Those pesky greenhouse gasses that are threatening to wreak havoc here on Earth may make other planets scattered throughout the universe more conducive to life, according to a paper published in the journal Science.
"It's really all about the greenhouse gases," the paper's author Sara Seager told Space.com. "The greenhouse gases are like a blanket that moderates the temperature at the surface."
Currently, most thinking regarding exoplanet habitability – a phrase that, unsurprisingly, is the title of Seager's paper – has focused on planets in the so-called "Goldilocks" zone: the orbital band in which the candidates would be neither too hot nor too cold to support liquid water.
That's too narrow a view, argues Seager. "If there is one important lesson from exoplanets, it is that anything is possible within the laws of physics and chemistry," she writes.
Those immutable, apparently universal laws would, for example, allow for planets to be much further from their stars than Earth is from the Sun, and still have conditions that could support life if their atmospheres were such that they could trap sufficient heat provided by their stars, much as does the Earth and the radiative forcing caused by greenhouse gasses.
One of those possibilities, Seager writes, would be a planet massive enough for it to hang onto molecular hydrogen gas (H2), which smaller planets such as ours don't have the sufficient gravitational pull to keep all but a relatively smallish amount from escaping.
The direct effect of gaseous hydrogen on radiative forcing has not been fully nailed down – at least this Reg hack has not been able to find definitive research – but it is known to interact with hydroxyl radicals (OH) and cause an increase in methane (CH4), a substantially more-potent greenhouse gas than our fossil-fuel buddy, carbon dioxide (CO2).
A large planet with a hydrogen-rich atmosphere could therefore conceivably trap a goodly amount of heat, possibly enough to warm the surface of the planet suffiently to allow for liquid water, and thus support life – at least until, as one wag commenting on the Space.com article pointed out, that life evolved long enough to discover fire.
Even more intriguing, Seager postulates, is the possibility that a planet could be a "wanderer", "nomad", or "rogue" – a body that orbits no star – and still have enough heat at its core to make it habitable, provided that its atmosphere were capable of trapping a sufficient amount of that heat.
If Seager's hypothesis is correct, there could be far more planets capable of supporting water-based life than exoplanetary researchers currently suggest. Speaking of her paper, she said "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." ®