Friction may save Earth-like planets: NASA

Planets in eccentric orbits can experience powerful tidal forces, and friction from the tides could be the key to the survival for some distant Earth-sized planets travelling in dangerous orbits, shows computer modelling by NASA scientists.

The findings are consistent with observations that Earth-sized planets appear to be very common in other star systems.

Although heat can be a destructive force for some planets, the right amount of friction, and therefore heat, can be helpful and perhaps create conditions for habitability.

"We found some unexpected good news for planets in vulnerable orbits," said Wade Henning, a University of Maryland scientist working at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"It turns out these planets will often experience just enough friction to move them out of harm's way and into safer, more circular orbits more quickly than previously predicted," Henning added.

Simulations of young planetary systems indicate that giant planets often upset the orbits of smaller inner worlds.

Even if those interactions are not immediately catastrophic, they can leave a planet in a treacherous eccentric orbit - a very elliptical course that raises the odds of crossing paths with another body, being absorbed by the host star, or getting ejected from the system.

Another potential peril of a highly eccentric orbit is the amount of tidal stress a planet may undergo as it draws very close to its star and then retreats away.

Near the star, the gravitational force is powerful enough to deform a planet, while in more distant reaches of the orbit, the planet can ease back into shape.

This flex action produces friction, which generates heat.

The findings appeared in the online edition of Astrophysical Journal.