Faint red stars can build water worlds drip by drip

Sheer number of M dwarf systems makes them likely to host habitable exoplanets

MOST COMMON  Most warm rocky planets around faint red stars (illustrated) are probably dry, but it’s likely that there are enough such worlds with oceans to outnumber all other habitable environments in the galaxy.


HONOLULU — The stars likely to have the most habitable planets may also have the hardest time importing water to these worlds, Gijs Mulders reported August 5 at a meeting of the International Astronomical Union.

Faint, red orbs known as M dwarfs are the most common type of star in the Milky Way, making up about 80 percent of the stars in the galaxy. But these dwarf systems tend not to build the giant planets that are good at flinging icy asteroids at dry worlds, as is thought to have happened to Earth (SN: 5/16/15, p. 18). Recent computer simulations, however, show that lots of small boulders working together within a planet nursery can assume Jupiter’s and Saturn’s role by relocating ice deposits that help build livable locales.

“Even though water delivery is less efficient,” said Mulders, a planetary scientist at the University of Arizona in Tucson, “for a fraction of these stars, it still takes place.”

Depending on how much water the typical asteroid carries and how far it has to travel before running into a potentially habitable planet, up to 20 percent of warm rocky worlds around M dwarfs could end up with an ocean, Mulders said. These same simulations put the chances of an Earthlike ocean showing up in a solar system like our own at nearly 100 percent.

The sheer number of M dwarfs makes up for their water-trekking challenges. Pick a warm, rocky, wet world at random, and it’s likely to come from one of these small red suns. 

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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