A planet outside our own solar system is called an exoplanet. With NASA's Kepler Space Telescope, thousands have been discovered over the past two decades.

There is a huge variety of sizes and orbits of these worlds. Some planets are giant and hug close to their parent stars; others are icy and some are rocky. NASA and other agencies are looking for a planet 9, six times the size of Earth that orbits the sun, but is it an exoplanet?.

The habitable zone is the range of distances from a star where liquid water oceans are possible on a planet. Current calculations of the habitable zone include many other factors, including a planet's atmosphere's greenhouse effect, in addition to simple thermal equilibrium. As a result, the boundaries of a habitable zone are fuzzy.

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