A computer model estimate says it might be easier than previously thought for a planet to overheat into the scorchingly uninhabitable "runaway greenhouse" stage. That may mean some planets thought to be habitable right now actually are not.

In such a 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 a numerical model, a group of astronomers came up with a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought.

"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," said paper co-author Tyler Robinson, a University of Washington post-doctoral researcher. 

But don't get nervous. This "single-column, clear-sky model," a one-dimensional measure averaged around a planetary sphere that does not account for the atmospheric effect of clouds, shows that while Earth will move into the runaway greenhouse stage, it won't be for a billion and a half years or so. 


If the model is good, a recalibration of where the habitable zone begins and ends might make sense. In doing so, some planets declared as as possible habitable worlds would have their candidacy revoked. 

"These worlds on the very edge got 'pushed in,' from our perspective — they are now beyond the runaway greenhouse threshold," Robinson said. And, the astronomers write, "As the solar constant increases with time, Earth's future is analogous to Venus's past."

Citation: Colin Goldblatt, Tyler D. Robinson, Kevin J. Zahnle&David Crisp, 'Low simulated radiation limit for runaway greenhouse climates', Nature Geoscience 6, 661–667 (2013) doi:10.1038/ngeo1892