Planet

A planet in common parlance is a large object in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. The International Astronomical Union lists nine planets in our solar system, based on historical consensus. Since the term planet has no scientific definition, many astronomers contest that figure, with some saying it should be lowered to eight (removing Pluto from the list), and others claiming it should be raised to fifteen, about twenty, or even dozens, depending on the definition used.

Extrasolar planets

:Main article: Extrasolar planet.

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Most extrasolar planets (those outside our solar system) discovered to date have masses which are about the same or larger than Jupiter's.

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Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12, the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized http://www.eso.org/outreach/press-rel/pr-2004/pr-22-04_pf.html, and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the earth and is probably rocky in origin.

Related Topics:
Pulsar - PSR B1257+12 - Mu Arae - 55 Cancri - GJ 436 - Gliese 876

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It is far from clear if the newly discovered large planets would resemble the gas giants in our solar system or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in our solar system, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.

Related Topics:
Hot Jupiter - Radiation - Chthonian planet

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The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.

Related Topics:
National Aeronautics and Space Administration - United States - Terrestrial Planet Finder - Artificial satellite - Drake equation - Intelligent, communicating civilizations

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Astronomers have recently http://www.nature.com/news/2005/050711/full/050711-6.html http://www.jpl.nasa.gov/news/news.cfm?release=2005-115 detected a planet in a triple star system, a finding that challenges current theories of planetary formation. The planet, a gas giant slightly larger than Jupiter, orbits the main star of the HD 188753 system, in the constellation Cygnus, and is hence known as HD 188753 Ab. The stellar trio (yellow, orange, and red) is about 149 light-years from Earth. The planet, which is at least 14% larger than Jupiter, orbits the main star (HD 188753 A) once every 80 hours or so (3.3 days), at a distance of about 8 Gm, a twentieth of the distance between Earth and the Sun. The other two stars whirl tightly around each other in 156 days, and circle the main star every 25.7 years at a distance from the main star that would put them between Saturn and Uranus in our own Solar System. The latter stars invalidate the leading hot Jupiter formation theory, which holds these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here, the outer star pair disrupting outer planet formation.

Related Topics:
Triple star system - HD 188753 - Cygnus - HD 188753 Ab

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Brown dwarf "planets"

The discovery of a planet-sized satelite of a brown dwarf has blurred the distinction between "planet" and "moon." A brown dwarf, though a star in theory, in practice is often described as in between a planet and a star. It is formally defined by the International Astronomical Union by its official statement that "Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located."

Related Topics:
Brown dwarf - Deuterium

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To the International Astronomical Union, the question of whether an object in orbit around a brown dwarf is a "planet" or a "moon" was simply not relevant, as it does not use the term "moon," only "satellite" and as yet has no official definition for "planet."

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Interstellar planets

Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a likely hypothesis based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass.

Related Topics:
Interstellar planet - Computer simulation

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Such objects are not formally called planets, however, since the International Astronomical Union has not defined the term "planet".

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