How many planets are there like Earth?

Data from the Kepler Space Observatory have been used to suggest that there may be more than two billion planets in our galaxy capable of supporting life.

The number comes from extrapolating from selection of Sun-like stars studied by the telescope, which found that 22% of them have planets that are potentially Earth-like.

An “Earth-like” planet is one that is small enough to be rocky (rather than being a gas giant like Jupiter or Saturn) and orbits its star within the “habitable zone”. Also known as the “Goldilocks zone”, this is the distance from a star at which the temperature is not too hot and not too cold, but just right for liquid water to exist.

The Goldilocks zone is dependent on the star’s luminosity – brighter, hotter stars have habitable zones with a larger radius. Although many searches for worlds like our own focus on Sun-like stars, it’s also possible for smaller, cooler red dwarf stars to have a habitable zone – although any planets orbiting close enough would likely be tidally locked, with one hemisphere permanently lit and other always in darkness.

The possibility of liquid water is the main criterion for whether a planet is suitable for life. All life on Earth depends on its unique and complex properties favourable to biochemistry. But it’s not the only chemical that could support life.

Any water that does exist around a planet can be detected via spectroscopy, as was the case when its signature was picked up in the debris orbiting a white dwarf 160 lightyears away, in a discovery announced last month.

That such a high proportion of Sun-like stars are likely to have Earth-like planets orbiting them increases the chances that we might eventually find other life somewhere in our galaxy.

Image credit: NASA, ESA, M. Kornmesser

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