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An artist places us on the surface of Gliese 876 d, a planet in or near the habitable zone around a red dwarf star. (Inga Nielsen, Hamburg OBS., Gate To Nowhere)

Earth-Like Planets Could be Right Next Door

Astronomers estimate that billions of habitable planets are orbiting red dwarf stars. What would it be like to live there?

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Let’s say our red dwarf planet is tidally locked. The common assumption is that one side of the planet, in eternal daylight, would be scorched while the side in perpetual night would be frozen solid. But that’s not necessarily true, says Kasting.

Astronomers working with Jill Tarter, the longtime champion of the search for extraterrestrial intelligence, have shown how tidally locked planets could support life. Tarter recently stepped down as director of the SETI Institute to lead the funding for the Allen Telescope Array in California (see “Can We Hear Them Now?” July 2007), which will spend time listening for extraterrestrial signals from red dwarf systems. For many years, says Seth Shostak, a senior astronomer at the SETI Institute, astronomers had ruled out red dwarfs as places to look for Earth-like planets because they believed that being tidally locked would make them uninhabitable. Tarter, among the first to seriously address whether red dwarfs could harbor planets viable for life, convened a workshop to consider the idea, publishing their conclusions in the journal Astrobiology in 2007.

Studies at the time suggested that on a planet with a substantial enough atmosphere, the climate would be dominated by currents that pull up water in the air and toward the surface in oceans on the hotter, day side—a process called upwelling. This circulation would result in thick clouds covering the sun-facing side; the clouds would prevent the persistent solar radiation from scorching the surface. The currents would also cause atmospheric churning that would spread warmth around the planet.

Furthermore, this thickening of the atmosphere on the dayside would provide an important defense against other radiation dangers as well. Young red dwarfs, in their first few billion years, tend to be very active, emitting flares and ultraviolet radiation. Tarter and her colleagues proposed that the constant cloud cover would help diffuse these violent outbursts—though not totally. Indeed, life on a planet orbiting a young red dwarf might be more likely found underwater, taking advantage of even more protective layers.

On some planets orbiting red dwarfs, a search for “hidden” water, like the one that has dominated NASA’s exploration of Mars, won’t be necessary. The nature of planet formation in these systems could result in some habitable planets being covered in water. One reason, says Kasting, is that Earth-size planets residing in the close-in habitable zone probably formed farther out and migrated inward.

Planets form in a star’s debris disk by sweeping up dust and ice particles in their orbits. A planet with a small orbit generally won’t be able to collect as many particles as one in a longer orbit, so it won’t grow as large (just look at Mercury and Venus, for example, both smaller than Earth). Additionally, the closer a planet is to the star, the stronger the star’s gravitational pull is on the planet, and the faster the planet has to travel to keep from getting pulled in. This speed leads to a violent early life. Frequent collisions with other large objects can strip away a young planet’s atmosphere, or keep a sizable planet from forming altogether.

Planets that form farther out might gather up an excess of ice and, once they shimmy into the habitable zone, essentially become water worlds. “Earth’s oceans are an average of three kilometers deep if you spread them out over entire surface, but you might have an ocean [on an exoplanet] that’s 300 kilometers deep,” Kasting says. “That might be okay for marine life, but it probably would preclude the presence of continents.” Any visitors are going to need a raincoat in addition to a boat, because the planet’s highly active water cycle, Tarter writes, would produce “intense cloudiness, as well as massive precipitation.”

If, however, there’s enough water for a cloudy sky and sizable oceans, and currents strong enough to distribute heat, you could very well see our imagined exoplanet having forested continents. But even here things can get a little strange. Nancy Kiang at NASA’s Goddard Institute for Space Studies in New York has investigated the color plants might have on a habitable planet orbiting red dwarf stars, and the answer is not green.

Earth plants survive by converting the light our sun gives off into energy through photosynthesis. In most plants, the chlorophyll that enables this process absorbs blue and red light while reflecting the green light. But a red dwarf radiates much less visible light than our sun. A plant on this red dwarf planet might need to absorb as many wavelengths of light as possible to maximize photosynthesis. The plants, reflecting back almost no visible light, would appear black.

In fact, all the life on our imaginary planet might not just look different because of its red dwarf sun, it could be much, much older. Red dwarfs can exist for hundreds of billions of years before they finish the slow depletion of their fuel—indeed, they live for so long that it’s likely no red dwarf that has come into existence since the beginning of the universe has died.

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