Companion Planet Gives Old World New Chance at Life

Companionship is apparently something that planets, like people, value in old age. It turns out that a companion planet might be useful in giving an old, less habitable world a new chance at life.

According to the study - published in the journal Monthly Notices of the Royal Astronomical Society - planets cool as they age. Their molten cores solidify and inner heat-generating activity dwindles, causing them to lose their habitability because they can no longer use carbon dioxide to prevent runaway heating or cooling.

But astronomers at the University of Washington (UW) and the University of Arizona have found that for certain Earth-sized planets, a companion may be their salvation.

The gravitational pull of an outer companion planet could generate enough heat - through a process called tidal heating - to effectively prevent that internal cooling, and extend the inner world's chance at hosting life.

"When the planet is closer to the star, the gravitational field is stronger and the planet is deformed into an American football shape. When farther from the star, the field is weaker and the planet relaxes into a more spherical shape," UW astronomer Rory Barnes explained in a news release. "This constant flexing causes layers inside the planet to rub against each other, producing frictional heating."

This tidal heating in fact happens between Jupiter and its moons lo and Europa. The researchers set out to prove that it could also happen between exoplanets outside our solar system.

Using computer models, they found that the effect can occur on older, Earth-sized planets in noncircular orbits in the habitable zone of low-mass stars - those less than one-quarter the mass of the Sun. The habitable zone is that strip of space around a star just right to allow an orbiting rocky planet to sustain liquid water on its surface, thus giving life a chance.

The study team notes that the outer planet is necessary to keep the potentially habitable planet's orbit noncircular - meaning, the gravitational pull is constant.

"Perhaps in the distant future, after our sun has died out, our descendants will live on worlds like these," Barnes said.