Life cycle

Dorian Abbot studies the water cycle of extrasolar planets.

Geophysical sciences assistant professor Dorian Abbot grew up in the coastal town of Yarmouth, Maine, where his engineer grandfather and uncle would take him out on ships and talk about the waves and weather. He now studies climate dynamics through Earth’s paleoclimate and exoplanet habitability, both of which often depend on the presence and behavior of water.

Abbot earned three degrees at Harvard—a bachelor’s in physics and a master’s and doctorate in applied mathematics—and completed postdoctoral fellowships there and at UChicago before joining the faculty in 2011. Because the University encourages crossing departmental boundaries, he can use both his physics and math backgrounds. “I’m not a guy who likes to stay working on the same thing forever,” he says.

The importance of clouds

In 2013, Abbot developed predictive models that defined more precisely the boundaries of the habitable zone, the area around a star where planets have the temperature and atmospheric pressure to maintain liquid water. Too close to the star, all water evaporates; too far, it freezes. He focused on red dwarfs, common stars smaller and cooler than the sun, and their closest planets, which are often tidally locked with one side always facing the star.

Original habitable zone climate models were 1-D and neglected clouds. Abbot and Jun Yang, a postdoc working with him, applied 3-D models that also incorporate cloud behavior. On Earth, and presumably on other planets with atmospheres, clouds have a cooling effect by reflecting light from the sun before it reaches the planet. They also have a warming effect by trapping energy leaving the planet, deflecting it back down.

“If the clouds were to stop doing their warming, then we would be a snowball Earth, and if they were to stop cooling, we would turn into Venus. Whole oceans would evaporate—we would just be frying,” says Abbot. “That’s how important clouds are.” According to his calculations, thick clouds would form under the star on tidally locked planets thought too close to their star to sustain life, cooling and stabilizing the climate and preventing water from boiling off. This model expands the inner edge of the predicted habitable zone. Crucially, this model makes predictions that will be testable with the James Webb Space Telescope, due to be launched in 2018.

In deep water

More recently, Abbot has been studying super-Earths, exoplanets with masses greater than Earth but less than giants like Neptune and Uranus. If a rocky super-Earth orbits within the habitable zone and contains water, it could theoreti¬cally support life. But geophysicists expect it to be completely covered by ocean; high mass plan¬ets should tend to have deeper oceans and higher surface gravity that would cause smoother topography. Land wouldn’t rise high enough to break the ocean surface. Abbot’s research, however, suggests that these presumed water worlds may in fact have exposed continents. This possibility matters because planets need dry land to activate the silicate weathering thermostat, a temperature-dependent process that regulates atmospheric carbon dioxide and makes possible a stable climate.

Story originally published in the Spring/Summer 2014 issue of Inquiry.