Dr. Nicolas Cowan, Northwestern University – Heat and Tidally-Locked Planets
In today’s Academic Minute, Dr. Nicolas Cowan of Northwestern University explains how cloud cover moderates the temperature of tidally-locked exoplanets orbiting red dwarf starts.
Nicolas Cowan is a research assistant professor in the Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University where his research is focused on understanding the climate of exoplanets. His research projects involve creating maps of distant planets by monitoring their brightness and color change. He earned his Ph.D. at the University of Washington.
Dr. Nicolas Cowan – Heat and Tidally-Locked Planets
Most of the stars in the Milky Way galaxy are smaller and redder than the Sun. Data from NASA's Kepler mission indicate that rocky planets orbiting such "red dwarfs" are ubiquitous. A planet orbiting a red dwarf must huddle close to its host star to gather as much sunlight as Earth receives from the Sun. The tidal forces exerted on a close-in planet are huge, so shortly after formation it will become tidally locked, with one hemisphere always facing its red dwarf host, and the other in perpetual night.
How close could a tidally locked planet orbit to its host star without boiling away its oceans? We tackled this question with a three-dimensional global climate model, the same kind used for predicting climate change on Earth. We found that tidally locked planets receiving a lot of sunlight from their star tend to be cloudier and hence nearly as cool as planets receiving modest sunlight. Most of the day-side of such a planet is covered in thick clouds like the ones found in Earth's tropics. The clouds are highly reflective, which keeps the planet cool.
Seen from far away in the thermal infrared, the day-side of such a planet would look downright frigid. Instead of seeing the room-temperature surface, you would see the cloud tops, which are about as high and cold as the summit of Mount Everest. The cloud-free night-side of the planet, on the other hand, would appear warm. The James Webb Space Telescope, which NASA plans to launch in 2018, will monitor the thermal radiation from rocky planets orbiting nearby red dwarfs. If a planet appears to have a cold day-side and warm night-side, we will infer that it has a cloud shield and hence liquid water at the surface.