Rocky extrasolar planets thought to be half frozen and half scorched might instead rock back and forth, creating large swaths of twilight with temperatures suitable for life.
Because of gravitational tugs with the objects they orbit, rocky bodies often settle into trajectories in which they always show the same face to their hosts. Such 'tidally locked' exoplanets would thus seem like bad candidates for life, since the hemisphere facing their host stars would roast and the dark side would freeze.
But a new computer model by Anthony Dobrovolskis of NASA Ames Research Center in California, US, suggests this is not always so. He finds that such planets can rock to and fro if they travel on elongated, or eccentric orbits, creating a 'twilight zone' that could be hospitable to life.
The Moon experiences a similar rocking motion. It always shows the same face to Earth, taking the same amount of time to rotate around its axis as it does to circle our planet once. However, because the Moon's path around the Earth is not perfectly circular, its orbital speed is sometimes faster or slower than its rotational speed. The difference between the two motions causes the Moon to rock slightly.
"If you're standing on the Moon, you'll see the Earth rock back and forth a little bit," Dobrovolskis told New Scientist.
He says extrasolar planets on very elongated orbits will experience pronounced rocking motions, or librations. Rather than being worlds of fire and ice, these 'rock-a-bye' planets could have much more temperate climes than previously thought.
If the planet rocks by 90° or more, "there is no permanent day or permanent night side anymore," Dobrovolskis told New Scientist. "The whole thing becomes a twilight zone."
The effect could increase the likelihood of life on rocky worlds orbiting small, dim stars called red dwarfs, said NASA Ames astronomer Jack Lissauer. That's because the dwarfs' habitable zone - where liquid water, and potentially life, could exist - lies so close to the small stars that any planets there would almost certainly be tidally locked to their hosts.
The results also have implications for attempts to directly observe new worlds, Dobrovolskis says. He says astronomers should look for planets whose temperatures are relatively even all across their surfaces, not just for planets sporting one very hot and one very cold hemisphere.
Astrobiology - Learn more in our out-of-this-world special report.
Journal Reference: Icarus (vol 192, p 1)
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