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Dwarf Planet Pluto: Facts About the Icy Former Planet

Hypothetical Pluto Craters with 'Star Trek' Names
Some craters and other surface features spotted on Pluto by NASA's New Horizons probe in 2015 may end up bearing the name of "Star Trek" characters, as this screenshot from a Google+ Hangout demonstrates.
Credit: SETI Institute (via Google+/YouTube)

Pluto is the only dwarf planet to once have been considered a major planet. Once thought of as the ninth planet and the one most distant from the sun, Pluto is now seen as one of the largest known members of the Kuiper Belt, a shadowy disk-like zone beyond the orbit of Neptune populated by a trillion or more comets. 

Pluto was reclassified as a dwarf planet in 2006, a change widely thought of as a demotion that has attracted controversy and debate that has continued in scientific communities for the last eight years.

American astronomer Percival Lowell first caught hints of Pluto's existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, suggesting that another world's gravity was tugging at them from beyond. He predicted its location in 1915, but died without finding it. Its discovery came in 1930 from Clyde Tombaugh at the Lowell Observatory, based on predictions from Lowell and other astronomers.

Pluto is the only world named by an 11-year-old girl, Venetia Burney of Oxford, England, who suggested to her grandfather that it get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto. 

Physical characteristics

Since Pluto is so far from Earth, little is known about the planet’s size or surface conditions. Pluto has an estimated diameter less than one-fifth that of Earth or only about two-thirds as wide as Earth's moon. The planets’ surface conditions probably consist of a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane and nitrogen frost coating its surface. NASA’s Hubble Space Telescope also revealed evidence that Pluto’s crust could contain complex organic molecules. Chemicals such as nitrogen and methane may lay frozen beneath the icy crust.

Pluto's orbit is highly eccentric, or far from circular, which means its distance from the sun can vary considerably and at times, Pluto’s orbit will take it within the orbit of the planet Neptune. When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds.

Pluto's surface is one of the coldest places in the solar system at roughly minus 375 degrees F (minus 225 degrees C). For a long time, astronomers knew little about its surface because of its distance from Earth, but more is coming, bit by bit, with the Hubble Space Telescope returning images of a planet that appears reddish, yellowish and grayish in places, with a curious bright spot near the equator that might be rich in carbon monoxide frost. When compared with past images, the Hubble pictures revealed that Pluto had apparently grown redder over time, apparently due to seasonal changes.

This is the most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.
This is the most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.
Credit: NASA, ESA, and M. Buie (Southwest Research Institute)

Orbital characteristics

Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. It actually gets closer to the sun than Neptune for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world. So after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet).

Composition & structure

Atmospheric composition: Methane, nitrogen

Magnetic field: It remains unknown whether Pluto has a magnetic field, but its small size and slow rotation suggest it has little to none.

Chemical composition: Probably a mixture of 70 percent rock and 30 percent water ice.

Internal structure: Probably a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane and nitrogen frost coating its surface.

Dwarf planet Pluto was discovered in 1930 and was once considered to be the ninth planet from the sun in Earth’s solar system.
Credit: Karl Tate, SPACE.com

Orbit & rotation

Average distance from the sun: 3,670,050,000 miles (5,906,380,000 km) — 39.482 times that of Earth

Perihelion (closest approach to the sun): 2,756,902,000 miles (4,436,820,000 km) — 30.171 times that of Earth

Aphelion (farthest distance from the sun): 4,583,190,000 miles (7,375,930,000 km) — 48.481 times that of Earth

Pluto's moons

In 1978, astronomers discovered Pluto had a very large moon nearly half its size, dubbed Charon, named for the mythological demon who ferried souls to the underworld in Greek mythology. The huge size of Charon sometimes leads scientists to refer to Pluto and Charon as a double dwarf planet or binary system.

Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking.

While Pluto appears reddish, Charon seems grayish. Scientists suggest Pluto is covered with nitrogen and methane while Charon is covered with ordinary water ice. In its early days, the moon may have contained a subsurface ocean, though it probably can’t support one today.

Compared with most of solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun. Also, Pluto's rotation is retrograde compared to the other worlds — it spins backward, from east to west.

In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission — the first spacecraft to visit Pluto and the Kuiper Belt — they discovered two other tiny moons of Pluto, now dubbed Nix and Hydra. These are two to three times farther away from Pluto than Charon, and they are thought to be just 31 to 62 miles (50 to 100 km) wide.

Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. This moon is estimated to be 8 to 21 miles (13 to 34 km) in diameter. P4's orbit is between the orbits of Nix and Hydra. On July 11, 2012, a fifth moon Styx, was discovered, fueling the debate about Pluto’s status as a planet.

The four newly spotted moons may have formed from the collision that created Charon, hurled away from Pluto by the gravity of the massive moon.

Research & exploration

Pluto's distance from Earth has made it hard to see with telescopes and a daunting challenge to explore with spacecraft — NASA's New Horizons mission will be the first probe to study Pluto, its moons, and other worlds within the Kuiper Belt. It was launched on January 2006, making its closest approach to Pluto on July 2015, and carries some of the ashes of Pluto's discoverer, Clyde Tombaugh.

The limited knowledge of Pluto creates unprecedented dangers for the exploration. Prior to the mission’s launch, scientists only knew about the existence of three moons. The discovery of Keberos and Styx during the spacecraft’s journey fueled the idea that more satellites could orbit the dwarf planet, unseen from Earth. Not only the moons, but the debris fields they may have created, could prove hazardous to New Horizons.

Pluto's formation & origins

The leading theory for the formation of Pluto and Charon is that a nascent Pluto was struck with a glancing blow by another Pluto-sized object. Most of the combined matter became Pluto, while the rest spun off to become Charon. [Infographic: Pluto: A Dwarf Planet Oddity]

Nola Taylor Redd contributed to this article.

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AUTHOR BIO
Charles Q. Choi

Charles Q. Choi

Charles Q. Choi is a contributing writer for Space.com and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.
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