‘Normal’ is a tricky word when you’re talking about extrasolar objects. As in ‘normal star,’ a phrase used during yesterday’s news briefing about the new planet detected around Gliese 876, and in much of the press coverage since. The planet’s low mass (about 5.9 times the mass of Earth) rules out the possibility that it is in any sense Jupiter-like, and the natural assumption is that this is a rocky world in a tight orbit around an M-class red dwarf.

Now it is true that M-class stars are normal, if by normal we mean abundant; in fact, some 70 percent of the stars in the Milky Way seem to be red dwarfs (maybe 15 percent are K-class, 3 percent G class, like our Sun, and 3 percent F class, all categories that might support life-bearing planets).

Image: An enlarged view of the rocky planet orbiting Gliese 876. Copyright 2005 by Lynette Cook (http://extrasolar.spaceart.org/), and used with her permission.

But what the scientists studying this object mean by a ‘normal star’ is a star that is still undergoing the processes of stellar burning. They’re making this distinction because until yesterday, the only rocky worlds found orbiting another star had been found around a pulsar, the savaged remnant of an exploded star. M-class red dwarfs are still burning, but they are not Sun-like, and we should keep that caveat in mind. These stars are small, cool, and quite long lived.

And any life found on planets around them might be exotic indeed. Take Proxima Centauri. Like our newfound planet around Gliese 876, a rocky world around Proxima would have to orbit close in to stay warm enough for possible life to form (admittedly, the Gliese 876 planet seems a bit too close in — its surface temperatures would be high enough to make life as we know it hard to imagine). Well inside the orbit of Mercury in our own Solar System is where the habitable zone is going to be around a red dwarf like this. In Proxima’s case, the frequent x-ray laden flares it emits might prove fatal to life, or they might serve as an evolutionary stimulus, depending on their severity.

And those temperatures, even the high ones on the surface of the new Gliese 876 planet, are still provocative. Any planet this close to a red dwarf is probably going to be tidally locked to it, so that one side remains in perpetual light, the other in darkness. Assuming an atmosphere, the hottest spot on the planet would be at the center of the day side; the coldest would be directly opposite, on the night side. Temperatures would drop steadily as one moved toward the dividing line between light and darkness. Ken Croswell paints an interesting scenario involving such a planet in the article “Red, Willing and Able,” which originally ran in New Scientist (January 27, 2001) and is now available online. The recent National Geographic TV program Extraterrestrial also showed an intriguing red dwarf planet with bizarre though perfectly feasible life-forms.

Scientists at NASA’s Ames Research Center studied a hypothetical planet around a red dwarf in the 1990s and concluded that even a sparse atmosphere might be enough to circulate heat to the dark side of the planet, keeping its atmospheric gases from freezing out. If water existed on such a world, the possibility of oceans underneath crustal ice, heated by geothermal activity from below, could mean life might be found even on the dark side.

On the day side, of course, most light would be in the infrared, and the sun would be a stationary red disk — no axial tilt, no seasons. And here’s the kicker when it comes to possible life around red dwarf stars: these are exceedingly long-lived objects. If our Sun has another eight to ten billion years left, a red dwarf might live 10 times as long, 100 billion years in which to let evolution work out the quirks of existing in such an unusual ecosphere. Gliese 876 doesn’t seem like a candidate for biology, but we’re getting to where we’ll be able to detect smaller rocky worlds around similar stars, and that makes the planet hunt get more and more interesting.

NOTE: This post originally mis-stated the expected life span of a red dwarf as 100 times as long as the Sun. The correct figure is ten times as long, or 100 billion years, as now corrected above.

At Princeton two weeks ago, I asked the university’s Jeremy Kasdin, who had just delivered a fine presentation on imaging extrasolar planets from space (with obvious reference to the Terrestrial Planet Finder mission), whether TPF would examine red dwarfs among its list of Sun-like stars in nearby space. And Kasdin said yes, though they wouldn’t be primary targets — “…they’re simply too cool,” he added. I can see his point, since resources will be limited, and it makes sense to concentrate on stars more or less like our own. But wouldn’t it be fascinating if we find that unusual forms of terrestrial biospheres do develop around some M-class stars, and that they may be places where intelligences far older than our own have found a way to survive?

The Ames work is described in Manoj Joshi, Robert Haberle, and R. Reynolds, “Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability,” Icarus 129 (1997), pages 450-65. See also Martin J. Heath et al., “Habitability of Planets Around Red Dwarf Stars,” Origins of Life and Evolution of the Biosphere 29 (1999), pages 405-24.