The pursuit of actual living organisms on other planets is a common theme in space exploration, but finding locations in space that could sustain life are just as exciting. Researchers at the University of Chicago and Northwestern University have released a new study about the possibility of finding exoplanets around red dwarf stars, the most common type of star in the universe. The study, which appears in Astrophysical Journal Letters, concluded that cloud behavior on red dwarf planets makes them more likely to sustain life.
Current data from NASA’s Kepler Mission suggests that there could be one Earth-size planet in the habitable zone of each red dwarf, but the UChicago-Northwestern study roughly doubles that estimate, meaning there is the possibility of over 60 billion habitable planets. “Most of the planets in the Milky Way orbit red dwarfs,” said Nicolas Cowan, a postdoctoral fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics. “A thermostat that makes such planets more clement means we don’t have to look as far to find a habitable planet.”
The habitable zone refers to the space around a star where orbiting planets can maintain liquid water at their surface. The formula for calculating that zone has remained much the same for decades. But that approach largely neglects clouds, which exert a major climatic influence.
“Clouds cause warming, and they cause cooling on Earth,” said Dorian Abbot, an assistant professor in geophysical sciences. “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.”
A planet orbiting a star like the sun would have to complete an orbit approximately once a year to be far enough away to maintain water on its surface. “If you’re orbiting around a low-mass or dwarf star, you have to orbit about once a month, once every two months to receive the same amount of sunlight that we receive from the sun,” Cowan added.
The team is now using the James Webb Telescope to test the validity of these findings by measuring the temperature of the planet at different points in its orbit. If a tidally locked exoplanet lacks significant cloud cover, astronomers will measure the highest temperatures when the dayside of the exoplanet is facing the telescope, which occurs when the planet is on the far side of its star. Once the planet comes back around to show its dark side to the telescope, temperatures would reach their lowest point.
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 this signal from an exoplanet, Abbot noted, “it’s almost definitely from clouds, and it’s a confirmation that you do have surface liquid water.”
Images via NASA