The only planet on which we know for sure that life exists is Earth. Even though this gives us just a sample of one, it is tempting to think that conditions here must be close to the ideal for life to emerge and that if there are other planets on which life exists, they must be Earth-like in terms of size and occupying the so-called ‘stellar habitable zone’ (SHZ) from its star so that the temperature range is similar to ours.
But a new paper in the journal Astrobiology titled Superhabitable Worlds by Rene Heller and John Armstrong suggests that it may not be the case that our planet’s characteristics make it the most hospitable, and in fact maybe merely marginally habitable, and that there may be so-called superhabitable planets that are two to three times bigger than Earth and orbiting stars that are somewhat less massive than our sun.
The authors look at the many factors that affect habitability: habitable surface area, total surface area, land-to-ocean fraction and distribution, plate tectonics, magnetic shielding, climatic thermostat, surface temperature, biological diversification, multihabitability and panspermia, localization in the stellar habitable zone, age, stellar mass, stellar UV irradiation, stellar lifetime, early planetary bombardment, planetary spin, orbital dynamics, and atmosphere.
After looking at the interplay of all these features, they conclude:
Eventually, just as the Solar System turned out to be everything but typical for planetary systems, Earth could turn out to be everything but typical for a habitable or, ultimately, an inhabited world. Our argumentation can be understood as a refutation of the Rare Earth hypothesis. Ward and Brownlee (2000) claimed that the emergence of life required an extremely unlikely interplay of conditions on Earth, and they concluded that complex life would be a very unlikely phenomenon in the Universe. While we agree that the occurrence of another truly Earth-like planet is trivially impossible, we hold that this argument does not constrain the emergence of other inhabited planets. We argue here in the opposite direction and claim that Earth could turn out to be a marginally habitable world. In our view, a variety of processes exist that can make environmental conditions on a planet or moon more benign to life than is the case on Earth.
As someone who hopes that there is life out there somewhere, I find this study encouraging.