Planets with hydrogen-rich atmospheres, even if they are far from their parent star, can generate enough heat to keep liquid water on their surfaces making it habitable.

According to recent research, rocky planets with hydrogen-rich atmospheres could experience the greenhouse effect which would be sufficient to allow liquid water on their surface despite their distant orbits.

Planets near their suns reap the benefits of light and heat, while those farther away must endure colder temperatures.

The area around a star in which water can be liquid rather than ice is known as the habitable zone, sometimes called the "Goldilocks zone." In this area, liquid water is not too hot for it to evaporate nor too cold for it to freeze.

"This is the kind of planet we know is habitable," explains Raymond Pierrehumbert of the University of Chicago, lead author on a paper published recently in the Astrophysical Journal Letters.

However, the size of a solar system's habitable zone varies; stars that are hotter or brighter the region stretches farther out into space, while its inner edge cannot be too close to the star.

Researchers said that the habitable zone for a G-type star such as the Sun, for instance, lies between 0.95 and 1.4 AUs or astronomical unit, the distance from the Earth to the Sun. The Earth falls within that region.

A smaller, dimmer M-type star will have a habitable zone that is closer, between 0.08 and 0.12 AUs.
But according to Pierrehumbert's research, , a rocky planet with a hydrogen atmosphere could have a habitable zone extending as far as 1.5 AUs for M-stars and 15 AUs for G-stars.

This means that for stars similar to the Sun, rocky planets beyond the reach of Saturn could contain oceans of water.