Planets need not just water but also UV rays to be habitable

For a planet to have liquid surface water and be habitable it must not only be the right distance from its star, but also receive the right amount of ultraviolet (UV) rays essential to forming the biochemical building blocks that are the basis of life.

These are the findings of an Italian study led by the University of Insubria in conjunction with the National Institute of Nuclear Physics, and they further narrow the range of planets that could potentially host extraterrestrial life. The research, online at arXiv and published in the journal Monthly Notices of the Royal Astronomical Society, shows that 75% of known stars are just too cold.

The researchers, led by Riccardo Spinelli of the University of Insubria, used Nasa's Swift Space Telescope to observe 17 stars hosting 23 planets in the habitable zone, namely at the right distance to have water in the liquid state.

"The planets discovered in the habitable zone of red dwarfs, which are the majority of stars, do not receive enough UV radiation to trigger some of the processes that lead to the formation of the fundamental building blocks of life, such as RNA," noted Spinelli. "To initiate such reactions, the star must have a surface temperature of at least 4,000 degrees, but 75% of the stars in the universe are cooler," he continued.

"On the other hand, we know that too much UV radiation is detrimental to life, because it damages DNA and destroys many proteins," Spineli continued.

There is, therefore, a belt around every star within which a planet receives enough UV radiation to trigger life, but not too much to destroy it. The authors of the study have called it the "habitable UV zone", and it must overlap, at least in part, with the zone allowing water.