Planet
A planet (from the Greek planetes or "wanderers") is a body of considerable mass that orbits a star and that doesn't produce energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 2004, 118 are known, with all of the new discoveries being extrasolar planets, aka "exoplanets."
Astronomers often call asteroids "minor planets," and call the larger planetary bodies (those which are popularly called planets) "major planets."
Planets within Earth's solar system can be divided into categories according to composition.
- Terrestrial or rocky: Planets that are similar to Earth—with bodies largely composed of rock
- Jovian or gas giant: Those with a composition largely made up of gaseous material, as with Jupiter
- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Titan).
The planets of our solar system (in increasing distance from the Sun) are
All of the planets in the solar system (except Earth) are named after Roman gods. Moons are also named after gods and characters from classical mythology or from the plays of Shakespeare. Asteroids can be named, at the discretion of their discoverers, after anybody or anything (subject to approval by the International Astronomical Union's panel on nomenclature). The act of naming planets and their features is known as planetary nomenclature.
Several hypothetical planets, like Planet X (supposedly beyond the orbit of Pluto) or Vulcan (thought to orbit inside the orbit of Mercury), were posited, and were subjects of intense searches that found nothing.
Almost all extrasolar planets (those outside our solar system) discovered to date have masses which are about the same or larger than the gas giants within the solar system. (The only exception is three planets discovered orbiting a burned-out star, or supernova remnant, called a pulsar. These are comparable in size to the terrestrial planets). This is largely because the gravitational effect of massive planets is larger, making them easier to detect. However, it is far from clear if the newly discovered planets would resemble gas giants in our solar system or if they are of an entirely different type or types which are unknown in our solar system. In particular, some of the newly discovered planets orbit extremely closely to their parent star sometimes in highly elliptical 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.
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 possibility of extraterrestrial intelligence.
Planets are thought to form from the collapsing nebula that a planet's star formed out of, aggregating from gas and dust that orbits the protostar in a dense protostellar disk before the star's core ignites and its solar wind blows the remaining material away.
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Referenced By
Astronomical body | Astronomical object | Jovian planet | Jovian system | List of astronomical topics | List of astronomical topics (N-Z) | List of solar system objects | M-theory Simplified
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