Pluto

| dimensions = (observations consistent with a sphere, predicted deviations too small to be observed)

| sidereal_day =

| rot_velocity =

| axial_tilt = (to orbit)

| atmosphere = yes

| temp_name1 = Kelvin

| min_temp_1 = 33 K

| mean_temp_1 = 44 K (−229 °C)

| max_temp_1 = 55 K

| surface_pressure = 1.0 Pa (2015) ()

| atmosphere_composition = Nitrogen, methane, carbon monoxide

In 1906, Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "Planet X". The name was published on May 1, 1930.

The name Pluto had received some 150 nominations among the letters and telegrams sent to Lowell. The first had been from Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.

The choice was further helped by the fact that the first two letters of Pluto were the initials of Percival Lowell; indeed, 'Percival' had been one of the more popular suggestions for a name for the new planet.

File:Pluto discovery plates.gif|alt=The same area of night sky with stars, shown twice, side by side. One of the bright points, located with an arrow, changes position between the two images.|Discovery photographs of Pluto

File:Clyde W. Tombaugh.jpeg|Clyde Tombaugh, in Kansas

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Symbol

Once named, Pluto's planetary symbol was then created as a monogram of the letters "PL". but not in a graph of planets, dwarf planets and moons from 2016, where only the eight IAU planets are identified by their symbols.

(Planetary symbols in general are uncommon in astronomy, and are discouraged by the IAU.) though it is still common in astrology. However, the most common astrological symbol for Pluto, occasionally used in astronomy as well, is an orb (possibly representing Pluto's invisibility cap) over Pluto's bident , which dates to the early 1930s.

The name 'Pluto' was soon embraced by wider culture. In 1930, Walt Disney was apparently inspired by it when he introduced for Mickey Mouse a canine companion named Pluto, although Disney animator Ben Sharpsteen could not confirm why the name was given.

{| class="wikitable plainrowheaders floatright" style="clear:right;"

|+ Mass estimates for Pluto

!scope="col"| Year

!scope="col"| Mass

!scope="col"| Estimate by

!scope="row"| 1915

| 7 Earths

| Lowell (prediction for Planet X)

!scope="row"| 1976

| less than 0.01 (1/100) Earth<br>0.002–0.005 (1/200–1/500) Earth

| Cruikshank, Pilcher, & Morrison

!scope="row"| 1978

| 0.0017 (1/588) Earth

| Christy & Harrington

Classification

From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the Kuiper belt. This made its official status as a planet controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System. In February 2000 the Hayden Planetarium in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later. Although the first Kuiper belt objects discovered were quite small, objects increasingly closer in size to Pluto were soon discovered, some large enough (like Pluto itself) to satisfy geological but not dynamical ideas of planethood.

In 1998, Brian G. Marsden of Harvard University's Minor Planet Center suggested that Pluto be given the minor planet number 10000 while still retaining its official position as a planet. The prospect of Pluto's "demotion" created a public outcry, and in response the International Astronomical Union clarified that it was not at that time proposing to remove Pluto from the planet list.

The debate became unavoidable when, in July 2005, these astronomers announced the discovery of a new object, Eris, which was substantially more massive than Pluto and the most massive object discovered in the Solar System since Triton in 1846. The press initially called it the tenth planet, although there was no official consensus at the time on whether to call it a planet. According to their proposal, there are three conditions for an object in the Solar System to be considered a planet:

The object must be in orbit around the Sun.
The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.
It must have cleared the neighborhood around its orbit. Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon).

There has been some resistance within the astronomical community toward the reclassification, and in particular planetary scientists often continue to reject it, considering Pluto, Charon, and Eris to be planets for the same reason they do so for Ceres. In effect, this amounts to accepting only the second clause of the IAU definition. He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.

Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University Applied Physics Laboratory for a conference that included back-to-back talks on the IAU definition of a planet.

This alone is not enough to protect Pluto; perturbations from the planets (especially Neptune) could alter Pluto's orbit (such as its orbital precession) over millions of years so that a collision could happen. However, Pluto is also protected by its 2:3 orbital resonance with Neptune: for every two orbits that Pluto makes around the Sun, Neptune makes three, in a frame of reference that rotates at the rate that Pluto's perihelion precesses (about degrees per year Pluto and Neptune's minimum separation is over 17 AU, which is greater than Pluto's minimum separation from Uranus (11 AU).

Other factors

Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change.

According to a paper released from the University of Arizona, this could be caused by masses of frozen nitrogen building up in shadowed areas of the dwarf planet. These masses would cause the body to reorient itself, leading to its unusual axial tilt of 120°. The buildup of nitrogen is due to Pluto's vast distance from the Sun. At the equator, temperatures can drop to , causing nitrogen to freeze as water would freeze on Earth. The same polar wandering effect seen on Pluto would be observed on Earth were the Antarctic ice sheet several times larger.

Geology

Surface

thumb|Sputnik Planitia is covered with churning nitrogen ice "cells" that are geologically young and turning over due to [[Convection cell|convection.]]

The plains on Pluto's surface are composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide. Pluto's surface is quite varied, with large differences in both brightness and color. there are obvious signs of glacial flows both into and out of the basin. It has no craters that were visible to New Horizons, indicating that its surface is less than 10 million years old. Latest studies have shown that the surface has an age of years.

The New Horizons science team summarized initial findings as "Pluto displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface–atmosphere interactions as well as impact, tectonic, possible cryovolcanic, and mass-wasting processes."

File:Pluto-01 Stern 03 Pluto Color TXT.jpg|A multispectral Visual Imaging Camera image of Pluto in enhanced color to bring out differences in surface composition

File:Pluto_Charon_crater_map_Robbins_Dones_2023.jpg|The distribution of numerous impact craters and basins on both Pluto and Charon. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity. Precisely for this reason, the confidence of numerous craters on Pluto remain uncertain. In September 2016, scientists at Brown University simulated the impact thought to have formed Sputnik Planitia, and showed that it might have been the result of liquid water upweling from below after the collision, implying the existence of a subsurface ocean at least 100 km deep.

In June 2020, astronomers reported evidence that Pluto may have had a subsurface ocean, and consequently may have been habitable, when it was first formed. In March 2022, a team of researchers proposed that the mountains Wright Mons and Piccard Mons are actually a merger of many smaller cryovolcanic domes, suggesting a source of heat on the body at levels previously thought not possible.

Mass and size

thumb|right|Pluto (bottom left) compared in size to the Earth and the MoonPluto's diameter is

With less than 0.2 lunar masses, Pluto is much less massive than the terrestrial planets, and also less massive than seven moons: Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of adaptive optics allowed them to determine its shape more accurately. which was later revised to be on July 24, and later to .

New Horizons observations showed that atmospheric escape of nitrogen to be 10,000 times less than expected.

In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016. In 2021, astronomers at the Southwest Research Institute confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.

The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with the average temperature of its atmosphere tens of degrees warmer than its surface, The satellites' orbits are circular (eccentricity < 0.006) and coplanar with Pluto's equator (inclination < 1°), and therefore tilted approximately 120° relative to Pluto's orbit. The Plutonian system is highly compact: the five known satellites orbit within the inner 3% of the region where prograde orbits would be stable. From any position on either body, the other is always at the same position in the sky, or always obscured.

Quasi-satellite

In 2012, it was calculated that 15810 Arawn could be a quasi-satellite of Pluto, a specific type of co-orbital configuration. and confirmed the earlier ones. However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on its orbital dynamics, since its orbit is primarily controlled by Neptune with only occasional perturbations by Pluto. knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.

Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers began to find small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is thought to be the source of many short-period comets. Pluto is the largest member of the Kuiper belt, a stable belt of objects located between 30 and 50 AU from the Sun. As of 2011, surveys of the Kuiper belt to magnitude 21 were nearly complete and any remaining Pluto-sized objects are expected to be beyond 100 AU from the Sun. It has been proposed that Pluto may have formed as a result of the agglomeration of numerous comets and Kuiper-belt objects.

Though Pluto is the largest Kuiper belt object discovered,

Like other members of the Kuiper belt, Pluto is thought to be a residual planetesimal; a component of the original protoplanetary disc around the Sun that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its position to a sudden migration undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself (Triton), locking others into resonances, and knocking others into chaotic orbits. The objects in the scattered disc, a dynamically unstable region overlapping the Kuiper belt, are thought to have been placed in their positions by interactions with Neptune's migrating resonances. because its angular diameter is maximum 0.11".

Since the New Horizons flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives. They include mapping the surface at per pixel, observations of Pluto's smaller satellites, observations of how Pluto changes as it rotates on its axis, investigations of a possible subsurface ocean, and topographic mapping of Pluto's regions that are covered in long-term darkness due to its axial tilt. The last objective could be accomplished using laser pulses to generate a complete topographic map of Pluto.

New Horizons principal investigator Alan Stern has advocated for a Cassini-style orbiter that would launch around 2030 (the 100th anniversary of Pluto's discovery) and use Charon's gravity to adjust its orbit as needed to fulfill science objectives after arriving at the Pluto system.

New Horizons imaged all of Pluto's northern hemisphere, and the equatorial regions down to about 30° South. Higher southern latitudes have only been observed, at very low resolution, from Earth. Images from the Hubble Space Telescope in 1996 cover 85% of Pluto and show large albedo features down to about 75° South. This is enough to show the extent of the temperate-zone maculae. Later images had slightly better resolution, due to minor improvements in Hubble instrumentation. The equatorial region of the sub-Charon hemisphere of Pluto has only been imaged at low resolution, as New Horizons made its closest approach to the anti-Charon hemisphere.

Some albedo variations in the higher southern latitudes could be detected by New Horizons using Charon-shine (light reflected off Charon). The south polar region seems to be darker than the north polar region, but there is a high-albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit.

thumb|center|upright=2|A panoramic view of Pluto's icy mountains and flat ice plains. Imaged by New Horizons, 15 minutes after its closest approach to Pluto. Distinct haze layers in Pluto's atmosphere can be seen backlit by the Sun.