Explore Pluto

Student Planet Facts

Pluto and Charon Pluto is a dwarf planet far from the sun. Here are some facts and other places you can find information about Pluto. When you are ready, click back to return to the Student Center or click below to explore again.

Pluto is a small rocky object that lies at the very edge of the solar system. The planet is so far out it takes light from the sun about 5 and one half hours to reach Pluto in contrast to the 8 minutes it takes to reach Earth. Its orbit of about 248 years sometimes takes it inside Neptune’s orbit. Pluto is so cold that nitrogen and oxygen, which we breathe so easily on Earth, become frozen solid. The planet is only about two-thirds the size of our moon and up until recently was the biggest known object in the Kuiper Belt.

This belt is in the same plane of the planets and are the millions of rocks, ice chunks and particles left over from the formation of the solar system. Comets with orbits of less than 200 years, short-period comets, come from this debris. Beyond the Kuiper Belt is a spherical cloud of dust, rocks and ice called the Oort Cloud where long-period comets hide and the solar wind still flows.

The best image we have of Pluto is the one above taken by the Hubble Space Telescope (HST). The smaller sphere is Pluto's moon, Charon. Pluto was found in 1930 by Clyde Tombaugh, an astronomer looking for "Planet X". Percival Lowell started the search for the unknown object from the Lowell Observatory in Arizona in 1905 after noticing differences in the calculated positions and the observed positions of the planets. The small size of the planet has been the discussion of how to define a planet and if Pluto should be called a planet. See more below about the definition of a planet.

Pluto has a moon, Charon, about half the size of the planet. The Hubble Space Telescope, see Telescopes in Space, has found two more moons orbiting Pluto. Called Nix and Hydra, they are named after the Greek goddess of darkness (Nyx) and a nine-headed serpent that in Greek mythology guards the underworld. Click image or here for larger image.

Quaoar and Other Kuiper Objects Found
NASA astronomers have discovered a large icy object in the Kuiper Belt beyond Pluto. Click image or here for Quaoar Object page. Another object, 2003 UB_313 and for a time unofficially called Xena, is larger than Pluto and added more to the controversy over whether Pluto was a planet or not. See more below. Astronomers think that more may be found as instruments become more precise. There is a mission underway, New Horizons: Pluto/Kuiper Belt Mission, that will study Pluto and the Kuiper Belt.

What is a Planet?
The International Astronomical Union recently decided on a definition of the word "planet". The IAU members gathered at the 2006 General Assembly agreed that a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

This means that the Solar System consists of eight "planets" Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called "dwarf planets" was also decided. It was agreed that "planets" and "dwarf planets" are two distinct classes of objects. The first members of the dwarf planet category are Ceres, Pluto and 2003 UB313, given the name Eris. More dwarf planets are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate dwarf planets are listed on IAU's dwarf planet watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The "dwarf planet" Pluto is recognized as an important proto-type of a new class of trans-Neptunian (beyond Neptune's orbit) objects. The IAU will set up a process to name these objects.

Gravity and You
Your weight on Earth is determined by your mass and Earth's mass. Would you weigh more or less on Pluto?

Planet Names
Why are the planets named for Roman gods? What is the story or myth about their names? Click image or here for Planet Myths and Lore.

New! Click here to find out about "The Professor's Telescope", a new book by Chris Moreau, one of our advisors. Take an adventure with Eric to Saturn!

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Quick Facts about Pluto

Topic

Data

Diameter

2320 km

Density

2.05 g/cm³

Mass

1.290 x 10²² kg

Volume

6.545 x 10⁹ km³

Temperature Range

-240° C to -218° C

Atmosphere

Methane

Winds

Not Measurable

Moons

Three

Average Distance from Sun

5,913,520,000 km

Orbital Period

248 Years, 197 Days, 5.5 Hours

Rotation

6 Days, 9.25 Hours

Tilt

122.52°

Rings

None

Composition

Frozen Methane and Other Ices

Magnetic Field

None

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The Nine Planets
NASA Planet Facts
New Horizons: Pluto/Kuiper Belt Mission
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