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Pluto's orbit is surprisingly unstable

In 1930, astronomer Clyde Tombaugh discovered the fabled "Ninth Planet" (or "Planet X") while working at the Lowell Observatory in Flagstaff, Arizona. The existence of this body had been predicted previously based on perturbations ...

Giant ice volcanoes identified on Pluto

Strange lumpy terrain on Pluto unlike anything previously observed in the solar system indicates that giant ice volcanoes were active relatively recently on the dwarf planet, scientists said on Tuesday.

Clues to Pluto's history lie in its faults

The world first glimpsed Pluto up close when NASA's New Horizons spacecraft whizzed by it in July 2015. One of the most exciting discoveries scientists made based on New Horizons data was that Pluto, despite orbiting at more ...

Moons are planets too

What makes a planet a planet? The answer turns out to be rather contentious. The official definition of a planet, as defined by the International Astronomical Union (IAU) is that a planet must satisfy three conditions:

Scientists confirm decrease in Pluto's atmospheric density

When Pluto passed in front of a star on the night of August 15, 2018, a Southwest Research Institute-led team of astronomers had deployed telescopes at numerous sites in the U.S. and Mexico to observe Pluto's atmosphere as ...

Explaining glaciers of solid methane and nitrogen on Pluto

Planetary scientist Dr. Helen Maynard-Casely and associates have reported for the first time how solid methane and nitrogen expand in response to temperature changes and resolved an historic ambiguity relating to the structure ...

10 cool things we learned about Pluto from New Horizons

Five years ago today, NASA's New Horizons spacecraft made history. After a voyage of nearly 10 years and more than 3 billion miles, the intrepid piano-sized probe flew within 7,800 miles of Pluto. For the first time ever, ...

SOFIA finds clues hidden in Pluto's haze

When the New Horizons spacecraft passed by Pluto in 2015, one of the many fascinating features its images revealed was that this small, frigid world in the distant solar system has a hazy atmosphere. Now, new data helps explain ...

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Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun. Originally classified as the ninth planet from the Sun, Pluto was recategorized as a dwarf planet and plutoid due to the discovery that it is one of several large bodies within the newly charted Kuiper belt.

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a 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. As of 2011, it is 32.1 AU from the Sun.

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto. On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.

Pluto has four known moons, the largest being Charon discovered in 1978, along with Nix and Hydra, discovered in 2005, and the provisionally named S/2011 P 1, discovered in 2011. Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body. However, the IAU has yet to formalise a definition for binary dwarf planets, and as such Charon is officially classified as a moon of Pluto.

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