ESA Science & Technology08-Jul-2005 16:35:51

Habitable Planets


A habitable planet must meet a number of criteria.

Depending on the stellar type, habitable planets will be at different distances from the parent star.

Firstly, it must possess a thin atmosphere, similar to that of Earth. To do this, the planet must be more than half, but less than ten times, the mass of the Earth. Too small a planet would not possess enough gravity to hold onto an atmosphere. Too large and its pull of gravity would be so strong that it would develop a thick hydrogen and helium atmosphere, like Jupiter and Saturn within our Solar System.

Secondly, the planet's surface temperature must be similar to Earth's so it is capable of sustaining liquid water. This depends upon how much energy the star gives out, known as its luminosity, and the planet's distance from the star. The range of orbits around each star that allow liquid water is known as the habitable zone.

This is not to say that a habitable planet will have developed life. It is simply a way of determining which planets might be capable of supporting life as we understand it.

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