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?
- By Bruce Lieberman
- Air & Space magazine, June 2013
Inga Nielsen (Hamburg OBS., Gate To Nowhere)
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So far, this planet on which we stand is a bit foreign compared to Earth, but not shockingly different. This is where any sense of familiarity ends, however, because another consequence of orbiting so close to its star is that the planet could be tidally locked, so that one side always faces its sun. Astronomers call this synchronous rotation, when an orbiting body takes the same time to rotate around its axis as it does to revolve around another body, just like our moon does as it orbits Earth.
Or it could be like Mercury, says Jim Kasting, an exoplanet researcher at Pennsylvania State University and the author of How to Find a Habitable Planet. Mercury, he explains, is caught in a “spin- orbit resonance” and rotates three times for every two orbits around the sun. This stable relationship between orbital and rotational periods is caused by the long shape, or eccentricity, of Mercury’s orbital path, which in turn is caused by the pull of gravity from the many outer planets—a situation that could also exist for a close-orbiting exoplanet. In other words, it’s possible that on our imaginary planet, we would enjoy only three sunrises every two years.
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.