Determining eta-Earth

Determining the frequency of Earth-like exoplanets.

One of the key goals of exoplanet research is to determine eta-Earth, the frequency of potentially habitable Earth-like exoplanets.  Although there are now a reasonable number of known rocky planets,  most are on orbits that are too close to their parent stars to allow for them to be habitable.  However, if this is a sub-population of the population of all rocky exoplanets, we may be able to use this population to infer the frequency of potentially habitable Earth-like exoplanets.  On the other hand, many of the known rocky exoplanets are close enough to their parents stars that they may have originally had substantial gaseous atmospheres that have been lost via photo-evaporation.

It is therefore important to understand if the population of known close-in rocky planets are objects that formed rocky and are, hence, simply the close-in portion of a population that likely extends to large radii.  If so, they could then be used to infer the frequency of rocky planets on longer orbits.  On the other hand, they may have initially been planets with substantial gaseous envelopes that they lost, via photoevaporation, after ending up on orbits so close to their parent star.  If so, then these would not be the inner portion of a population of rocky planets that probably extend to larger orbits and could not be used to infer the frequency of such planets. 

Therefore, a key area of research for the centre will be to understand the origin of the known close-in rocky planets so as to determine if they can be used to infer the frequency of rocky planets on longer orbits.