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IF YOU WANT to find extraterrestrial intelligence, you’re going to have to
look in the right place. In our Galaxy alone there are more than 100 billion
stars, so you might expect to find a profusion of alien abodes. But which suns
do you point your telescope at? Bright, yellow stars like our own Sun have
always seemed the obvious place to start. In the past few years, though,
researchers have begun to wonder if they’ve been neglecting a whole class of
likely targets: red dwarfs.

Smaller, cooler and fainter than the Sun, red dwarfs give out just a feeble
red glow. More than a dozen of these puny stars reside within as many light
years of Earth, yet they’re so faint that not a single one is visible to the
unaided eye. It was always thought that any planet orbiting a red dwarf would be
an extremely unlikely place to find life. But it now looks as though these dim
red suns could harbour most of the Galaxy’s life-bearing worlds.

This is great news for anyone hoping to find hospitable planets outside the
Solar System. While stars like the Sun are relatively rare, four out of five
stars in our Galaxy are red dwarfs. “We all want to find habitable planets out
there,” says Laurance Doyle, an astronomer at the SETI Institute in Mountain
View, California. “The fact that we can now rule in 80 per cent of the stars is
a positive note for almost everybody.”

For decades, the arguments against finding life around red dwarfs have seemed
secure. These stars owe their dimness to a misfortune of birth—when they
formed they only acquired between 8 and 60 per cent as much mass as the Sun. As
a result, their cores are cool and the nuclear reactions take place at a slow
rate, providing little energy. The nearest red dwarf—Proxima Centauri,
which is 4 light years from Earth—emits less visible light in a century
than the Sun does in a week.

No problem, you may say. The Earth is hospitable to life because it lies at
just the right distance from the Sun. So although red dwarfs may be fainter than
the Sun, an alien planet orbiting one could still have a balmy climate if it
huddled close enough to its star. For a red dwarf with one-hundredth of the
Sun’s brightness, for instance, a planet would be at a suitable temperature if
it circled ten times closer to its parent star than the Earth does to the
Sun.

Such a planet would not, however, be just like the Earth. Although it would
enjoy terrestrial temperatures, its proximity to the star would come at a price.
A planet in such a tight orbit would become tidally locked to its star, just as
our own Moon is locked to Earth. One side would perpetually face the star, while
the other faced away.

The Moon’s Earth-facing side suffers little more than the occasional visiting
astronaut, but the day side of a red dwarf planet would fry. Worse, the night
side would be so frigid that you would expect the gases in the atmosphere to
freeze, and snow onto the dark surface, where they would remain locked up
forever. Only if the atmosphere was sufficiently thick would a planet be spared
such a fate. Researchers calculated that gases circulating in the atmosphere
would then be able to transport heat from the planet’s day side to its night
side, warming the night air so that it wouldn’t freeze out.

Until recently, though, most researchers believed that to do this the
atmosphere would have to be so thick that it would prevent the sun’s rays from
reaching the surface. And that would rule out photosynthesis on the planet, a
serious blow for the development of life as we know it. No wonder, then, that
those looking for life have pointed their telescopes elsewhere.

But in the 1990s, Robert Haberle and Manoj Joshi of NASA’s Ames Research
Center in Moffett Field, California, found something unexpected. They simulated
the atmosphere of a red dwarf planet, and calculated that even a thin atmosphere
would do the trick. If the planet had only 15 per cent as much air as the Earth,
they said, that would still ferry enough heat around to the dark side to keep
the atmosphere from freezing out.

“When I first heard that the atmosphere wasn’t going to freeze out, I found
it tremendously exciting,” says Martin Heath of Greenwich Community College in
London. But there was still a problem with the water cycle. Even in some of
Joshi and Haberle’s models, there remained freezing conditions on the planet’s
dark side. Heath was worried that even if the gases didn’t freeze, the planet’s
water might still migrate from the day side to the night side, killing off any
prospects of life.

Then in 1997, Heath began to wonder whether deep ocean basins might solve the
problem. With deep enough seas, even though the surface of the ocean might
freeze on the planet’s dark side, you could still have a liquid layer beneath,
kept from freezing by the planet’s geothermal heat. This would allow liquid
water to flow back to the day side.

Perpetual light

Although it now looks as if a planet orbiting a red dwarf can offer oceans,
atmospheres and a mild climate, such a world would still differ greatly from
Earth. It would have no seasons, because the tidal pull of the star would
prevent its spin axis from tilting. And one side would be in perpetual light,
while the other was in perpetual darkness.

The hottest part of a red dwarf planet would be just one spot on the
equator—the centre of the day side, where the sun is overhead. On a
habitable planet, the temperature at the hot spot might soar to 40 or 50 °C
(see Diagram).
Moving away from this spot, temperatures would drop, falling
towards freezing near the dividing line between the day and night sides. On the
night side there would be an ice cap covering the coldest part, directly
opposite the hot spot.

“The daylight hemisphere is going to be where the action is,” says Heath.
“For one thing, it’s going to be pretty cold on the dark side. We know that
there are organisms that can sit in water pockets in the ice and carry out
photosynthesis, but they can’t do that if there’s no light getting there.”

Wherever you were on the planet’s day side, the red dwarf sun would never
set. Instead it would hover perpetually at the same place in the sky. Plants and
trees might orient themselves towards it as they grew. But because the sun is
stationary some regions would never see direct sunlight. A region in the shadow
of a mountain, for example, would be forever in shade, preventing photosynthesis
there.

And even for the regions in sunlight, photosynthesis might be difficult. Red
dwarfs are so cool that they emit most of their energy at infrared wavelengths,
giving off relatively little at the visible wavelengths that support life on
Earth.

As if all this weren’t enough, red dwarfs subject their planets to other
challenges. They often display spots far larger than those seen on the Sun.
These “starspots” can cause the star to dim by up to 40 per cent for several
months at a time. Would this be enough to precipitate the big freeze? Joshi
thinks not, as long as the planet wasn’t at the extreme edge of the star’s
comfort zone. Plants might cope with starspots by changing their colour,
absorbing more light when their sun dims.

At other times, red dwarfs brighten dramatically, spewing large flares that
can more than double the star’s brightness in a matter of minutes. Such flares
might damage life, but they might also help it evolve, by increasing the
mutation rate. In any case, the number of flares often decreases as a red dwarf
ages, and many old red dwarfs don’t flare at all.

One clear advantage that red dwarfs have over Sun-like stars is their
longevity. Although they were born with less fuel than the Sun, they burn it so
frugally that some will survive for more than 1000 billion years. In contrast,
the Sun will die within a mere 8 billion years. It has taken terrestrial
intelligence 4.6 billion years to evolve since the Solar System formed, but life
on Earth may be atypical. If intelligence generally requires more time to
emerge, then planets orbiting red dwarfs may be ideal.

So do red dwarfs really have suitable rocky planets like the Earth for life
to occupy? We already know that they can have larger planets, more akin to
Jupiter. Astronomers have found two Jupiter-sized worlds circling a nearby red
dwarf called Gliese 876, which lies just 15 light years from Earth. These
particular planets are unlikely to harbour life, however, since Jupiter-sized
planets—at least in our Solar System—consist mostly of hydrogen and
helium.

Even chance

Still, it’s perfectly possible that red dwarfs could have smaller planets
too. Doyle and his team think they may have detected Earth-sized worlds around
another star, CM Draconis. This “star” is actually a binary system composed of
red dwarfs orbiting each other. The plane of this orbit is edge-on to the Earth,
so the stars eclipse each other every 30 hours. If it has planets, they too
should lie in this plane, meaning they will cross the stars’ faces and block out
some of their light. And because the stars are so small, even a planet with just
three times the Earth’s diameter would dim the light noticeably. But does CM
Draconis have such planets? “I think it’s about 50:50,” says Doyle. His team
published a paper last year reporting two possible candidates, but they still
have nothing definite. “The candidates we have need to be observed more,” says
Doyle.

Even if such planets exist, researchers admit that many questions remain
about whether red dwarfs can support life. “It’s very early days,” says Heath.
“What we’ve shown is that there is a case to be answered. That’s a very
different thing from demonstrating that there is actually life on a planet
around a red dwarf star.”

But they are cautiously optimistic. “Our approach to this whole subject has
gotten more catholic over the years rather than more selective,” says SETI
pioneer Jill Tarter, who is searching for signs of life around all stars within
16 light years of the Sun, most of which are red dwarfs. “Those are our
next-door neighbours, and we really ought to look down the street before we try
and hike across the country,” she adds. When it comes to SETI surveys of more
distant systems, however, Tarter still prefers Sun-like stars.

“If you’d asked me a few years ago,” says David Soderblom of the Space
Telescope Science Institute in Baltimore, Maryland, “I would have said that red
dwarfs have a very low probability of having life-bearing planets. But given
what we’ve seen here on Earth and the rather hostile conditions under which life
can flourish, I would say it’s pretty good odds.”

And there is good reason to believe that the first extraterrestrial
civilisation that we find will differ greatly from our own. Ten years ago, when
astronomers knew no planets beyond the Solar System, they believed that other
solar systems would resemble our own. Then, in 1991, they accidentally
discovered the first extrasolar planets, circling not a living star like the Sun
but a type of dead star known as a pulsar. And in 1995, when they found the
first extrasolar planet around a Sun-like star, it took them completely by
surprise. In our Solar System, giant planets like Jupiter and Saturn orbit far
out from the Sun. But this giant was astonishingly close to its star, and
astronomers have since found many others like it.

Which leads to an intriguing thought. Any planets that circle red dwarfs may
have given rise to astronomers as parochial as those on Earth. These alien
observers may have concluded that only red dwarfs can support life, blessed as
they are with stable planets where suns never set and seasons never disrupt the
climate. Indeed, their SETI programs may ignore Sun-like stars altogether. After
all, they might argue, any temperate planet orbiting such a star would lie so
far out that it would rotate freely, subjecting life to a relentless cycle of
light and dark. Any tilt of the axis would cause severe summers and winters, and
changes in axial tilt might induce ice ages, with mighty glaciers smothering
much of the globe. How on Earth could life possibly arise on such a hostile
world?