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 the 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 a guy who likes to stay working on the same thing forever," he says.

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 colling, 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.

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 theoretically support life. But geophysicists expect it to be completely covered by ocean; high mass planets 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.

abbot-teaching_0.jpgLike cloud-based climate control, this stabilization, is also related to water cycle. But instead of atmospheric cycling, Abbot and Northwestern University astrophysicist Nicolas Cowan developed models based on water cycling between ocean and mantle—the deep water cycle.

Earth maintains dry continents by partitioning and cycling water between oceans and the rocky mantle beneath its crust. In this process, mantle rock is exposed at mid-ocean ridges—underwater mountain ranges created by plate tectonics—and degasses, which releases water trapped in the rock into the ocean. Water is also incorporated into oceanic crust, which plate tectonics pushes into the mantle, closing the water cycle.

Based on these processes, Abbot and Cowan developed a hydrosphere model, which indicates that a terrestrial planet, regardless of mass, could maintain dry continents like on Earth. Seafloor pressure is proportional to surface gravity, and higher seafloor pressure reduces water degassing and increases ocean crust hydration. So a super-Earth would theoretically store more water in its mantle than on the surface.

This proposition, however, depends on several assumptions: that the rocky planet has plate tectonics; that Earth's mantle actually stores a considerable amount of water, which thus far has been impossible to quantify; and that the planet's mass is less than 0.2 percent water (about ten times more water than Earth has). Any more would create a true water world where the mantle doesn't store enough water to create continents or an ocean planet covered with water hundreds of miles deep where no amount of mantle reserve could expose dry land.

All of this theorizing also rests on a massive assumption—"that water is essential for life," says Abbot. "All of biology is hampered by the fact that we only know about one type of life. It's hard for people to define what life is. Every time someone comes up with what they think is a good definition, people will find some counterexample." But scientists have to start somewhere. "If there are other types of life that aren't at least somewhat similar to us, we'll never find them anyway because we won't know what we're looking for or where to find it." So we look for the best possible candidates: planets with liquid water and a stable climate. —M.S.