Pluto and its Moons
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Click for Live Display showing current positions of Pluto and its moons.
Click for Display showing a replay of New Horizons as it sped through the Plutonian System.
Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA’s New Horizons spacecraft, taken on July 13, 2015 when the spacecraft was 476,000 miles (768,000 kilometres) from the surface. Click for full story. Credits: NASA/APL/SwRI
Pluto is the second largest dwarf planet after Ceres, and is about 1/6 the mass of the Moon. It has an diameter of 2370km and is made of rock and ice with a thin atmosphere of nitrogen, methane and carbon monoxide. It has a temperature of around -230 degrees C.
Its 248 year elliptical orbit occasionally takes it inside the orbit of Neptune. However the orbit remains stable because it orbits exactly 2 times for every 3 orbits of Neptune. The orbit is also inclined by 17 degrees to the plane of the ecliptic ensuring it remains distant from Neptune.
For more information on the moons see our Plutonian System page.
Pluto has five moons, the latest of which is Styx which was discovered in 2012 in a Hubble space telescope image. The largest Moon is Charon (1200km diameter). It is so large (12 percent the mass of Pluto) that it is forms a binary system with Pluto in which both objects orbit about each other every 6 days. The other moons orbit around both Pluto and Charon. The outer moons are quite small - between 10km to 100km diameter - and orbit with periods of (generally) less than a month.
Pluto and Man
Pluto is of course the most well known dwarf planet because of its previous classification as the outermost planet of the solar system. Rather like Ceres before it, it has suffered because of the number of similar objects now discovered in the same region mean that it can no longer be thought of as one of the major planets.
Pluto was discovered by Clyde W. Tombaugh on the 18th February 1930 after a long search for "Planet X" which was thought to exist due to perturbations in the orbit of Neptune.
It was initially thought to be as large as Neptune but as better observations became possible its estimated mass was constantly reduced until , in the 1970s, it was found to weigh less that 1% that of the Earth.
The name Pluto, after the god of the underworld, was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology. She suggested it in a conversation with her grandfather Falconer Madan who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States. It was then chosen after a vote against other candidate names. Listen to Venetia here.
Pluto was visited for the first time ever by the New Horizons spacecraft which reached its closest approach on July 14, 2015. Scientific observations of Pluto began 5 months before closest approach and continued for a month after the encounter.