[LINK] "Astronomers Say Super-Earths Are Actually Mini-Neptunes"
io9's George Dvorsky reported on a recent study suggesting that Super-Earth planets--worlds more massive than our Earth, but less massive than ice giants like Neptune and Uranus (~15 Earth masses each)--are unlikely to lose their dense primordial hydrogen atmospheres even if they orbit their stars closely. Super-Earths in habitable zones are even less likely to do so, making their resemblances to Earth decidedly limited.
The new study, which was led by Helmut Lammer of the Space Research Institute (IWF) of the Austrian Academy of Sciences, suggests that low density super-Earths are clinging to an extended hydrogen-rich atmospheric layer. These planets, therefore, are unlikely to ever become Earth-like.
Lammer and his team reached this conclusion after analyzing the effects of radiation on the upper atmospheres of super-Earths orbiting the stars Kepler-11, Gliese 1214, and 55 Cancri. All these planets are in relatively close orbits with their parent stars, and they're all suspected of containing solid cores surrounded by an atmosphere rich in hydrogen, water, and methane. The astronomers theorize that these primordial gasses were captured from nebulae during planet formation but have not had a chance to escape — nor will they ever.
Their subsequent analysis showed that the short wavelength extreme ultraviolet light coming in from the planets' respective stars are heating up the atmospheres. As a result, the envelopes are bloating up to a massive size — over several times the radius of each planet. Some of these gasses have escaped into space (in a complex process called "hydrodynamic blow-off"), but most of the protoatmosphere remains intact.
"The atmospheric mass loss of the studied super-Earths is one to two orders of magnitude lower compared to that of hot Jupiters," they write," so one can expect that these exoplanets cannot lose their hydrogen envelopes during their remaining lifetimes."