Roughly three quarters of the stars in the galaxy are red dwarfs, but planet searches have typically passed over these tiny faint stars because they were thought to be unfriendly to potential life forms.
But this prejudice has softened lately. Preliminary results from a dedicated research program have shown that planets around red dwarfs could be habitable if they can maintain a magnetic field for a few billion years.
Red dwarfs — also called M dwarfs — are between 7 and 60 percent as massive as our sun. Their lower mass means they don't burn as hot or as brightly, emitting less than 5 percent as much light as the sun. However, they have strong magnetic activity, which makes them relatively bright in X-rays and UV radiation and causes them to flare frequently.
To understand the environment around these common stars, the "Living with a Red Dwarf" program was started three years ago. It is piecing together observational data to provide a profile of how red dwarfs vary in brightness and magnetic activity as they age.
"This is the information that you would want to know to model the suitability for life on a nearby planet," says Ed Guinan of Villanova University, a scientist working with the program.
As habitability goes, red dwarfs were thought to be the bad roommates of the cosmos.
Because they are so faint, the habitable zone — the distance from a star where liquid water can exist — is in many cases closer than the orbital distance between Mercury and our sun. When a planet orbits a star this closely, the gravitational pull of the star may cause the planet to become tidally locked with the same side always facing the star (similar to the Moon's fixed gaze on the Earth).
Previously, scientists speculated that the dark side of a tidally locked planet would become so cold that it would freeze up the entire atmosphere, leaving even the sun-lit side with little air for breathing. But more recent models have shown that winds would distribute the heat sufficiently to avoid this atmospheric collapse.
Still, life might not be a picnic around a red dwarf. Several times per day flares shoot off the star, causing the UV radiation to jump by 100 to 10,000 times normal. For several minutes, the star appears blue instead of red. This increased radiation could sterilize the surface of a nearby planet.
"You probably want to live on the dark side," Guinan says. "Or at least along the twilight zone where you would have less exposure."
Even between flares, the combination of UV light and stellar winds can strip away the atmosphere if nothing is protecting or replenishing it.
However, all hope is not lost. The high-energy radiation is predominantly emitted by young stars. As they age, red dwarfs become less magnetically active, while continuing to shine steadily at visible wavelengths for 100 billion years or more.
Therefore, if an orbiting planet can just hold onto its atmosphere through the wild early years of its red dwarf roommate, it could end up being a decent place to live.
Turning back the stellar clock
But just how long are red dwarfs dangerous?
To develop a model for how a star's magnetic activity changes with time, Guinan and his colleague Scott Engle looked at the rotation rates of a large sample of red dwarfs. As expected, faster spinning stars had more X-ray and UV emission, as well as more flares. The rotation causes charged material inside the star to be churned around, and this "dynamo" action generates a magnetic field. Gas around the star becomes trapped in this field and heated to millions of degrees. This hot gas produces the observed high energy radiation.
By estimating the ages of stars in their sample, the researchers were able to build up a typical red dwarf life history.
The data show that a red dwarf is born spinning rapidly, and it exhibits the corresponding magnetic activity. However, the magnetic field also creates strong winds that carry away angular momentum, and thus slow the star down with time.
The conclusion is that a red dwarf will calm down after about 2 or 3 billion years. In comparison, our sun (a typical G star) was magnetically very active (with 2 to 5 big flares per day) for its first half a billion years.
A planet with a substantial magnetic field, like Earth's, can deflect stellar winds and thereby avoid having its atmosphere stripped away.
"This could protect the planet for the 2 to 3 billion years that a red dwarf is active," Guinan says.
He is not completely optimistic, however. The fact that potentially habitable planets around a red dwarf are tidally locked implies they are rotating slowly around their axis. By the same physics that applies to stars, slow rotation will mean a weak magnetic field that could shut down completely.
This is what happened to Mars. It had a magnetic field 3.5 billion years ago, but when its liquid iron core solidified, the field turned off. Without this protective shield, the solar wind stripped away most of the planet's atmosphere and liquid water.
To avoid this fate around a red dwarf, Guinan speculates that a planet might need to be more massive than Earth. The large liquid iron core inside a super Earth (with a mass between 2 and 10 times Earth's) could perhaps maintain a magnetic field in spite of the slower rotation rate.
Interestingly, three of the two dozen planets detected so far around red dwarfs are super Earths. More will presumably be found in future searches: The MEarth Project is a planned survey of 2000 M stars using ground-based telescopes, and the Kepler spacecraft that launched in March has added more red dwarfs to its target list.
"M dwarf stars were overlooked in the past, but they have become more popular as people realize that life could potentially arise around them," Guinan says.