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Pluto : a dwarf planet occupying an orbit that crosses the orbit of Neptune — in 2006 the International Astronomical Union defined planet in such a way as to exclude Pluto, reclassifying it instead as a dwarf planet. — Webster   See also   OneLook


Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It was the first object discovered in the Kuiper belt and remains the largest known body in that area. After Pluto was discovered in 1930, it was declared the ninth planet from the Sun. However, beginning in the 1990s, its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt and the scattered disc, including the dwarf planet Eris, leading the International Astronomical Union (IAU) in 2006 to define the term planet formally—excluding Pluto and reclassifying it as a dwarf planet.

Pluto is the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit, ranging from 30 to 49 astronomical units (4.5 to 7.3 billion kilometers; 2.8 to 4.6 billion miles) from the Sun. Therefore, Pluto periodically comes closer to the Sun than Neptune. Still, a stable orbital resonance with Neptune prevents them from colliding.

Pluto has five known moons: Charon (the largest, whose diameter is just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body. — Wikipedia

Pluto (Encyclopædia Britannica)




The New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, becoming the first and, to date, only spacecraft to do so. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen, and other gases released from the atmosphere of Pluto and transferred 19,000 km (12,000 mi) to the orbiting moon. — Wikipedia

Pluto’s ocean is capped and insulated by gas hydrates (Shunichi Kamata, et al., Nature Geoscience)

Pluto (Jet Propulsion Laboratory, NASA)
Pluto’s Moons (Jet Propulsion Laboratory, NASA)

New Horizons Mission (NASA)
New Horizons: NASA’s Mission to Pluto and the Kuiper Belt (Johns Hopkins University Applied Physics Laboratory)
New Horizons (Wikipedia)

Pluto and the Developing Landscape of Our Solar System (International Astronomical Union)
Pluto, The Kuiper Belt’s Most Famous Dwarf Planet (Planetary Society)

Pluto (Eric Weisstein’s World of Astronomy, Wolfram Research)
Pluto (Wolfram Alpha)




History of Pluto’s Discovery (Lowell Observatory)
Clyde W. Tombaugh (New Mexico Museum of Space History)
Pluto (National Air and Space Museum)
Clyde Tombaugh (Encyclopædia Britannica)


Library of Congress # QB701 Pluto (UPenn Online Books)

WorldCat, Library of Congress, UPenn Online Books, Open Library




The journey to Pluto, the farthest world ever explored (Alan Stern, TED-Ed)
Chasing New Horizons: Inside the Epic First Mission to Pluto (Alan Stern and David Grinspoon)

All About Pluto (Space Place, NASA)
Pluto (Cosmos4Kids)

MERLOT: Multimedia Educational Resource for Learning and Online Teaching
OER Commons: Open Educational Resources



International Astronomical Union (IAU)
Minor Planet Center (International Astronomical Union)


Pluto (Astronomy Magazine)
Pluto (Science Daily)
Pluto (
Pluto (NPR Archives)


Pluto (ISBNdb)


Pluto (NASA)


Pluto (



Here are links to pages about closely related subjects.

Knowledge Realm


Law (Constant) Relativity
Force Gravity, Electromagnetism (Light, Color)
Matter (Microscope) Molecule, Atom (Periodic Table), Particle

Universe (Astronomical Instrument)
Galaxy Milky Way, Andromeda
Planetary System Star, Brown Dwarf, Planet, Moon

Our Neighborhood
Solar System Sun
Terrestrial Planet Mercury, Venus, Earth (Moon), Mars
Asteroid Belt Ceres, Vesta
Jovian Planet Jupiter, Saturn, Uranus, Neptune
Trans-Neptunian Object
Kuiper Belt Pluto, Haumea, Makemake
Scattered Disc Eris, Sedna, Planet X
Oort Cloud Etc. Scholz’s Star
Small Body Comet, Centaur, Asteroid


1.   The resources on this page are are organized by a classification scheme developed exclusively for Cosma.