Styx, Pluto's fifth moon, was uncovered on June 26 in 2012 by a Showalter-led Hubble team searching for potential hazards for the New Horizons flyby. The IAU approved the names Kerberos and Styx in 2013 following an unprecedented internet vote held in February 2012 that received almost 500,000 votes, including 30,000 write-in suggestions.

Pluto's atmosphere is thin and tenuous, composed mainly of molecular nitrogen, with methane and carbon monoxide also present. It behaves in a manner resembling a comet's, expanding as the dwarf planet draws closer to the Sun and collapsing as it moves farther away. Its density is roughly 100,000 times smaller than that of Earth's atmosphere, and it could freeze out onto the surface as Pluto recedes from the Sun.

Looking back at Pluto on departure, New Horizons discovered multiple haze layers wrapping completely around the planet and extending over several hundred kilometers in height.

NASA's New Horizons was the first spacecraft to explore Pluto up close, flying by the dwarf planet and its moons on 14 July 2015. It was the first and so far the only mission to Pluto, and the first to explore the Kuiper Belt, the solar system's third zone. The spacecraft launched in January 2006 and received a gravity assist from Jupiter in February 2007. That Jupiter encounter increased the spacecraft's velocity by about 9,000 miles per hour (14,000 kilometers per hour), shortening its trip to Pluto by three years.

At closest approach New Horizons came about 7,750 miles (12,500 kilometers) from Pluto and about 17,900 miles (28,800 kilometers) from Charon. In early 2019 the mission flew past Arrokoth, the most distant object ever explored up close. Pluto and Charon form a binary planet, and although binary planets are thought to be common in the galaxy, no spacecraft had explored one until New Horizons visited the Pluto-Charon system during its 2015 flyby.

New Horizons revealed Pluto to be a geologically and meteorologically active world, and it remains the physically largest world in the Kuiper Belt. Charon's surface is dominated by water ice and has no atmosphere, and New Horizons also confirmed ammonia ice across much of the surface.

The International Astronomical Union defines a dwarf planet as a celestial body orbiting the Sun with enough mass for hydrostatic equilibrium that has not cleared its orbital neighbourhood and is not a satellite. Pluto was long considered the ninth planet, but in 2006 the IAU reclassified it as a dwarf planet. It failed one of the three criteria for a full-sized planet, having not cleared its neighbouring region of other objects, and it was reclassified because it lies in the Trans-Neptunian region where other objects might cross its orbital path. The IAU explains that after a vigorous debate the first draft of the planet definition was presented at the XXV General Assembly in Prague, and Pluto was found not to meet the criteria, so it is a dwarf planet. In August 2006 the downgrade left the rocky inner worlds and the giant planets as the only planets, so the Solar System has eight planets instead of nine. Resolution 6A was passed with 237 votes in favour, 157 against and 17 abstentions, while Resolution 6B on the name of the category had 183 votes in favour and 186 against.

An IAU draft resolution in 2006 would have kept Pluto a planet and enlarged the Solar System to 12 planets, adding Ceres, Charon and 2003 UB313 while keeping Pluto the prototype of a new category of plutons. A Planet Definition Committee of seven astronomers, writers and historians formed by the IAU Executive Committee reached unanimous consensus in July 2006 on a proposed new definition of planet. Resolution 6A declares Pluto a dwarf planet by the IAU definition and recognizes it as the prototype of a new category of trans-Neptunian objects. The first members of the dwarf planet category were Ceres, Pluto and the object 2003 UB313 that was later named Eris.

The discovery of Pluto in early 1930 was made by a farm boy from Kansas with no formal training in astronomy, and the announcement that March excited both scientists and the public.

He died in 1997 without seeing Pluto demoted to dwarf planet status. Tombaugh lived to within three weeks of his 91st birthday and died on January 17 in 1997 in Las Cruces, New Mexico.

• 20 August 1909
  Earliest known pre-discovery photograph
  The oldest of sixteen known pre-discovery photographs of Pluto was taken by Yerkes Observatory.^{5Why is Pluto no longer a planet? · Library of Congress ↗}
• 18 February 1930
  Discovery
• 1 May 1930
  Name announced
  The name Pluto was announced.^{5Why is Pluto no longer a planet? · Library of Congress ↗}
• June 1978
  Charon discovered
• 1979–1999
  Near perihelion
  Pluto was near perihelion and was actually closer to the Sun than Neptune.^{3Pluto: Facts · NASA Science ↗}
• June 2005
  Nix and Hydra discovered
  A large team led by New Horizons science team members Hal Weaver and Alan Stern discovered Nix and Hydra using the Hubble Space Telescope.^{2The Pluto System · Johns Hopkins University Applied Physics Laboratory (New Horizons) ↗}
• January 2006
  New Horizons launched
  New Horizons blasted off from Cape Canaveral.^{19The Year of Pluto · Lowell Observatory ↗}
• 2006
  Reclassification
  The IAU reclassified Pluto from planet to dwarf planet.^{1Pluto · NASA Science ↗}
• February 2007
  Jupiter gravity assist
  New Horizons received a gravity assist from Jupiter.^{13New Horizons · NASA Science ↗}
• 28 June 2011
  Kerberos discovered
  A large team led by Mark Showalter discovered Kerberos while astronomers searched for rings around Pluto with the Hubble Space Telescope.^{2The Pluto System · Johns Hopkins University Applied Physics Laboratory (New Horizons) ↗}
• 26 June 2012
  Styx discovered
  A Showalter-led Hubble team searching for potential hazards for the New Horizons flyby uncovered Styx, Pluto's fifth moon.^{2The Pluto System · Johns Hopkins University Applied Physics Laboratory (New Horizons) ↗}
• 2013
  Moon names approved
  The IAU approved the names Kerberos and Styx following an unprecedented internet vote held in February 2012 that received almost 500,000 votes, including 30,000 write-in suggestions.^{2The Pluto System · Johns Hopkins University Applied Physics Laboratory (New Horizons) ↗}
• 14 July 2015
  New Horizons flyby
  New Horizons flew by the dwarf planet and its moons.^{13New Horizons · NASA Science ↗} At closest approach it came about 7,750 miles (12,500 kilometers) from Pluto and about 17,900 miles (28,800 kilometers) from Charon.^{12The Path to Pluto and Arrokoth · Johns Hopkins University Applied Physics Laboratory (New Horizons) ↗}
• Early 2019
  Arrokoth flyby
  New Horizons flew past Arrokoth, the most distant object ever explored up close.^{13New Horizons · NASA Science ↗}