MKGF
ABOlocked planets with simple atmospheres of
carbon dioxide. If these simulated worlds
had at least 10 percent as much atmosphere
(by mass) as Earth, enough heat would be
shuttled to the dark side to prevent freez-
ing. If the atmosphere was somewhat
thicker, oceans could slosh — unboiled and
unfrozen — over most of the planet.
In addition, the oceans themselves
would provide a further mechanism for
smoothing out temperature extremes. Just
as the Gulf Stream ferries tropical heat to
England and Scandinavia on Earth, so
would oceanic currents on a dwarf ’s planet
prevent water on the dark side from turn-
ing into a granite-hard mass of ice.
Recently, researchers at the University of
Chicago have calculated that any tidally
locked planet with water will quickly pro-
duce thick clouds above its sun-facing sur-
face. This would reduce the planet’s
temperatures on the hot side and is another
mechanism that could increase the fraction
of habitable planets orbiting red dwarfs.
As a result of these and other simula-
tions, the view of tidally locked worlds is
considerably more upbeat than it once was.
Instead of an airless bipolar planet veneered
by scalding deserts and frozen seas, we now
envision a strip of land with mild tempera-
tures straddling the sunny and darkened
hemispheres — a ring-world where the cli-
mate is as salubrious as our own and where
both liquid water and a gaseous atmosphere
are available to spawn and sustain life.
If life exists in these oases, it might dis-
play some interesting adaptations. Without
nighttime or seasons, plant growth can
proceed uninterrupted. Since the sun is
always in the same place in the sky, there’s
no necessity to adjust the position of leaves
(or other light-gathering appendages). And
for worlds with sophisticated inhabitants,
erecting high-efficiency solar-power instal-
lations would be a cinch, given the fixed
(albeit, low) position of the sun.
The tidally locked world we know best
— our own Moon — is as sterile as an auto-
clave. Consequently, we are biased to think
that if a world doesn’t have a diurnal cycle
— if it doesn’t spin faster than it orbits
— it’s probably not a decent place
for brainy biology. But the truth
seems quite different.No-fry zones
Even if scientists are
optimistic about the
chances for life on red
dwarfs’ planets, they
need to know how
many such planets
actually exist. In
our solar system,
the habitable zonestretches from roughly 0.8 to 2 astronomi-
cal units from the Sun (1 astronomical
unit, or AU, is the distance between Earth
and our star). In other words, it exists from
just beyond Venus to the vicinity of Mars.
But for red dwarfs, the Goldilocks zone is
less than half that, typically a few tenths of
an AU or less across. With this reduced
size, habitable spaces around most of the
red dwarfs may be ungratifyingly empty.
But consider our solar system. The inner
region is stuffed with planets compared toA planet that orbits a red dwarf likely would not be uniformly habitable. It would be tidally locked, like the Moon is to Earth, rotating on its axis and revolving
around its sun in the same amount of time. A strip of land with mild temperatures would form a belt around the planet. Such a ring-world, half dark and half light,
would be able to sustain both liquid water and a gaseous atmosphere. The author created this rendition in his garage with flashlights and lamp shades. SETH SHOSTAK
Red dwarfs, or M-type stars, are 10 percent as mas-
sive as G-type Sun-like stars, whereas O-type stars
are 16 times as massive as G-type stars. But what M
dwarfs lack in size and brightness, they make up in
number. ASTRONOMY: ROEN KELLYStellar sizes