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An exoplanet is any planet discovered outside of our solar system. Our solar system has 9 planets, in order of distance from our Sun they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Outside of our solar system, other kinds of planets orbit other stars. Most of these planets are not like the planets in our solar system. But since we know the planets nearest to us best, we use our solar system to classify these exoplanets. For example, rocky planets that are ten times the size of Earth are called Super Earths.

Exoplanets are discovered with telescopes, satellites, and some ground instruments. Most exoplanets published on our site are discovered by NASA's TESS telescope, the Kepler satellite, or the James Webb Space Telescope. When a planet orbits its host star, it blocks out some light from the star. These telescopes measure the amount of light blocked out and use that to estimate the planet's radius. The planet's transit speed gives some information on its mass. And finally, the spectra of the light absorbed by the planet can give some information about the kind of elements on that planet. Most planetary scientists are interested on liquid water, since that would be a sign of life!

We use NASA's exoplanet classification here at The Exoplanet Project. NASA classifies exoplanets into these main types:

  • Gas Giant A gas giant is a large planet mostly composed of helium and/or hydrogen. These planets, like Jupiter and Saturn in our solar system, don’t have hard surfaces and instead have swirling gases above a solid core. Gas giant exoplanets can be much larger than Jupiter, and much closer to their stars than anything found in our solar system.
  • Neptune Like Neptunian exoplanets are similar in size to Neptune or Uranus in our solar system. Neptunian planets typically have hydrogen and helium-dominated atmospheres with cores of rock and heavier metals.
  • Super Earth Super-Earths – a class of planets unlike any in our solar system – are more massive than Earth yet lighter than ice giants like Neptune and Uranus, and can be made of gas, rock or a combination of both. They are between twice the size of Earth and up to 10 times its mass.
  • Terrestrial In our solar system, Earth, Mars, Mercury and Venus are terrestrial, or rocky, planets. For planets outside our solar system, those between half of Earth’s size to twice its radius are considered terrestrial and others may be even smaller. Exoplanets twice the size of Earth and larger may be rocky as well, but those are considered super-Earths.

The most common exoplanets are rocky planets larger than Earth called Super Earths or gaseous planets smaller than Neptune called sub-Neptune planets.

When we call an exoplanet habitable we mean that it has the possibility to sustain life. So far, scientists have observed that life as we know it requires liquid water to exist. On this site, we characterize any planet that is at a temperature condusive to liquid water as habitable. This means that the exoplanet is in the Goldilock's Zone: not too hot (where water is vapor) and not too cold (where water is frozen). If a planet's temperature meets that requirement, then we say that there is a chance that its habitable. There are habitable planets in our universe! However, we would need further study to confirm that life exists on those planets.