Luke Skywalker’s home planet “Star Wars” is science fiction. But Tatooine-like planets orbiting pairs of stars may be our best bet in the search for habitable planets outside our solar system.
Many stars in the universe come in pairs. And around many of them have rotate the planets. This means that many more planets can orbit binary systems than single stars like ours. But until now, no one had a clear idea of whether the environment of these planets could be favorable for life. New computer simulations show that in many cases life can imitate art.
Earth-like planets orbiting some binary star configurations can remain in stable orbits for at least a billion years , researchers reported on January 11 at a meeting of the American Astronomical Society. The researchers suggest that this stability would be enough to potentially allow life to develop, provided the planets are not too hot or too cold.
Of the planets that got stuck, about 15 percent stayed in their habitable zone—the temperate region around their stars where water could remain liquid—most of the time.
The researchers simulated 4,000 configurations of binary stars, each of which revolves around an Earth-like planet. The team varied things like the stars’ relative masses, the sizes and shapes of the stars’ orbits around each other, and the size of the planet’s orbit around the binary pair.
The scientists then tracked the motions of the planets over a billion-year simulation to see if the planets would remain in orbit for the timescales that could allow life to emerge.
A planet orbiting a binary star can be ejected from a star system by complex interactions between the planet and the stars. In the new study, the researchers found that for planets with large orbits around star pairs, only 1 in 8 were ejected from the system. The rest were stable enough to continue orbiting for a billion years. About 1 in 10 settled in their residential areas and stayed there.
Of the 4,000 planets the team modeled, about 500 maintained stable orbits that kept them in the habitable zone at least 80 percent of the time.
“Residential area. . . as I’ve characterized it so far, ranges from freezing to boiling,” said Michael Pedowitz, a graduate student at the College of New Jersey at Ewing, who presented the study. Their definition is too strict, he said, because they chose to model Earth-like planets without atmospheres or oceans. This is easier to model, but it also allows the temperature to fluctuate wildly on the planet as it rotates.
“The atmosphere and oceans are pretty good at smoothing out temperature fluctuations,” said study co-author Mariah McDonald, an astrobiologist at the College of New Jersey. A large amount of air and water would potentially allow the planet to maintain habitable conditions, even if it spent most of its time outside the nominal habitable zone around the binary star system.
The number of potentially habitable planets “will increase as we add atmospheres,” Macdonald says, “but I can’t say by how much yet.”
She and Pedowitz hope to build more complex models in the coming months, and to extend their simulations beyond a billion years to include changes in stars that can affect conditions in the solar system as it ages.
The possibility of stable, habitable planets in binary star systems is a pressing issue, says University of Pennsylvania astrophysicist Jason Wright, who was not involved in the study.
“At the time of release ” Star Wars ,” he says, “we haven’t known any planets outside the solar system, and we won’t for 15 years. Now we know that there are many of them and that they revolve around these binary stars.”
These simulations of planets orbiting binary systems could provide guidance for future experiments, Wright says. “This is an understudied population of planets. There’s no reason why we can’t pursue them, and studies like this probably tell us it’s worth trying.”
