60 Billion Planets in Milky Way Galaxy Could Support Life

There may be more habitable planets out there than we once thought. Astronomers have discovered that cloud behavior on planets orbiting red dwarfs actually doubles how many are habitable. The finding means that in the Milky Way galaxy alone, there are approximately 60 billion planets that may be orbiting red dwarf stars in the habitable zone.

In order to better study planets that orbit red dwarfs and their clouds, the researchers used computer simulations of cloud behavior on alien worlds. Because red dwarfs are smaller and cooler than our own sun, planets need to have a closer orbit in order to be able to support life. In fact, these worlds orbit their stars about once every two months in order to receive the same amount of sunlight as Earth. This, in turn, means that these planets eventually become tidally locked with their sun; the worlds always keep the same side facing the sun, like the moon does toward the Earth. This, in turn, would mean that the star-facing side would experience vigorous convection and highly reflective clouds.

So what does this mean exactly? With the models, the scientists found out the effects of these clouds on the planet. In fact, they found that if there is any surface water on the planet, water clouds result. This, in turn, has a significant cooling effect on the inner portion of the habitable zone, which allows planets to sustain water on their surfaces much closer to their sun. The findings have exciting implications for further studies.

"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, in a news release. "A thermostat that makes such planets more clement means we don't have to look as far to find a habitable planet."

The astronomers can also test these simulations. By using the James Webb Telescope, they'll be able to measure the temperature of planets at different points in its orbit. If a tidally locked exoplanet lacks significant cloud cover, they will measure the highest temperatures when the dayside of the exoplanet is facing the telescope. Yet if clouds are present, they would measure the coldest temperatures when the planet is on the opposite side and the warmest temperatures when looking at the night side.

"Clouds cause warming, and they cause cooling on Earth," said Dorian Abbot of UChicago in a news release. "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 findings are crucial for better understanding exoplanets and finding habitable ones nearby. They could mean that habitable planets are in our very own Milky Way, just waiting to be discovered.

The findings are published in Astrophysical Journal Letters.