Cloudy Worlds Climate
Clouds are pretty to see. They are also much more potent than previously perceived in modifying climate. This is particularly important when considering habitable planets near red dwarf stars. A new study that calculates the influence of cloud behavior on climate doubles the number of potentially habitable planets orbiting red dwarfs, the most common type of stars in the universe. This finding means that in the Milky Way galaxy alone, 60 billion planets may be orbiting red dwarf stars in the habitable zone. Researchers at the University of Chicago and Northwestern University based their study, which appears in Astrophysical Journal Letters, on rigorous computer simulations of cloud behavior on alien planets. This cloud behavior dramatically expanded the estimated habitable zone of red dwarfs, which are much smaller and fainter than stars like the sun.
Current data from NASA’s Kepler Mission, a space observatory searching for Earth-like planets orbiting other stars, suggest there is approximately one Earth-size planet in the habitable zone of each red dwarf. The UChicago-Northwestern study roughly doubles that estimate. It also suggests new ways for astronomers to test whether planets orbiting red dwarfs have cloud cover.
Previously many factors appear to indicate that red dwarfs have a very low probability for hosting indigenous life. Planets in the habitable zone of most red dwarfs were expected to experience such a strong tidal heating that the hydrogen necessary for water and all known life would be baked out of the planets before a stable orbit could be achieved. Combined with other problems, such as those created by tidal locking, the variable radiation of red dwarfs, lack of planetary axial tilts, small habitable zones due to low energy output etc. the probability of a habitable world was low.
A red dwarf is a small and relatively cool star on the main sequence, either late K or M spectral type. Red dwarfs range in mass from a low of 0.075 solar masses (the upper limit for a brown dwarf) to about 50% of the Sun and have a surface temperature of less than 4,000 K.
Red dwarfs are by far the most common type of star in the Milky Way galaxy, at least in the neighborhood of the Sun, but due to their low luminosity, individual red dwarfs cannot easily be observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are twenty of the next thirty nearest. According to some estimates, red dwarfs make up three-quarters of the stars
"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 the effects of clouds, which exert a major climatic influence.
"Clouds cause warming, and they cause cooling on Earth," said 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."
Planets in a tight orbit eventually would become tidally locked with their sun. They would always keep the same side facing the sun, like the moon does toward Earth. Calculations of the UChicago-Northwestern team indicate the star-facing side of the planet would experience vigorous convection and highly reflective clouds at a point that astronomers call the sub-stellar region. At that location the sun always sits directly overhead, at high noon.
The team’s 3-D global calculations determined for the first time the effect of water clouds on the inner edge of the habitable zone. The simulations are similar to the global climate simulations that scientists use to predict Earth's climate.
"There’s no way you can do clouds properly in one dimension," Cowan said. "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."
These new simulations show that if there is any surface water on the planet, water clouds result. The simulations further show that cloud behavior has a significant cooling effect on the inner portion of the habitable zone, enabling planets to sustain water on their surfaces much closer to their sun.
Astronomers observing with the James Webb Telescope will be able 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 day side 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.
But if highly reflective clouds dominate the day side of the exoplanet, they will block a lot of infrared radiation from the surface, said Yang, a postdoctoral scientist in geophysical sciences at UChicago. In that situation "you would measure the coldest temperatures when the planet is on the opposite side, and you would measure the warmest temperatures when you are looking at the night side, because there you are actually looking at the surface rather than these high clouds," Yang said.
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."
For further information see Red Dwarf Clouds.
Red Dwarf World image by Lynette Cook via Northwestern University.