The Milky Way appears to be peppered with so-called Super-Earths — distant exoplanets that are between two to 10 Earth masses. Many of them have even been detected within habitable zones. But a new study shows it doesn't matter. They're all just dead worlds.
Super-Earths — many of which are Mini-Neptunes — can be terrestrial planets like Earth, or gaseous blobs like Uranus and Neptune. Astronomers have catalogued nearly 400 of these exoplanets (many of which were identified in a recent Kepler study).
Because of their (relatively) reasonable size, some astrobiologists have wondered if Super-Earths are habitable. But a new analysis by researchers at the Space Research Institute (IWF) of the Austrian Academy of Sciences indicates this is likely not the case.
When planets from from hydrogen, helium, and heavier elements that orbit in the protoplanetary disk around stars, dust and rocky material clump together over time, eventually forming rocky cores that become planets. The gravity of these proto-planets draws hydrogen from the disk around them, some of which is stripped away by ultraviolet light.
The mass of the initial rocky core determines whether the final planet is potentially habitable. On the top row of the diagram, the core has a mass of more than 1.5 times that of the Earth. The result is that it holds on to a thick atmosphere of hydrogen (H), deuterium (H2) and helium (He). The lower row shows the evolution of a smaller mass core, between 0.5 and 1.5 times the mass of the Earth. It holds on to far less of the lighter gases, making it much more likely to develop an atmosphere suitable for life. Caption and image: NASA / H. Lammer.
But according to Helmut Lammer, Super-Earths, unlike Earth-sized planets, hold on to almost all of their hydrogen. The resulting planets end up as mini-Neptunes with atmospheres much thicker than what we see here on Earth. The same holds true for Super-Earths within habitable zones, such as Kepler-62e and -62f.
"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," notes Lammer in a statement. "With such thick atmospheres, the pressure on the surfaces will be huge, making it almost impossible for life to exist." The ongoing discovery of low density super-Earths affirms these conclusions.
Of course, that excludes the potential for extremophiles to exist in super-dense atmospheres.
Read the entire study at Monthly Notices of the Royal Astronomical Society.