Pluto is a dwarf planet (or plutoid) that usually orbits past the orbit of Neptune. It was classified as a dwarf planet in 2006; before that it was considered to be a planet, the smallest planet in our solar system. There are many other dwarf planets in our Solar System. Pluto is smaller than a lot of the other planets’ moons, including our moon. Pluto has not been visited by spacecraft yet; we only have blurry pictures of its surface; even the Hubble Space Telescope orbiting the Earth can only get grainy photos because Pluto is so far from us. In 2015, a spacecraft called New Horizons (launched by NASA in 2006) will visit Pluto.
Pluto is about 1,413 miles (2274 km) in diameter. This is about 1/5 the diameter of the Earth.
Pluto is smaller than the 8 planets in our Solar System.
Mass and Gravity
Pluto’s mass is about 1.29 x 1022 kg. This is about 1/500th of the mass of the Earth. The gravity on Pluto is 8% of the gravity on Earth.
Pluto is the least massive planet in our Solar System (and is now classified as a dwarf planet).
A 100 pound person on Pluto would weigh only 8 pounds.
Length of a Day and Year
Each day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth years for Pluto to orbit the Sun once).
Pluto is 39 times farther from than the sun than the Earth is. Pluto ranges from 2.8 to 4.6 billion miles (4.447 billion to 7.38 billion km) from the Sun. From Pluto, the sun would look like a tiny dot in the sky.
Occasionally, Neptune’s orbit is actually outside that of Pluto; this is because of Pluto’s highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.
Pluto has a very eccentric orbit; that means that its distance from the sun varies a lot during its orbit around the sun. Sometimes it is even closer to the Sun than the planet Neptune (it was that way from January 1979 to February 11, 1999)! Pluto also rotates about its axis in the opposite direction from most of the other planets.
Pluto’s orbit is tilted from the plane of the ecliptic. This angle, its orbital inclination, is 17.15°. This is a much larger inclination of those of the eight planets.
Pluto is VERY, VERY cold. Its temperature may range from between -396°F to -378°F (-238°C to -228°C, or 35 K to 45 K). The average temperature is -393°F (-236°C = 37 K).
Pluto’s composition is unknown. It is probably made up of about 70% rock and 30% water. This is determined from density calculations; Pluto’s density is about 2,000 kg/m3. There may be methane ice together with frozen nitrogen and carbon dioxide on the cold, rocky surface.
Not much is known about Pluto’s atmosphere. It is probably mostly nitrogen with a little carbon monoxide and methane - definitely not breatheable by humans. The atmospheric pressure is probably very low. The atmosphere forms when Pluto is closest to the Sun and the frozen methane is vaporized by the solar heat. When it is farther from the Sun, the methane freezes again. From Pluto, the sky would appear black, even when the Sun (the size of a star) is up.
Pluto has one large moon, named Charon; two small moons (called Nix and Hydra) were discovered in 2005 and another was discovered in 2011 (it is called P4).
Although Charon is small, about 1,172 km (728 miles) in diameter, it’s about half of the size of Pluto itself. Charon orbits about 19,640 km from Pluto on average. It may be covered by water ice and probably has no atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on Pluto; Charon’s orbit takes exactly one Pluto day.
Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried people across the mythological river Styx into Hades.
The two tiny moons, Nix and Hydra, are from 30 and 100 miles (45 to 160 km) in diameter, and orbit Pluto about 27,000 miles (44,000 km) from Pluto, more than twice as far as the orbit of Charon. Another moon, P4, was discovered by the Hubble telescope in 2011.
Pluto was discovered after the 8 planets and was originally considered a planet itself (until 2006). In the early 1900s, Planet “X” was the temporary name given to the then-unknown planet beyond Neptune that disturbed the orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet “X’s” orbit, but died in 1916 before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto. He did his observations at the Lowell Observatory in Arizona.
Name and Symbol
Pluto was named after the Roman god of the underworld, Pluto.
Its symbol is the combined letters “P” and “L,” either for Percival Lowell or for Pluto.
The name Pluto was suggested by Venetia Burney of England, who was 11 years old at the time. She suggested the name to her grandfather, who was Librarian at Oxford. He passed her idea to the astronomers who were trying to name the newly-discovered planet.
Planet, Asteroid, or TNO?
Pluto’s unusual orbit makes some scientists think that Pluto is not a regular planet, but a “dwarf planet,” “minor planet” or a Trans Neptunian Object (TNO) [Kuiper Belt objects left over from the formation of the solar system]. In 2006, Pluto was re-classified as a dwarf planet. Alternatively, Pluto could have been be listed as an asteroid (it would have been given the asteroid number 10,000). It also may be the first TNO - it will also still be considered a dwarf planet.