According to Sara Seager, astronomer at MIT, such a narrow focus would exclude many potentially life-supporting exoplanets. Exoplanets show an enormous diversity, from enormous ‘hot Jupiters’ to ‘rogue planets’ that drift through the cold depths of space.
“If there is one important lesson from exoplanets, it is that anything is possible within the laws of physics and chemistry,” Seager writes in the Science article. “Planets of almost all masses, sizes and orbits have been detected, illustrating not only the stochastic nature of planet formation but also a subsequent migration through the planetary disk from the planet’s place of origin.”
The typical definition of the habitable zone includes an Earth-like atmosphere made up of nitrogen, carbon dioxide and water vapour. However, Seager argues that a broader way of thinking is needed. For example, large and/or cold exoplanets could have retained their gaseous molecular hydrogen, which is a powerful greenhouse gas that could make liquid water possible on a number of planets far beyond the recognized habitable zone.
According to Seager, we should consider alien planets’ potential to support life individually, on a case-by-case basis. She says: ‘I hope it gets people to realize that so many types of worlds could be habitable, and that our chance of finding one is higher when we accept that”.