Studies show many 'habitable planets' may actually be too hot for life

Not all planets predicted to be habitable may be capable of harbouring life as their atmospheres keep them too hot, say scientists including one of Indian-origin. When looking for planets that could harbour life, scientists look for planets 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 planets orbiting stars like our Sun, in a similar way to Earth. However, recent research has turned to small planets orbiting very close to stars called M dwarfs, or red dwarfs, which are much smaller and dimmer than the Sun.
M 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. This month, both the Belgian robotic telescope TRAPPIST and NASA's Kepler planet-hunting spacecraft have announced the discovery of multiple near-Earth-sized planets orbiting M dwarf stars, some within the habitable zones.

New research from Imperial College London and the Institute for Advanced Studies in US showed that although they orbit smaller and dimmer stars, many of these planets might still be too hot to be habitable. The scientists suggest that some of the planets might still be habitable, but only those with a smaller mass than Earth, comparable to Venus or Mars. "It was previously assumed that planets with masses similar to Earth would be habitable simply because they were in the 'habitable zone'," said lead author James Owen, from the Institute for Advanced Studies. "However, when you consider how these planets evolve over billions of years this assumption turns out not to be true," Owen said.

It was known previously that many of these planets are born with thick atmospheres of hydrogen and helium, making up roughly one percent of the total planetary mass. In comparison, the Earth's atmosphere makes up only a millionth of its mass. The greenhouse effect of such a thick atmosphere would make the surface far too hot for liquid water, rendering the planets initially uninhabitable. However, it was thought that over time, the strong X-ray and ultraviolet radiation from the parent M dwarf star would evaporate away most of this atmosphere, eventually making the planets potentially habitable. The new analysis shows that this is not the case.

Detailed 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, said researchers including Subhanjoy Mohanty, from Imperial College London. The study was published in the journal Monthly Notices of the Royal Astronomical Society.