Luna to the Rescue: Large moons – and Other Companions – May Be a Saving Grace

Rory Barnes, University of Washington

Rory Barnes, University of Washington

Time takes its toll – on people, on stars and on planets. As planets age, they cool. Low internal temperatures can terminate global plate tectonics, freezing the carbon cycle and spelling disaster for life. However, as the authors of a recent astronomy paper pointed out, it’s possible to escape this fate with the aid of a friend.

In Tides, planetary companions, and habitability: habitability in the habitable zone of low-mass stars, Laerhoven, Barnes and Greenberg point out that the majority of Earth-sized exoplanets are discovered close to their stars – too close for life to be comfortable when stars are as bright as ours. However, exoplanets up close to dimmer stars might be sitting pretty. Low-mass M or K stars shed enough light to support life, but not so much that they scorch the surface. Thus, close-in habitable zones (HZ) around these stars could provide a happy home for life in the universe.

Naturally, there’s a catch.

“Despite their location within a HZ,” wrote the authors, “the extreme ages of such planets present a challenge to habitability because such planets are likely to have cooled internally.”

This is where a companion planet or massive moon can come to the rescue. Companion planets or satellites tug on their parent planets, producing a sort of friction between the two bodies. The result of that interplay causes tidal heating.

“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,” said Rory Barnes, second author on the paper. “This constant flexing causes layers inside the planet to rub against each other, producing frictional heating.”

This back-and-forth around a low mass stars between an exoplanet and a very large moon – or more likely, between two planets in a sort of binary system – could keep one if not both planets warm at heart as they age.

An artist’s rendition of a planet around a red dwarf. Credit: NASA.

An artist’s rendition of a planet around a red dwarf. Credit: NASA.

The authors used this observation and their model to make the following argument: Exoplanets around low mass stars with companions are the most likely to have been kept alive by tidal heating, and are therefore the best candidates for long-term life dependant on, “heat-driven plate tectonics to maintain the carbon cycle and to moderate the greenhouse effect.” Therefore, once potentially habitable planets are found around low-mass stars, a search for a companion should swiftly follow.

As luck would have it, a model for such searches was recently proposed. Coincidentally, this method works best around low-mass stars such as the ones used in Laerhoven et al’s model.

It seem that focusing the search for life in orbit around low-mass stars might be a circular argument. A companion may confer long-term survival to life on the exoplanets we’re finding around low mass stars. In turn, these exoplanets are the best candidates for finding planetary companions and exomoons.

In addition to the relative abundance of low mass stars and the extended possibility for life in the presence of a companion, there may at least one additional advantages to looking for planets around low mass stars. Once again, the answer has to do with age.

“Given the long lifetimes of low-mass stars, a substantial fraction of [the low mass stars] are very old, some more than twice as old as the Earth,” wrote Laerhoven et al, “Thus the most readily discoverable Earth-scale, HZ planets are likely to be ∼10 Gyr old.”

As stars like ours age, they become hotter and more volatile. Eventually, they annihilate nearby planets. By contrast, red dwarfs and other low-mass stars are slow and steady in terms of power output. This grants them long life. It also gives life nearby a long lead time to develop. Perhaps, as some have calculated, a very long lead time.

“My primary interest was other habitable planets; how long would these other worlds be temperate for?” said Andrew Rushby of the University of East Anglia, author on a paper that estimated the output of energy from a star as it ages. “In some cases (planets around small red dwarfs), we predict over 40 billion years.

Even if the brightest star in their sky started to dim, life on planets around low-mass stars might be in luck if they have a companion or two on their horizon.

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

The research was funded through the NASA Earth, the Space Science Fellowship Program and the National Science Foundation.