New Austrian Study Suggests Super-Earths Are Unfit For Life

John P. Millis, Ph.D. for – Your Universe Online

The last two decades have brought unprecedented understanding of worlds outside of our solar system. The launch of the Kepler space telescope and other instruments have allowed astronomers to find and characterize hundreds of new planets, with thousands more candidates waiting to be confirmed.

As our technology continues to improve, we will be able to isolate smaller and smaller planets, eventually identifying Earth-sized examples with regularity, and hopefully some with habitable conditions. To this point, however, nearly all of the discoveries have been of planets several times the size of Earth — many of these larger even than Jupiter. Most of these worlds are quite different than the planets in our little corner of the galaxy, so astronomers are now working to understand how they form and evolve.

A team of scientists have created a new model that suggests that super-Earths, even those in the habitable zone – the region around a star where the temperature would allow for the existence of liquid water on a planet – would be unfit for life.

Solar systems form out of clouds dominated by hydrogen and helium, and trace amounts of other elements. Over time dust and rocky material heat up and clump together forming what will eventually be planets around the emerging star. As these cores increase in size and mass, their gravitational influence accretes hydrogen gas from the dwindling cloud. Some of this gas will be ejected from the forming world by the ultraviolet light of the star.

This battle of accumulation and removal proceeds until a relative equilibrium is reached, a process that will be dependent on the masses of the planet and star, and their relative distance from each other (other factors such as the stellar brightness in ultraviolet radiation are also important).

Dr. Helmut Lammer, from the Space Research Institute (IWF) of the Austrian Academy of Sciences, reports in a new study that modeled the accumulation of gas for planetary cores that are between 0.1 and 5 times the mass of the Earth.

Planets with similar density to earth will struggle to capture gas until they surpass 0.5 times the Earth’s mass. In the case where the forming object is similar in mass to Earth, there is a tug-of-war between the accumulation and dissipation of gas, so the atmospheres are not likely to become highly dense.

However, for the highest mass cores – the aptly named super-Earths – almost all of the gas is bound to the surface because of their stronger gravitational fields. In fact, the so-called super-Earths may be more like mini-Neptunes.

“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 only lose a few percent of it over their lifetime,” reports Lammer. “With such thick atmospheres, the pressure on the surfaces will be huge, making it almost impossible for life to exist.”

Lammer’s study is published in the Monthly Notices of the Royal Astronomical Society