Companion planets
August 2, 2014

Companion Planets May Help Create Tidal Heating On Older Worlds

April Flowers for - Your Universe Online

For humans, having a companion in our later years can help increase our lifespan. A new study led by the University of Washington shows that having a companion might also extend the life of some Earth-sized planets.

As a planet ages, it cools and the molten core solidifies. This leads to a dwindling of the inner heat-generating activity that keeps the planet habitable by regulating carbon dioxide to prevent runaway heating and cooling.

The team of astronomers, including researchers from the University of Arizona, has found that for certain planets about the size of Earth, the gravitational pull of an outer companion planet could generate enough heat to prevent that internal cooling. This process, called tidal heating, can extend the inner world’s chance at hosting life.

UW Department of Astronomy research assistant professor Rory Barnes said that tidal heating is the result of the gravitational push and pull of the outer planet on its inner neighbor. The effect is, relatively, local. For example, Jupiter’s moon Io affects Europa. The team, which included UW graduate student Christa Van Laerhoven and University of Arizona planetary scientist Richard Greenberg, found that tidal heating can also occur on exoplanets — planets outside our solar system.

The team used computer modeling to determine that tidal heating occurs on Earth-sized planets in noncircular orbits in the habitable zone of low-mass, or less than one-quarter the mass of our Sun, stars. A star’s habitable zone, better known as the Goldilocks zone for being “just right,” is that region of space that puts the planet just far enough from the star to sustain liquid water on its surface, and not far enough to freeze solid.

“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,” Barnes said in a recent statement. “This constant flexing causes layers inside the planet to rub against each other, producing frictional heating.”

Barnes notes that the outer planet is necessary to keep a potentially habitable planet’s orbit non-circular. A planet with a circular orbit never changes shape because the gravitational pull of the star remains constant, and so tidal heating will not occur.

Further research after the discovery of any Earth-sized planets in the habitable zone of old, small stars should be done to find out of the planet has a companion to understand the chances of the inner planet’s ability to host life.

Barnes commented that the combined effect of the tectonics of the ancient planet, and the tidal heating created by the outer companion, could create the right environment for hosting the longest-lived surface habitats in the universe.

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

The findings were published in a recent issue of the Monthly Notices of the Royal Astronomical Society.