Look at a big spiral galaxy like the Milky Way and its blazing with the blue-white light of hot, young stars. But the vast majority of stars in our galaxy are the smaller, redder types. Of the estimated 400 billion suns in our galaxy, astronomers believe at least half of them are red dwarfs. One problem with red dwarfs is they tend to be active, they can flare and dim dramatically over short periods of time compared to our reliable, stable yellow sun. Because they are so much dimmer, planets in the Goldilocks zone where water might exist as a liquid on the surface would need to huddle much closer to the red stars. Not only would those planets would be pummeled by stellar outburst, they’d become tidally locked, always the same side facing the sun, similar to the way our moon always presents the same face to earth. All things being equal, a planet that takes a few weeks to rotate instead of a day would have a less powerful magnetic field to ward off the solar wind. Both Mars and Venus have mostly lost whatever primeval magnetic field they once had and that probably played a role in vaporizing whatever oceans they may have started out with.
But there’s a newer consideration, one which could help protect the planets of red dwarfs, now being modeled and tested. This increases the number of potentially habitable exo-planets to 60 billion! How? Think of a cold Venus:
Link — The team’s three-dimensional 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. These required several months of processing, running mostly on a cluster of 216 networked computers at UChicago. Previous attempts to simulate the inner edge of exoplanet habitable zones were one-dimensional. They mostly neglected clouds, focusing instead on charting how temperature decreases with altitude.
“There’s no way you can do clouds properly in one dimension,” Cowan says. “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 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.
But if highly reflective clouds dominate the dayside of the exoplanet, they will block a lot of infrared radiation from the surface, says Yang, a postdoctoral scientist in geophysical sciences. 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 says.