The most common type of star in the universe may savage planets near them, potentially stripping away their atmospheres and dooming chances for life as we know it, scientists say.
Red dwarf stars, also known as M dwarf stars, are up to 50 times dimmer than the sun and are just 10 to 20 percent as massive. They are the most common kind of star, making up to 70 percent of the stars in the universe.
That red dwarfs are so common has made scientists wonder if the area around them they might be the best chance for discovering planets habitable to alien life. More and more planets are getting discovered around red dwarfs — recent findings from NASA's Kepler space observatory reveal that at least half of these stars host rocky planets that are one-half to four times the mass of Earth.
"Since red dwarfs are very common, if we find that planets are habitable around them, then it might be very likely we have life in the cosmos," said lead study author Ofer Cohen, a space physicist at the Harvard-Smithsonian Center for Astrophysics.
Scientists typically define planets as potentially habitable if their surfaces are warm enough to sustain liquid water on their surfaces. Red dwarfs are relatively cold, which means their habitable zones are closer than Mercury is to the sun — just 0.1 to 0.2 astronomical units (AU), or one-tenth to one-fifth the average distance from Earth to the sun.
The problem is "when you put a planet so close to its star, the space environment it will face is very extreme," Cohen told Space.com. "The planet has to face the stellar wind of particles streaming from the red dwarfs."
The extreme space weather would trigger spectacular auroras, or northern lights. The auroras on a red-dwarf planet could be 100,000 times stronger than ones on Earth, and extend from the poles halfway to the equator.
"If Earth were orbiting a red dwarf, then people in Boston would get to see the northern lights every night," Cohen said.
Earth is protected from space weather by its magnetic field, which deflects incoming energy blasts a bit like the shields of Star Trek's Enterprise. "When Mars lost its magnetic field, its atmosphere was very quickly stripped away, even though Mars is farther away from the sun than Earth is," Cohen said.
To see if alien planets around red dwarfs might also get their atmospheres torn off, Cohen and his colleagues modeled three planets ranging from 0.92 to 1.69 times Earth's diameter orbiting a red dwarf at distances of 0.06 to 0.16 AU. The red dwarf was relatively average, one that was 0.3 the diameter of the sun, 0.35 the mass of the sun, and about 5,660 degrees Fahrenheit in temperature.
The researchers found that even when these planets had magnetic fields as strong as Earth's, they would lose their atmospheres on the order of a couple of million years.
"Even though the planet is within the conventional definition of the habitable zone, it most likely does not have an atmosphere," Cohen said. "Without an atmosphere, all water would be lost on the surface."
However, not all hope may be lost for life as we know it around red dwarfs. The scientists are now seeing if red-dwarf planets might be habitable if they started off with thick atmospheres, just like Venus has.
The scientists detailed their findings Monday at the American Astronomical Society meeting in Boston.