Earth-sized worlds are probably too hot.
When scientists look for planets in the “habitable zones” around stars, they often look for planets orbiting stars like our sun. However, this traditional view is shifting, with researchers looking 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 per cent of all 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. What’s more, planet hunting telescopes TRAPPIST and Kepler recently discovered multiple Earth-sized planets orbiting M dwarf stars, some within the habitable zones.
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However, new research from Imperial College London and the Institute for Advanced Studies in Princeton, published in the Monthly Notices of the Royal Astronomical Society, has revealed that although these planets orbit smaller and dimmer stars, many of them might still be too hot.
But it is not all bad news. The scientists also suggest that some of the planets might still be habitable, as long as they have a smaller mass than Earth — closer in size 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.' However, when you consider how these planets evolve over billions of years this assumption turns out not to be true," explained James Owen, Hubble fellow and lead author of the study, in an Imperial College London news release.
Many of these planets are born with thick atmospheres of hydrogen and helium, and the greenhouse effect of such a thick atmosphere would make the surface too hot for liquid water. However, it was thought that, over time, strong X-ray and ultraviolet radiation from the parent M dwarf would evaporate most of this atmosphere away, making the planet potentially habitable.
But it turns out that is only true for smaller planets. According to computer simulations, hydrogen and helium atmospheres cannot escape the gravity of planets that are similar to, or larger in mass, than Earth, meaning many of them likely keep their hot atmospheric temperatures. On the other hand, smaller planets can lose them to evaporation.
"There are hints from recent exoplanet 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 Subhanjoy Mohanty, senior lecturer in astrophysics at the Imperial College London and co-author of the study.
Future space missions should provide an answer.
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