Pluto (minor-planet designation 134340 Pluto) is the largest object in the Kuiper belt, and the tenth-most-massive body observed directly orbiting the Sun. It is the second-most-massive known dwarf planet, after Eris. Like other Kuiper-belt objects, Pluto is composed primarily of rock and ice and is relatively small, approximately one-sixth the mass of the Moon and one-third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune, but an orbital resonance with Neptune prevents the bodies from colliding. In 2014 it was 32.6 AU from the Sun.
Discovered in 1930, Pluto was originally classified as the ninth planet from the Sun. Its status as a major planet fell into question following further study of it and the outer Solar System over the ensuing 75 years. Starting in 1977 with the discovery of the minor planet 2060 Chiron, numerous icy objects similar to Pluto with eccentric orbits were found. The most notable of these was the scattered disc object Eris, discovered in 2005, which is 27% more massive than Pluto. The understanding that Pluto is only one of several large icy bodies in the outer Solar System prompted the International Astronomical Union (IAU) to define formally in 2006 what it means to be a "planet". This definition excluded Pluto and reclassified it as a member of the new "dwarf planet" category (and specifically as a plutoid). A few astronomers hold that Pluto should have remained classified as a planet, and that other dwarf planets and even moons should be added to the roster of planets along with Pluto.Back at Top
Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Nix, Hydra, Kerberos, and Styx. Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body. The IAU has yet to formalise a definition for binary dwarf planets, and Charon is officially classified as a moon of Pluto.
What is Pluto
Pluto was discovered in 1930 by an astronomer from the United States. An astronomer is a person who studies stars and other objects in space.
Pluto was known as the smallest planet in the solar system and the ninth planet from the sun.
Today, Pluto is called a "dwarf planet." A dwarf planet orbits the sun just like other planets, but it is smaller. A dwarf planet is so small it cannot clear other objects out of its path.
On average, Pluto is more than 3.6 billion miles (5.8 billion kilometers) away from the sun. That is about 40 times as far from the sun as Earth. Pluto orbits the sun in an oval like a racetrack. Because of its oval orbit, Pluto is sometimes closer to the sun than at other times. At its closest point to the sun Pluto is still billions of miles away.
Pluto is in a region called the Kuiper (KY-per) Belt. Thousands of small, icy objects like Pluto are in the Kuiper BeltBack at Top
Pluto, the Ninth Planet That Was a Dwarf
Pluto's History & Naming
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.
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.
Facts about Pluto
Pluto is named after the Greek god of the underworld:
This is a later name for the more well known Hades and was proposed by Venetia Burney an eleven year old schoolgirl from Oxford, England.Back at Top
Facts on Pluto
Pluto was reclassified from a planet to a dwarf planet in 2006:
This is when the IAU formalised the definition of a planet 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 neighbourhood around its orbit.”
Pluto was discovered on February 18th, 1930 by the Lowell Observatory:
For the 76 years between Pluto being discovered and the time it was reclassified as a dwarf planet it completed under a third of it’s orbit around the Sun.
Pluto has five known moons:
They are Charon (discovered in 1978,), Hydra and Nix (both discovered in 2005), Kerberos originally P4 (discovered 2011) and Styx originally P5 (discovered 2012) official designations S/2011 (134340) 1 and S/2012 (134340) 1.
Pluto is the second largest dwarf planet:
Eris is the largest with an average diameter of 2,326km while Pluto has a diameter of 2,306km.
Pluto is smaller than a number of moons:
These are Ganymede, Titan, Callisto, Io, Europa, Triton, and Earth’s moon. Pluto has 66% of the diameter of the Earth’s moon and 18% of its mass.Back at Top
Pluto has a eccentric and inclined orbit:
This takes it between 4.4 and 7.4 billion km from the Sun meaning Pluto is periodically closer to the Sun than Neptune.
No spacecraft have visited Pluto:
Though in July 2015 the spacecraft New Horizons, which was launched in 2006, is scheduled to fly by Pluto on it’s way to the Kuiper Belt.