'Super-Earths' may be dead worlds: Being in habitable zone is not enough

Otis has pointed out a study a year ago that has a important influence on the discovery of Kepler 452b. This relates to the point I made in a previous post regarding the heavy atmosphere that Kepler 452b is most likely to have. Otis notes:

 

I would like to point to a study published last year that found that super Earths in the Habitable Zone are very unlikely to be habitable.  The reason is that any rocky planet that is larger than about 1.15 Earth radius will not lose its primordial hydrogen atmosphere.   The researchers even predicted that many planets will be found in the HZ like the one recently reported by NASA, but will have hydrogen-dominated atmospheres.  The recently found planet Kepler 452b has an estimated radius of 1.6 Earth's radius.

 

Here is an excerpt from the conclusions in the MNRAS paper:

"Therefore, we suggest that 'rocky' habitable terrestrial planets, which can lose their nebula-captured hydrogen envelopes and can keep their outgassed or impact delivered secondary atmospheres in HZs of G-type stars, have most likely core masses with 1 ± 0.5 M⊕ and corresponding radii between 0.8 and 1.15 R⊕.  Depending on nebula conditions, the formation scenarios, and the nebula lifetime, there may be some planets with masses that are larger than 1.5M⊕ and lost their proto atmospheres, but these objects may represent a minority compared to planets in the Earth-mass domain. We also conclude that several recently discovered low density 'super-Earths' with known radius and mass even at closer orbital distances could not get rid of their hydrogen envelopes. Furthermore, our results indicate that one should expect many 'super-Earths' to be discovered in the near future inside HZs with hydrogen-dominated atmospheres."

 

Otis

 

The ScienceDaily article Otis refers to can be found here:

 

'Super-Earths' may be dead worlds: Being in habitable zone is not enough