Runaway Greenhouse Stage More Easily Triggered on Planets Than Once Thought: A Study

The process by which a planet overheats and enters the uninhabitable phase known as the "runaway greenhouse" stage might occur much more easily than previously thought, according to new research by astronomers at the University of Washington and the University of Victoria.

In the runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium and, as a result, the world overheats and its oceans boil, filling the atmosphere with steam. When this happens, the planet becomes forever uninhabitable, as seen in the case of Venus.

One estimate of the inner edge of a star's "habitable zone," or the area around a star in which water is able to remain in a liquid form, is where the runaway greenhouse process begins. However, revisiting this classic planetary science scenario with new computer modeling, the researchers discovered a lower thermal radiation threshold for the runaway greenhouse process, which, if correct, would mean a change in this decreed region.

"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," Tyler Robinson, a UW astronomy postdoctoral researcher, explained in a press release.

Though more research is needed, the scientists point out that their findings could lead to a recalibration of where the habitable zone begins and ends, with some planets being knocked off the list of those potentially inhabited by life.

"These worlds on the very edge got 'pushed in,' from our perspective -- they are now beyond the runaway greenhouse threshold," Robinson said.

Furthermore, the scientists argue, 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. Though, they add, this will not occur for another billion and a half years or so.

Still, the researchers point out that, "As the solar constant increases with time, Earth's future is analogous to Venus's past."

Ultimately, subsequent research is necessary, the astronomers say, in part because their computer modeling was done in a "single-column, clear-sky model," or a one-dimensional measure averaged around a planetary sphere that does not account for the atmospheric effect of clouds.

The study is published in journal Nature Geoscience.