Astronomers have claimed that it may be easier for a planet to overheat into searing uninhabitable ‘runaway greenhouse’ stage than previously believed.
In the runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium. As a result, the world overheats, boiling its oceans and filling its atmosphere with steam, which leaves the planet glowing-hot and forever uninhabitable, as Venus is now.
One estimate of the inner edge of a star’s ‘habitable zone’ is where the runaway greenhouse process begins.
The habitable zone is that ring of space around a star that’s just right for water to remain in liquid form on an orbiting rocky planet’s surface, thus giving life a chance.
Revisiting this classic planetary science scenario with new computer modeling, the astronomers found a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought.
Tyler Robinson, a UW astronomy postdoctoral researcher and second author on the paper, said that the habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse.
The lead author is Colin Goldblatt of the University of Victoria.
“These worlds on the very edge got ‘pushed in,’ from our perspective -- they are now beyond the runaway greenhouse threshold,” Robinson said.
The findings apply to planet Earth as well, as the Sun increases in brightness over time, Earth, too, will move into the runaway greenhouse stage -- but not for a billion and a half years or so.
The findings have been published in Nature Geoscience.