What’s becoming clear is that any habitable locales around
these stars will probably be quite different from Earth.
An optimistic list
M dwarfs make up about 70 percent of the several hundred
billion stars in the galaxy. They are cool and tiny — at least for a
star. Proxima Centauri, an M dwarf and the closest star to the
sun, is roughly 2,800° Celsius. That’s about 2,700 degrees cooler
than the sun, giving Proxima a soft glow. Many M dwarfs aren’t
much bigger than Jupiter, which is only about one-tenth as wide
as the sun. All this means that none are visible from Earth to the
unaided eye. Proxima Centauri is about one one-hundredth as
bright as the faintest stars our eyes can see without a telescope.
Because M dwarfs are so lightweight, they don’t burn
through their nuclear fuel as fast as their heavier cousins. So
they live for an extraordinarily long time. A star that weighs
about one-tenth as much as the sun, for example, has a projected lifetime of roughly 12 trillion years — more than 800
times the current age of the universe. That’s plenty of time for
life to arise and evolve on any planets orbiting these stars.
M dwarfs appear to be prolific planet producers. The dim
stars harbor 3. 5 times as many small planets, defined as planets between one and 2. 8 times as wide as Earth, as do stars
more like our sun. Compared with slightly warmer stars called
K dwarfs and with suns like ours, M dwarfs probably have the
lead on habitable worlds.
“Habitable” doesn’t mean inhabited, nor does it necessarily
mean a pleasant place to live. For most exoplanets, astronomers cannot directly measure anything about the climate or
atmosphere. All they know is that the planet receives the right
amount of solar energy to conceivably have liquid water on its
surface. Though aliens might have very different needs than
Earth-based critters, and may not even require water, scientists
lean on a go-with-what-works approach in the search for life.
By one conservative estimate, 13 known exoplanets are
“habitable,” and 11 are around M dwarfs. That estimate comes
from the Planetary Habitability Laboratory at the University
of Puerto Rico at Arecibo. A precise number is elusive, however, because there are different ways to estimate the boundaries of the habitable zone. It’s also not clear how massive
a planet can be and still have a solid surface for life to take
hold. Broadening their criteria to include larger planets and
a wider habitable zone, the Arecibo researchers identified an
additional 39 habitable exoplanets ( 20 orbiting M dwarfs and
six around sunlike stars). That puts the more optimistic list of
potential habitables at 52.
M dwarfs feature prominently partly because it’s easier to
find habitable planets around these stars. For a planet to stay
warm around such a cool star, it has to huddle up close, offer-
ing the best chance to get noticed. A close-in planet will have a
stronger gravitational tug on its star, making it easier to detect
the star’s wobble. And because these planets loop around their
star faster than remote worlds, dips in the star’s brightness
as well as changes in the star’s speed will be seen more often.
M dwarf planets will probably continue to dominate lists of
habitable worlds. But when it comes to a climate that’s suitable
for liquid water, M dwarf planets have plenty of hurdles, says
James Kasting, a geophysicist at Penn State.
One of the biggest challenges — uncovered only recently — is
how to survive the star’s early years. M dwarfs are faint, but
they don’t start off that way. When they first begin to shine,
M dwarfs can be roughly as bright as our sun, up to about 100
times as bright as they’ll eventually become, Barnes says. It
can take several hundred million years for an M dwarf to settle
down to the low-level luminosity it will maintain for the rest
of its life. Stars like the sun also start brighter than they end
up, but they fade much faster, needing only about one-tenth
as much time as M dwarfs.
A small world that today sits in the habitable zone of an
M dwarf spent hundreds of millions of its early years blasted
with more intense light. Using computer simulations, Barnes
and Washington graduate student Rodrigo Luger showed that
prolonged exposure to bright starlight could strip a planet’s
atmosphere of its water, leaving behind a barren world. The
amount of water lost depends on the planet’s mass, proximity to its star and initial inventory of water, the team reported
in 2015 in Astrobiology. A “habitable” M dwarf world such as
Gliese 667Cc, roughly 3. 7 times as massive as Earth and about
one-twelfth the distance from its star as Earth is from the sun,
could have lost as much as 10 times the amount of water as is
currently found in Earth’s oceans.
“Once you’ve lost all the water, you’re sunk,” Barnes says.
A once-promising planet could “potentially turn into Venus,
and Venus is not a good place to live,” he adds. While Venus
Huddle up In our solar system, the habitable zone (green) extends
more than 160 million kilometers, from just beyond Venus (at about
110 million kilometers from the sun) to beyond Mars. Around the relatively cool star TRAPPIST- 1, a mere 11 percent as wide as the sun, the
habitable zone is much closer and narrower. (Planets are not to scale.)
SOURCES: NASA SPACE SCIENCE DATA COORDINATED ARCHIVE; PLANETAR Y HABI TABILI T Y LAB/
UNIV. OF PUERTO RICO AT ARECIBO; M. GILLON ET AL/ NATURE 2017
TRAPPIST-1d TRAPPIST-1f TRAPPIST-1g TRAPPIST-1h TRAPPIST-1e TRAPPIST-1c TRAPPIST-1b