In the past couple of decades, nearly 900 planets have been identified outside of our Solar System with thousands more candidates to be considered. Among the most exciting of these exoplanets are the so-called “super-Earths” – planets somewhat larger than the Earth, yet some of which might be capable of supporting life. Unfortunately, a team led by Helmut Lammer at the Space Research Institute (IWF) of the Austrian Academy of Sciences has produced new models that indicate some of these super-Earths may really be mini-Neptunes, with deep, hydrogen-rich envelopes covering a small rocky core.

Super-Earths are a promising sign that life may exist elsewhere in our galaxy. Finding a rocky planet similar to Earth, though larger, raises the hopes of many a SETI enthusiast – especially if it’s found orbiting inside a star system’s habitable zone. However, the IWF team points out that super-Earths evolved differently than the terrestrial planets of our Solar System and the question remains unanswered whether they are, or could ever become, Earth-like.

To find the answer, the team looked at the impact of radiation on the upper atmospheres of seven super-Earths orbiting the stars Kepler-11, Gliese 1214 and 55 Cancri. These particular super-Earths are certainly no candidates for the home of some extraterrestrial civilization because they are as uninhabitable as a blast furnace. They orbit closer to their stars than Mercury does the Sun and are very hot, with temperatures ranging from 475º K (202º C, 395º F) to 2,360º K (2,086º C, 3,788º F). Worse, their mass to size ratio indicates atmospheres of hydrogen or hydrogen-rich mixtures of gases.

Interior of a mini-Neptune "super-Earth" (Image: H. Lammer)

These planets were chosen because their mass and radii are known, so it was possible to develop mathematical models of their atmospheres based on known facts about the planets, such as their temperatures, distance from their stars, the nature of their stars, orbits, tidal forces and other factors. The models indicate that the soft X-rays and extreme ultraviolet light from the host star heats up the planet’s atmosphere, causing it to expand to several times the radius of the planet itself. Some of the atmosphere escapes and is blown away by stellar winds, but not fast enough, so most of it is retained throughout the life of the planet.

"Our results indicate that, although material in the atmosphere of these planets escapes at a high rate, unlike lower mass Earth-like planets many of these super-Earths may not get rid of their nebula-captured hydrogen-rich atmospheres,” said Dr Lammer.

Neptune (Image: NASA)

The upshot of this is that many of these super-Earths end up as scaled down versions of Neptune, the eighth planet of our Solar System. Ironically, the conditions in a star’s habitable zone make it more likely that the planet will be uninhabitable because, according to the team, it will be more likely to retain its massive atmosphere.

The next step for the team is to test their findings in 2017 when ESA launches the the CHaracterising ExOPlanets Satellite (CHEOPS) – an unmanned probe designed to make a close study of known exoplanets.

The team’s results were published in the Monthly Notices of the Royal Astronomical Society.

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