Are Super-Earths Actually Mini-Neptunes?
‘Super-Earths’ follow a different evolutionary track from the planets found in our solar system, but an open question is whether they can evolve to become rocky bodies like the ‘terrestrial planets’ Mercury, Venus, Earth and Mars. To try to answer this, Dr. Lammer and his team looked at the impact of radiation on the upper atmospheres of super-Earths orbiting the stars Kepler-11, Gliese 1214 and 55 Cancri.
This group of planets are all a few times more massive and slightly larger than the Earth and orbit very close to their respective stars. The way in which the mass of planets scales with their sizes suggests that they have solid cores surrounded by hydrogen or hydrogen-rich atmospheres, probably captured from the clouds of gas and dust (nebulae) from which the planets formed.
The new model suggests that the short wavelength extreme ultraviolet light (much ‘bluer’ than the blue light we see with our eyes) of the host stars heats up the gaseous envelopes of these worlds, so that they expand up to several times the radius of each planet and gas escapes from them fairly quickly. Nonetheless most of the atmosphere remains in place over the whole lifetime of the stars they orbit.
Rather than becoming more like the Earth, the super-Earths may more closely resemble Neptune, which together with Uranus, is a smaller ‘gas giant’ in our solar system. If the scientists’ results are right, then super-Earths further out from their stars in the ‘habitable zone’, where the temperature would allow liquid water to exist, would hold on to their atmospheres even more effectively. If that happens, they would be much less likely to be habitable.
The team’s findings will be put to the test in 2017 when the European Space Agency launches the CHaracterising ExOPlanets Satellite (CHEOPS). This will study super-Earths in more detail and should be able to tell whether some of these exotic worlds could one day be more like our own.