Evaporated planet (image: APOD, Inga Nielsen)
To date, 443 exoplanets (extrasolar planets) are discovered orbiting other stars in our Milky Way galaxy. In 1992 the discovery of planets around another pulsar was announced. After 3 years the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi) was confirmed.
Many known exoplanets are hot Jupiters: planets of Jupiter-like mass on very small orbits with periods of only a few days.
A hot Neptune is a giant planet in an orbit close to its star ( less than 1 AU). The mass of its core and envelope are similar to Uranus and Neptune.
The majority of planets found outside our solar system have been gas giants since they produce more pronounced wobbles in the host stars and are thus more easily detectable. However, a number of extrasolar planets are suspected to be terrestrial (rocky).
A Super-Earth represent the upper-end of the terrestrial planet with a mass between that of Earth and the Solar System’s gas giants (upper bound of 10 Earth masses). Smaller terrestrial planets can be Iron planets (similar to Mercury) or Coreless planets.
A Chthonian planet is a class of celestial objects resulting from the stripping away of a gas giant’s hydrogen and helium atmosphere and outer layers (evaporated remnant cores). CoRot-7b may be the first in this class.
Goldilocks planet is a planet that falls within a star’s habitable zone, often specifically used for planets close to the size of Earth. Goldilocks planets are of key interest to researchers looking either for existing (and possibly intelligent) life or for future homes for the human race. Only two such planets are discovered, but finding Goldilocks planets is a key part of the Kepler Mission.
An ocean planet ( waterworld) is a hypothetical type of planet whose surface is completely covered with an ocean of water. The oceans on such planets would be hundreds of kilometers deep. GJ 1214 b is the most likely known candidate for an ocean planet.
An Eccentric Jupiter is a Jovian planet that orbits its star in a highly eccentric orbit, much like a comet. Eccentric Jupiters may disqualify a planetary system from having Earth-like planets in it because a massive and highly eccentric gas giant may remove all Earth mass planets from the habitable zone. These planets more even common than Hot Jupiters.
Cold Neptune is a type of planet with a mass ranging from about 10 Earth masses (a Super-Earth) to less than Saturn. Cold Neptunes should be located beyond a host star’s snow line where temperatures are much cooler. Only few planets of this class are discovered to date.
Cold Neptune (image: ESA)
Cold Jupiters (Jupiter-twins) are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter and orbit their suns in a roughly similar pattern. Within our own solar system, Jupiter and Saturn would be the prime examples of this planetary class.
A rogue planet ( an interstellar planet, free-floating planet or orphan planet) is an object which has equivalent mass to a planet and is not gravitationally bound to any star and orbits the galaxy directly.
An extragalactic planet is a planet that is outside the Milky Way Galaxy. One suspected planet is announced in the Andromeda Galaxy.
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