An exoplanet is any planet beyond our solar system. Most orbit other stars, but free-floating exoplanets, called rogue planets, orbit the galactic center and are untethered to any star.

First extrasolar planets ?

The first extrasolar planets to be discovered in 1991 were (to most astronomers' amazement) around a pulsar! Why this amazed astronomers was that no one could imagine how a planet could get to be around a pulsar. Any planets around the original star should have been destroyed in the supernova explosion which created the pulsing neutron star. Since then various theories about how the planets could be there have been passed around. In any case, radiation from the neutron star would make this stellar system very hostile to life as we know it. At present we can only detect Earth sized planets if they are around pulsars.

Types of exoplanets

Each planet type varies in interior and exterior appearance depending on composition.

Gas giants

Gas giants are planets the size of Saturn or Jupiter, the largest planet in our solar system, or much, much larger. Hot Jupiters, for instance, were among the first planet types found – gas giants orbiting so closely to their stars that their temperatures soar into the thousands of degrees (Fahrenheit or Celsius).

Neptunian planets

Neptunian planets are similar in size to Neptune or Uranus in our solar system. They likely have a mixture of interior compositions, but all will have hydrogen and helium-dominated outer atmospheres and rocky cores. We’re also discovering mini-Neptunes, planets smaller than Neptune and bigger than Earth. No planets of this size or type exist in our solar system.


Super-Earths are typically terrestrial planets that may or may not have atmospheres. They are more massive than Earth, but lighter than Neptune.In the range of 3-10 times the mass of Earth, there might be a wide variety of planetary compositions, including water worlds, snowball planets, or planets that, like Neptune, are composed largely of dense gas.

Terrestrial planets

Terrestrial planets are Earth sized and smaller, composed of rock, silicate, water or carbon. Further investigation will determine whether some of them possess atmospheres, oceans or other signs of habitability.