Number of potentially habitable planets might be less than predicted: study

The number of potentially habitable planets could be fewer than predicted since many of them have thick atmospheres that keep them too hot, according to a study published Thursday by the Imperial College London.

Scientists usually look for planets that could harbor life in the "habitable zones" around their stars -- at the right distance from the stars to allow water to exist in liquid form. Traditionally, this search has focused on looking for planets orbiting stars like our Sun.

But recent research has turned to small planets orbiting very close to stars called red dwarfs, which are much smaller and dimmer than the Sun. Red dwarfs make up around 75 percent of all the stars in our galaxy, and recent discoveries have suggested that many of them host planets, pushing the number of potentially habitable planets into the billions.

Researchers from Imperial College London and the Institute for Advanced Studies in Princeton found out that although these planets orbited smaller and dimmer stars, many of them might still be too hot to be habitable.

It was known previously that many of these planets are born with thick atmospheres of hydrogen and helium. The greenhouse effect of such a thick atmosphere would make the surface far too hot for liquid water.

However, it was thought that over time, the strong X-ray and ultraviolet radiation from the parent red dwarf star would evaporate away most of this atmosphere, eventually making the planets potentially habitable.

The new study reveals that this might not be the case. Instead, computer simulations show that these thick hydrogen and helium envelopes cannot escape the gravity of planets that are similar to or larger in mass than the Earth, meaning that many of them are likely to retain their stifling atmospheres.

But some smaller planets, comparable to Venus or Mars, could still lose them to evaporation, according to the researchers.

"There are hints from recent exoplanet [planet revolving around a star other than the Sun] discoveries that relatively puny planets may be even more common around red dwarfs than Earth mass or larger ones, in which case there may indeed be a bonanza of potentially habitable planets whirling around these cool red stars," said Dr Subhanjoy Mohanty from Imperial College London, one of the authors of the study.

The study has been published in the journal Monthly Notices of the Royal Astronomical Society. Enditem