Habitable zone

In Astronomy the term habitable zone (alternately goldilocks zone) refers to a range of possible orbits around a star where a suitably sized rocky planet could harbor life similar to that found on Planet Earth.

The size of a star's habitable zone varies according to the star's mass. It took about a billion years for Earth to go through it Hadean period, when it would not have been habitable. Stars approximately 1.5 times larger than Sol, our sun, run out of Hydrogen, within a billion years, so they are too large to have a habitable zone. The mass of dim red dwarf stars, is so low that, if they have a habitable zone, it will be so close the star so one face of their planets is locked to their sun.

Planets in the habitable zone have to have surface temperatures where liquid water exists.

Since the term was coined scientists have realized that worlds covered in ice may have liquid oceans, where life might exist, powered by chemosynthesis, not photosynthesis. There are moon and other objects, found in the Solar system, known to have Oceans of liquid water, underneath an icy surface. Space probes, have photographed geysers, spraying out of the surface of Europa, and other objects, including several other moons, Ceres, the largest asteroid, and Pluto, the first Kuiper Belt object, are candidates for oceans, beneath their icy surfaces