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.
Death lock
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.
Magnetic shield
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.
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