Super-Earths likely to be dead worlds

Many of the super-Earths discovered in recent planet-hunting missions are likely to be dead, uninhabitable worlds say Austrian researchers. Although these planets are in the so-called ‘habitable zone’ and appear to have similarities to Earth at first glance, researchers from the Space Research Institute of the Austrian Academy of Sciences say they are a lot less clement than previously thought. The research – published in Monthly Notices of the Royal Astronomical Society – suggest that for some of the recently discovered super-Earths like Kepler-62e and -62f, being in the habitable zone is not enough to make them agreeable for life. “Our results suggest that worlds like these two super-Earths may have captured the equivalent of between 100 and 1000 times the hydrogen in the Earth’s oceans, but may lose only a few percent over their lifetime,” said Dr Helmut Lammer. “With such thick atmospheres, the pressure on the surfaces will be huge, making it almost impossible for life to exist.” Planetary systems are believed to form from hydrogen, helium and heavier elements that orbit their parent stars in protoplanetary disks. Dust and rocky materials bundle together over time; eventually forming the rocky cores that go on to form planets. The gravity of these cores attracts hydrogen from the disk around them, some of which is stripped away by ultraviolet light of the star they orbit. Lammer and his team modelled the balance of capture and removal of hydrogen for planetary cores between 0.1 and 5 times the mass of the Earth, located in habitable zones of a Sun-like star. Protoplanets with the same density of the Earth but less than 0.5 times its mass were found to capture little gas from the disk, while planetary cores with a similar mass to Earth can capture but also lose their enveloping hydrogen. Higher mass cores – similar to the super-Earths found in many stars – hold onto almost all of their hydrogen, and usually end up as mini Neptunes with much thicker atmospheres than ours. Scientists using the next generation of giant telescopes will now have to look even harder to find places where life might exist, say researchers. Origin and loss of nebula-captured hydrogen envelopes from ‘sub’- to ‘super-Earths’ in the habitable zone of Sun-like stars