The number of potentially habitable planets orbiting red dwarf stars has doubled after a new study on cloud cover from researchers at the University of Chicago and Northwestern University.
This cloud behavior dramatically expanded the estimated habitable zone of red dwarfs, which are much smaller and fainter than stars like the sun. This finding means that in the Milky Way galaxy alone, 60 billion exoplanets may be orbiting in the habitable zones of red dwarf stars.
The habitable zone refers to the orbital distance where a planet can maintain liquid water on its surface. But the calculation used for this neglects the effect of clouds on climate.
“Clouds cause warming, and they cause cooling on Earth,” said Dorian Abbot, an assistant professor in geophysical sciences at UChicago. “They reflect sunlight to cool things off, and they absorb infrared radiation from the surface to make a greenhouse effect. That’s part of what keeps the planet warm enough to sustain life.”
The team’s 3-D calculations -- Similar to the global simulations scientists use to predict Earth's climate -- determined for the first time the effect of water clouds on the inner edge of the habitable zone.
“There’s no way you can do clouds properly in one dimension,” said study co-author Nicolas Cowan. “But in a three-dimensional model, you’re actually simulating the way air moves and the way moisture moves through the entire atmosphere of the planet.”
A planet orbiting a low mass star like a red dwarf has to orbit once every month or two to receive the amount of light we get on Earth. Planets in such a tight orbit eventually become tidally locked with their star, meaning the same side always faces the star.
If a tidally locked exoplanet lacks cloud cover, astronomers will measure the highest temperatures on the dayside. But if there is cloud cover, it will block a lot of infrared radiation from the surface.
Astronomers observing with the James Webb Telescope will be able to test the validity of these findings, published in Astrophysical Journal Letters, by measuring planet temperatures at different points in orbit.
Earth-observing satellites have documented this effect. “If you look at Brazil or Indonesia with an infrared telescope from space, it can look cold, and that’s because you’re seeing the cloud deck,” Cowan said. “The cloud deck is at high altitude, and it’s extremely cold up there.”
If the James Webb Telescope detects these temperature discrepancies on an exoplanet, Abbot noted, “it’s almost definitely from clouds, and it’s a confirmation that you do have surface liquid water.”