Alien life may evolve on planets with satellites far more often than on worlds lacking large companions in their solar system.
Planets coalesce from disks of hot gas and dust, like swirls of milk in a teacup. As the worlds age, they cool, although surfaces can be repeatedly melted by collisions with minor bodies like asteroids and comets. As planets cool, declining thermal energy from the core cools the surface, and the atmosphere. This can make it more difficult for geological processes to control a runaway greenhouse effect created by excess carbon dioxide in the atmosphere.
Tidal forces from a companion world could help warm the surfaces of these worlds, making alien life more likely, according to new research. Tidal heating is created due to the relationship between gravity and distance. For instance, as the Moon pulls on the Earth, places on the surface of our planet nearest the Moon are pulled toward the lunar surface to the greatest degree. The region on the exact opposite side of the Earth experiences the least amount of pull. This causes the Earth to continually stretch and bend as our planet rotates every day, and the Moon circles our planet every month. That same flexing, and heating, also takes place in the satellite.
"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. This constant flexing causes layers inside the planet to rub against each other, producing frictional heating," Rory Barnes of the University of Washington, said.
Io, one of the four giant moons of Jupiter, orbits closer to the giant than any other major body in the system. This world, along with its companion, Europa, develop significant heat from these movements.
Astronomers at the University of Washington and the University of Arizona believe this same heating make take place in worlds and moons surrounding other stars. Computer simulations run during the study suggest this heating could be significant on older planets, roughly the size of the Earth, in highly-elliptical (non-circular) orbits around low-mass stars.
A larger planet, located outside the orbit of a potentially habitable world could prevent the planet from settling into a circular orbit cooling the world, according to this study. If life evolved long ago on one of these bodies, this tidal heating may have extended habitability of the planet, meaning the oldest forms of life may be found on these alien worlds.
Investigation of the role tidal forces can play on alien worlds and how that process could affect extraterrestrial life was profiled in the journal Monthly Notices of the Royal Astronomical Society.
