Earth-sized Planets Survive and May be Habitable Thanks to Friction

Friction is one of the forces that shape our world and our universe. Now, NASA scientists have found out that it may play a larger role than expected; it could be key to survival for distant Earth-sized planets traveling in dangerous orbits.

Earth-sized planets are somewhat common in other star systems and while heat can be destructive for some planets, the right about of friction (and heat) can actually be helpful to others. In fact, it could lead to habitability.

While Earth-sized planets are common, they don't always have perfect orbits. In fact, simulations of young planetary systems show that giant planets often upset the orbits of smaller inner worlds. While these interactions may not immediately be catastrophic, they can leave a planet in an eccentric orbit that raises the odds of it crossing paths with another body, being absorbed by its host star, or getting ejected from the system altogether. These planets also undergo high amounts of tidal stress as gravitational forces impact it.

In this latest study, the researchers explored the effects of tidal stresses on planets that have multiple layers. In the end, the researchers found that some planets had the potential to move into a safer orbit about 10 to 100 times faster than previously expected-as little as a few hundred thousand years as opposed to millions of years. This is due to the fact that these planets would be driven close to melting due to the friction caused by tidal forces. Because this almost-melted layer would flex easily, it would generate a lot of friction-induced heat. By throwing off this heat, the planet would lose energy at a fast rate and relax into a circular orbit.

"In this case, the longer, non-circular orbits would increase the 'habitable zone,' because the tidal stress will remain an energy source for longer periods of time," said Terry Hurford in a news release. "This is great for dim stars or ice worlds with subsurface oceans."

The findings reveal how responsive layers in a planet can actually lead to a better outcome for the world. This could mean that there may be more habitable planets in the universe than scientists first thought.

The findings are published in The Astrophysical Journal.