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Red Dwarf Planets

Seventy-five percent of the stars in the universe are red dwarfs—relatively small, dim stars. Typically, scientists have discounted any planets that might orbit red dwarfs from the search for life. But this might be a mistake, Doyle says.



Red dwarfs are less than half the mass of our sun, and emit less than 5 percent as much light. So to be in the habitable zone (the distance from the sun at which liquid water can exist), a red dwarf planet would have to be extremely close to its star. For example, he says, if you had a red dwarf star with one one-hundreth the brightness of our sun, its planet would have to be 10 times closer to it than we are to our sun—but such a planet would experience a whopping 1000 times the tidal force. This means that the gravitational pull of the star on the planet, which contributes to the ebb and flow of the ocean here on Earth, would be so strong that the planet would be locked in place, unable to rotate as it orbited. One side would always face the sun; the other would be permanently dark.



This scorching-on-one-side, freezing-on-the-other planet wouldn't be very welcoming to life. "The front burns off and the back burns out, and you can't have anything habitable," Doyle says. But he thinks that there's still a way red dwarf planets could harbor life.



Doyle has done modeling experiments in which he pumps extra carbon dioxide into a red dwarf planet's environment. With enough of a CO2 boost, processes such as the greenhouse effect can take place, creating an atmosphere that holds in and redistributes heat. This could create a planet that's livable in spite of its locked position. If this model is correct, it means that some of the copious red dwarfs in the universe could have habitable planets. To find out more, Doyle says, one new mission of the planet-hunting telescope Kepler will be to search the sky for red dwarf planets.



Weather patterns on a red dwarf planet like the ones in Doyle's model would be incredibly bizarre. Weather patterns emanate from the sun, and here on Earth, they have a sort of hula-hoop formation around our planet because it rotates, he says. But on a tidally locked red dwarf planet, the dim red sun would be a permanent fixture, never moving from its spot in the sky. Weather patterns would propagate downward from it in a spiral pattern, like "a ribbon wrapped around an orange from the top to the bottom," Doyle says. "They would have all sorts of weather that we can't even relate to."