Planets with odd tilts -- those whose rotational axis leans first one way and then another over short periods of geological time -- could be surprisingly good candidates for hosting life, NASA scientists say.
The "wobbling" of these worlds could create climate effects that keep them from freezing up, even though many such worlds are farther from their parent stars than is currently considered a good distance within a star's so-called "habitable zone," they say.
If the wobble allowed water on their surface to remain liquid for extended periods of time, it could provide a habitat in which life could begin and be maintained, NASA astrobiologists say.
"Planets like these are far enough from their stars that it would be easy to write them off as frozen, and poor targets for exploration, but, in fact, they might be well-suited to supporting life," says Shawn Domagal-Goldman, at NASA's Goddard Space Flight Center in Maryland. "This could expand our idea of what a habitable planet looks like and where habitable planets might be found."
Computer models suggest a planet of nearly Earth size, orbiting a star like our sun in company with one or more gas giant planets, could have its rotational axis changed by the larger planets' gravitation pull within just tens of thousands of years or hundreds of thousands, an extremely quick phenomenon on a geologic timescale.
In thousands of computer simulations, the research team created 17 differenet planetary systems in which they could adjust the tilts of orbits and the leaning of a planet's rotational axis.
NASA scientists say such "wobble" planets aren't just a hypothesis; such extreme systems do in fact exist, and one has been spotted around a star known as Upsilon Andromedae, where two planets are in orbits that differ from each other by 30 degrees.
"Knowing that this kind of planetary system existed raised the question of whether a world could be habitable under such conditions," says University of Washington researcher Rory Barnes, who took park in the study of the two planets at Andromedae.
The possibility of planetary "wobbles" creating habitable conditions has been put forward in a paper the researchers submitted to the journal Astrobiology.
"In those cases, the habitable zone could be extended much farther from the star than we normally expect," says study lead author J.C. Armstrong. "Rather than working against habitability, the rapid changes in the orientation of the planet could turn out be a real boon sometimes."
