As every comic-book fan knows, Superman was born on the planet Krypton, which orbited a red star. Scientists are now learning that the Superman legend may contain a kernel of truth: the best places to find life in our galaxy could be on planets that circle the small but common stars known as red dwarfs.
Last June astronomers reported the discovery of Krypton's real-life counterpart, a planet orbiting a red dwarf called Gliese 876, about 15 light-years from our sun. Although researchers had previously identified two other planets orbiting Gliese 876, the detection of the third body made headlines because it is so much like Earth. Nearly all the planets found outside our solar system to date are gas giants comparable in size to Jupiter or Neptune. The newly discovered world, in contrast, is most likely a rocky body only about twice as large as Earth.
Red dwarfs, also known as M dwarfs, have less than half the mass of the sun and are hundreds of times dimmer. Scientists had long doubted that life could arise near such faint stars because the "Goldilocks zone"--the area around the star that is neither too hot nor too cold for liquid water--would extend no more than about 40 million kilometers from the M dwarf, less than the distance between the sun and Mercury. At such close quarters, the planet could become tidally locked: one side would always face the star while the other would be turned away. Researchers had previously assumed that the planet would inevitably lose its atmosphere because of freezing on the bitterly cold night side, but computer models have shown that an atmosphere containing modest amounts of carbon dioxide would spread enough heat to the night side to prevent freezing.
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M dwarfs are by far the most abundant stars in the Milky Way galaxy, outnumbering sunlike G stars more than 10 to one, so the possibility that they could harbor habitable worlds has excited researchers involved in the search for extraterrestrial intelligence (SETI). In July the SETI Institute in Mountain View, Calif., held a workshop in which scientists debated whether life could emerge on an M-dwarf planet. "No one found any showstoppers to habitability," says Gibor Basri of the University of California, Berkeley. One concern was that because M dwarfs frequently produce flares, the resulting torrents of charged particles could strip the atmosphere off any nearby planet. If the planet had a magnetic field, though, it would deflect the particles from the atmosphere. And even the slow rotation of a tidally locked M-dwarf planet--it spins once for every time it orbits its star--would be enough to generate a magnetic field as long as part of the planet's interior remained molten.
Unfortunately, the new planet found near Gliese 876 is only three million kilometers from the star, so its surface would be far too hot for life as we know it. But the sheer number of M stars in the Milky Way--an estimated 300 billion--has made them a prime target for SETI investigators, who are now building the Allen Telescope Array in northern California to scan the galaxy for radio signals. Besides their abundance, M dwarfs have another advantage over G stars as possible sites for intelligent life: because they shine longer before exhausting their hydrogen fuel and do not brighten with age, they provide a more stable long-term environment for life-bearing planets. One billion years from now, intensifying solar radiation will make Earth uninhabitable, but the galaxy's M dwarfs will burn steadily for hundreds of billions of years. It's enough to make Superman homesick.