lutoFrom Wikipedia, the free encyclopediaThis article is about the dwarf planet. For other uses, see Pluto (disambiguation).

Pluto 

Near-true-color image composite of Pluto taken by the New

Horizons spacecraft on 14 July 2015 from a distance of

450,000 kilometers

Discovery

Discovered by Clyde W. Tombaugh

Discovery date 18 February 1930

Designations

MPC   designation 134340 Pluto

Pronunciationi /ˈpluːtoʊ/

Named after Pluto

Minor   planet category Dwarf planet

Trans-Neptunian object

Plutoid

Kuiper belt  object

Plutino

Adjectives Plutonian

Orbital characteristics [4] [a]

Epoch J2000

Aphelion 49.319 AU

(7311000000 km)

Perihelion 29.656 AU

(4437000000 km)

(5 September 1989)[1]

Semi-major axis 39.487 AU

(5874000000 km)

Eccentricity 0.24897

Orbital period 247.94 years [2]

90570 d[2]

14164.4 Plutonian solar days [3]

Synodic period 366.73 days[2]

Average orbital speed 4.67 km/s[2]

Mean anomaly 14.85 deg

Inclination 17.1405°

(11.88° to Sun's equator)

Longitude   of ascending   node 110.301°

Argument   of perihelion 113.777°

Known satellites 5

Physical characteristics

Mean radius 1186±2 km[5]

0.18 Earths

Surface area 1.77×107 km2[b]

0.035 Earths

Volume 6.99×109 km3[c]

0.0064 Earths

Mass (1.305±0.007)×1022 kg[6]

0.0022 Earths

0.178 Moons

Mean density 1.87±0.02 g/cm3[d]

Surface gravity 0.620 m/s 2 [e]

0.063 g

Escape velocity 1.212 km/s[f]

Sidereal rotation period 6.387230 d

6 d, 9 h, 17 m, 36 s

Equatorial rotation velocity 47.18 km/h

Axial tilt 119.591°±0.014° (to orbit)[6][g]

North poleright ascension 132.993°[7]

North poledeclination −6.163°[7]

Albedo 0.49 to 0.66 (geometric, varies by

35%)[2][8]

Surface temp. min mean max

Kelvin 33 K 44 K (−229 °C) 55 K

Apparent magnitude 13.65[2] to 16.3[9]

(mean is 15.1)[2]

Absolute magnitude   (H) −0.7[10]

Angular diameter 0.06″ to 0.11″[2][h]

Atmosphere

Surface pressure 0.30 Pa (summer maximum) to

1.0 Pa [12]

Composition by volume Nitrogen, methane, carbon

monoxide [11]

Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. It is the largest

and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, a dwarf planet in the scattered disc. Like other Kuiper belt objects, Pluto is primarily made of ice and rock[13] and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.3 billion km) from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding. In 2014, Pluto was 32.6 AU from the Sun. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.4 AU).[14]

Pluto was discovered in 1930 by Clyde Tombaugh, and was originally considered the ninth planet from the Sun. After 1992, its status as a planet fell into question following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered, which led the International Astronomical Union (IAU) to define the term "planet" formally for the first time the following year.[15] This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid).[16] Some astronomers think that Pluto and the other dwarf planetsshould be considered planets.[17][18][19]

Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra.[20] Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.[21] The IAU has not formalized a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.[22]

On 14 July 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto.[23][24]

[25] During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons.[26]

Contents

  [hide] 

1 Historyo 1.1 Discoveryo 1.2 Nameo 1.3 Planet X disprovedo 1.4 Classification

1.4.1 IAU classification 2 Orbit

o 2.1 Relationship with Neptune 2.1.1 Other factors

o 2.2 Quasi-satellite 3 Rotation

o 3.1 Daylight 4 Geology

o 4.1 Surfaceo 4.2 Internal structure

5 Mass and size 6 Atmosphere 7 Satellites 8 Origin 9 Observation and exploration

o 9.1 Observationo 9.2 Exploration

10 Galleryo 10.1 Videos

11 See also 12 Notes 13 References 14 External links

HistoryDiscoveryFurther information: Planets beyond Neptune

Discovery photographs of Pluto

Clyde Tombaugh, in Kansas

In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptuneafter analysing perturbations in the orbit of Uranus.[27] Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.

In 1906, Percival Lowell—a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X".[28] By 1909, Lowell andWilliam H. Pickering had suggested several possible celestial coordinates for such a planet.[29] Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on 19 March and 7 April 1915, but they were not recognized for what they were.[29][30] There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on 20 August 1909.[31]

Percival's widow, Constance Lowell, attempted to wrest the observatory's million-dollar portion of his legacy for herself. Because of that ten-year legal battle, the search for Planet X did not resume until 1929.[32] Vesto Melvin Slipher, the observatory director, summarily handed the job of locating Planet X to 23-year-old Clyde Tombaugh, who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.[32]

Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position. Using a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On 18 February 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on 23 and 29 January of that year. A lesser-quality photograph taken on 21 January helped confirm the movement.[33] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on 13 March 1930.[29]

NameSee also: Venetia Burney

The discovery made headlines around the globe. The Lowell Observatory, which had the right to name the new object, received more than 1,000 suggestions from all over the world, ranging from Atlas to Zymal.[34] Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.[34]Constance Lowell proposed Zeus, then Percival and finally Constance. These suggestions were disregarded.[35]

The name Pluto, after the god of the underworld, was proposed by Venetia Burney (1918–2009), a then eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.[36] She suggested it in a conversation with her gran