Where’s our second Earth?

To date, we’ve found over 1500 exoplanets and have detected more than two thousand possible candidates for that status that have yet to be confirmed. So where’s our second Earth? Unfortunately, we haven’t found it yet. And according to researchers led by Helmut Lammer of the Space Research Institute of the Austrian Academy of Sciences, that planet may be even more elusive than we thought.

Two of the main criteria for whether are planet could harbor life are its size and its location. The planet must be small enough to have a rocky surface, as opposed to being a giant ball of gas like Jupiter. It must also be located at a distance from its star that allows the presence of liquid water. That orbital distance is often referred to as ‘the habitable zone’ or sometimes ‘the goldilocks zone’.

Lammer and his team think we should add internal composition, or more specifically, density to that list of requirements. 

The mass of the initial rocky core determines whether the final planet is potentially habitable.
Top: 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).
Bottom: 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.
Credit: NASA / H. Lammer

As it’s being formed from the dust and gas around a young star, a protoplanet captures a hydrogen envelope around it. Depending on conditions, most or all of that hydrogen can be stripped away, as was the case with the Earth. However, if a planet is dense enough, it will retain a thick blanket of hydrogen. If so, the pressure on the surface of that planet will be too great to sustain life. Thus, a planet might appear similar to the Earth and even orbit within the habitable zone, but be completely inhospitable. 

Lammer, H., Stokl, A., Erkaev, N., Dorfi, E., Odert, P., Gudel, M., Kulikov, Y., Kislyakova, K., & Leitzinger, M. (2014). Origin and loss of nebula-captured hydrogen envelopes from 'sub'- to 'super-Earths' in the habitable zone of Sun-like stars Monthly Notices of the Royal Astronomical Society DOI: 10.1093/mnras/stu085.