Exoplanet orbits that seem just right for life could be bent out of shape by pushy neighbors. New simulations of extrasolar planetary systems may mean the definition of "habitable" planets needs to be completely overhauled.

When astronomers talk about the "habitable zone," they mean the shell around a star where the temperatures are right for liquid water. Any closer, and oceans will boil. Any farther, and the planet will freeze. But this definition assumes that most planets have roughly circular orbits, like the Earth and most other planets in the solar system.

"What we know from studying exoplanets is that that is definitely not the rule," said Rory Barnes of the University of Washington at the meeting of the American Astronomical Society in Miami. Many of the 454 exoplanets discovered to date have highly elliptical orbits, meaning the planets are not always the same distance from their parent star. Thanks to this uneven geometry, the planet spends more time closer to its star, which tends to make for warmer planets.

Adding another planet, especially a bullying Jupiter-sized planet, can mess with orbits and make a once-hospitable planet move in and out of the habitable zone over time. Using computer simulations of several hypothetical planetary systems, Barnes showed that a giant neighbor can pull an Earth-like planet's orbit like a rubber band, shifting it from circular to elliptical and back to circular again in as little as a few thousand years.

"It's a very stable, repetitive process," Barnes said.

The Earth's orbit actually does feel similar nudges from Jupiter, known as Milankovitch Cycles. But luckily for us, these orbital shape shifts are subtle.

Barnes' simulations predicted more-dire consequences for extrasolar planets near the edge of their habitable zones, though. If the planet is on the cooler edge of the habitable zone, it could go through cycles of freezing and thawing. If it's on the warmer side, the temperature could fluctuate from comfy to boiling from one millennium to the next.

"The inner edge is much more dangerous," Barnes said. All the water could boil off and be lost forever, or the warming planet could experience a "runaway greenhouse" effect and end up a scorched wasteland like Venus.

But it's not all bad news. Barnes suggests that some planets we might dismiss as snowballs could just be going through an eccentric phase.

"Our own Earth has gone through stages of glaciation – we call them snowball Earth phases – and we managed to pull out of it," he said. "On a planet like that, on the outer edge, you will have reservoirs of life, and there will be habitats that will persist."

For planets around dim, low-mass stars, which have to be especially close to be in the habitable zones, neighboring giant planets could wreak havoc with the length of the day, and the gravitational pull could cause cycles of volcanic activity and earthquakes interspersed with relative calm.

"These are fascinating worlds to think about," Barnes said. "It will do lots of interesting things as far as how climates might evolve and how evolution might happen on such a planet."

The results suggest that the current definition of "habitable zone" may be too simplistic. Astronomers may have to consider the whole family of exoplanets in a system before determining if one is habitable or not.

"One of the things that this new work is emphasizing is that one needs to be very careful about defining habitability," commented Phil Armitage of the University of Colorado, Boulder. "Those ideas about terrestrial planet formation and habitability of terrestrial planets will need to be re-evaluated from scratch."

Image: NASA

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