For ancient planets orbiting old stars, having a companion planet around can keep them warm on the inside. And for Earth-sized planets, a companion can help them keep life-hosting conditions for a longer period of time.
Planets get cooler as they get older: Their molten cores begin to solidify, their inner heat-generating activity dwindles, and they stop regulating carbon dioxide, giving way to a runaway heating or cooling effect on the surface. In other words, they become less habitable. But for certain Earth-sized worlds, the influence of a companion planet could help to generate enough heat to prevent that internal cooling -- prolonging the planet’s chances of hosting life.
A trio of researchers led by Christa Van Laerhoven from the University of Arizona examined a process called tidal heating, which results from the gravitational push and pull of one body on another. This effect can be seen on Jupiter’s moons Io and Europa, where Jupiter’s mighty pull creates heat inside the moons. Stars have the same effect on planets.
The team used computer modeling to show that this phenomenon can occur on exoplanets as well. In particular, they looked at older, Earth-sized planets in non-circular orbits in the habitable zone of low-mass stars (those that are less than a quarter the mass of our sun). The habitable zone is that area around a star with just the right temperature to allow an orbiting rocky planet to sustain liquid water on its surface.
According to the researchers, having a companion planet keeps the potentially habitable planet in a non-circular orbit. That’s important because a planet needs a non-circular orbit to experience this kind of tidal heating from the star.
“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,” says study coauthor Rory Barnes from the University of Washington in a news release.
Barnes adds: “This constant flexing causes layers inside the planet to rub against each other, producing frictional heating.” By contrast, when a planet’s orbit is a perfect circle, the gravitational pull is constant and there’s no tidal heating.
A combination of the ancient inner planet’s tectonics with the tidal heating generated by the host star -- with an assist from a companion planet -- could yield long-lived habitable surfaces.
“Perhaps in the distant future, after our sun has died out,” Barnes says, “our descendants will live on worlds like these.”
The work was published in Monthly Notices of the Royal Astronomical Society.