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Published in New Scientist (January 27, 2001, page 28).
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.
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 100 or 120 Fahrenheit. 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 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?
Ken Croswell is an astronomer in Berkeley, California, and the author of Magnificent Universe.
"Elegant and eloquent"--Washington Post. See all reviews of Magnificent Universe here.
"Habitability of Planets Around Red Dwarf Stars" by Martin Heath, Laurance Doyle, Manoj Joshi and Robert Haberle, Origins of Life and Evolution of the Biosphere, vol 29, p 405 (1999)
"Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability" by Manoj Joshi, Robert Haberle and R. Reynolds, Icarus, vol 129, p 450 (1997)
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