Astronomy Wiki
Register
Advertisement

Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet located in the Kuiper belt, a zone of objects beyond the orbit of Neptune. It was the first Kuiper belt object to be discovered, and is the largest known. It was discovered by Clyde Tombaugh in 1930, and was originally considered to be the ninth planet from the Sun before 2006. In 2005, Eris, a dwarf planet in the scattered disc, was discovered. It was 27% more massive than Pluto, which led to the International Astronomical Union (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly. That definition excluded Pluto, and reclassified it as a dwarf planet.

Pluto is the largest known member of the Kuiper belt, and is second-most-massive dwarf planet in the Solar System, behind Eris. Like the other Kuiper belt objects, Pluto is made primarily of ice and rock and is very small - it has one-sixth the mass of the Moon and one-third of its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 AU (4.4-7.4 killion km) from the Sun. Pluto periodically goes closer to the sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding.

Pluto currently has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Some astronomers consider Pluto and Charon to be a binary system, or a "double planet", because the barycenter of their orbits do not lie within either body.

On July 14, 2015, the New Horizons spacecraft became the first spacecraft to flyby Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere of Pluto and transferred about 19,000 km (12,000 mi) to the orbiting moon.

History[]

Discovery[]

In 1906, Percival Lowell - a wealthy businessman and amateur astronomer who had founded the Lowell Observatory in Flagstaff, Arizona in 1894 - began a search for an unknown planetary body, which he dubbed "Planet X"[1]. He theorized that wobbles in the orbits of Uranus and Neptune were being caused by the gravitational pull of an unknown planetary body. He referred to this effort as the "The Invariable Plane Search"[2]. He carried out a series of several calculations based on the observed perturbations of Uranus. By 1910, he and William H. Pickering published a series of possible celestial coordinates for the theoretical planet. Lowell and his observatory continued the search, but were ultimately unable to locate anything. Lowell continued his search until his death in 1916. Unknown to Lowell, he had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized until much later on.[3]

Upon Lowell's death in 1916, his wife, Constance Lowell, entered a 10-year long legal battle with the Lowell Observatory over her husband's legacy, and the search for "Planet X" was suspended until 1929[4]. Vesto Melvin Slipher, the director of the observatory, gave 23-year-old Clyde Tombaugh the job of resuming the search for Planet X. Slipher decided to hire Tombaugh after being impressed with his astronomical drawings.

Tombaugh's job was to photograph the same areas of the sky several nights apart, then examine each photo and determine if any objects had shifted position. Using a blink comparator, he would rapidly shift back and forth between views of each of the photos to detect movement of any objects that would change position between photographs.

On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographs taken between January 23 and January 29. Through a lesser-quality photo taken on January 21, the movement was confirmed. After the observatory obtained further confirmation, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930 - both on the day of Lowell's birthday and William Herschel's discovery of Uranus. It was the first Solar System planet found in 84 years.

Name[]

The discovery made headlines around the globe, and the Lowell Observatory held rights to name the object. They received over 1,000 suggestions worldwide.[5] Constance Lowell proposed the object be named Zeus, Percival, and finally, Constance, which were all disregarded.

Venetia Burney, an eleven-year-old girl from Oxford, England, proposed the name Pluto during a conversation about the discovery with her grandfather Falconer Madan, who passed it onto an astronomy professor, Herbert Hall Turner, who cabled it to his colleagues in the United States.[6]

The final proposed names were Minerva (which was already in use for another asteroid), Cronus, and Pluto. Pluto received an unanimous vote. The name was announced on May 1, 1930.

Aftermath[]

After Pluto was discovered, there was considerable doubt that it was Lowell's Planet X because of its faintness and lack of a resolvable disc.

The initial estimate of Pluto's mass was based on its presumed effect on Neptune and Uranus. The initial estimate proposed by Lowell was 7 Earth masses. By 1931, it was estimated that the mass of Pluto was roughly the mass of Earth, or 1 Earth mass.[7] By 1948, the mass was down to roughly that of Mars.[8]

In 1976, Dale Cruikshank, Carl Pilcher, and David Morrison of the University of Hawaii calculated Pluto's albedo for the first time and found that it matched that of methane ice. This meant that Pluto had to be exceptionally luminous for its size and therefor was not more than 1% the mass of Earth.

In 1978, a Plutonian moon, Charon, allowed the measurement of Pluto's mass for the first time: roughly 0.2% of the Earth's, which meant it was far too small to account for the discrepancies in the orbit of Uranus. Subsequently, searches for Planet X resumed, notably by Robert Sutton Harrington, but all ultimately failed.

In 1992, Myles Standish used data from Voyager 2's flyby of Neptune in 1989 to calculate its gravitational effect on Uranus. Voyager 2 had discovered that Neptune's mass was about 0.5% less than previously thought. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.

Today, most scientists agree that Planet X, or at least how Lowell defined it, does not exist. Lowell had made a prediction of Planet X's orbit and position in 1915 that was fairly close to Pluto's actual orbit and position at that time. Ernest W. Brown concluded soon after Pluto's discovery that this was a coincidence.

Planetary status[]

There is a lot of controversy on whether or not Pluto is really a planet or not. In August 2006, the IAU declared that Pluto is a "dwarf planet". Others consider it to be a planet. A recent scientific debate on September 26, 2014 states that Pluto should be given back its planetary title, but it is unlikely that it will be given back this title.

Orbit[]

Pluto Orbit

Animation of a full orbit of Pluto

Pluto's orbital period is approximately 248 years. Pluto has not completed a single orbit around the Sun since its discovery. Pluto's orbit is very different to that of the planets. Its orbital path does not lie in the same plane, and it is somewhat inclined relative to the ecliptic (over 17°) and moderately eccentric (elliptical). This means that, at some points, Pluto's orbit actually lies closer to the Sun than Neptune's. Pluto came near perihelion on September 5, 1989. The last time Pluto was closer to the Sun than Neptune was between February 7, 1979 and February 11, 1999. It will not be closer until around 2227. [9][10]

Rotation[]

Like Uranus, Pluto rotates on its "side", with an axial tilt of 120°. This makes it have extreme variations in seasons, and during winter, one-fourth of its surface is in continuous daylight, while the other fourth is in continuous darkness.[11]

Atmosphere[]

Pluto has a very weak atmosphere, generated by the vaporization of surface ices. The atmosphere mostly consists of nitrogen, methane, and carbon monoxide. The surface pressure of Pluto was calculated by New Horizons to be around 10 microbars (1 pascals), or, 1/100,000 of Earth's surface pressure.

The temperature on the surface is approximately 40 K (-230 °C; -382 °F), however, because of a methane-generated greenhouse effect, this increases depending on the altitude. At 30 km, the temperature is approximately 110 K (-163 °C; -261 °F).[12]

Pluto is the only trans-Neptunian object known to possess an atmosphere. It shares many aspects with the atmosphere of Titan, and even the atmosphere of Mars.

Geology[]

Surface[]

Nearly all of the Plutonian surface is nitrogen ice; 98% of it is composed with nitrogen ice, along with traces of methane and carbon monoxide. [13]

Moons[]

Main article: List of Pluto moons

Pluto currently has five known moons. The closest moon to Pluto is Charon. However, some astronomers consider Charon to be a double planet system rather than its own moon.

Aside from Charon, four other much smaller circumbinary moons exist. In order of distance from Pluto, they are: Styx, Nix, Kerberos, and Hydra.

References[]

Advertisement