Johnson replies: Richard Mielbrecht raises an interesting and important topic. The first point to be made in response to his concern is that we do not understand the conditions for life on our own planet, let alone others. Thus many questions remain about the habitability of planets orbiting red dwarfs or any star other than the Sun, given that our planet is the only location in the galaxy where life is known to exist.
The job of planet hunters like me is to identify where Earth analogues exist, having first characterized the host stars and planetary orbits. Then we need to determine whether any have an equilibrium temperature that may allow for the existence of liquid water. That step is merely the first in a long process of assessing the potential habitability of a planet.
Other concerns include assessing whether the planet has plate tectonics; a suitable atmospheric mass and composition; enough but not too much water; and, as Mielbrecht points out, whether it has a strong enough magnetic field to shield it from energetic flares. All those points were outside the scope of my article, which focused on the search for distant worlds. But those topics are certainly worthy of a follow-up article.
