Close neighbors on Earth might exchange sugar and eggs, but when planets are close neighbors, they might instead share the ingredients necessary for life. Such friendliness could not only result in life evolving on both worlds, it could also mean survival when one of those planets undergoes hard times. If a traumatic event wipes out life on one, the close quarters may allow it to rapidly be reseeded by its neighbor.
For close planets, “you have another reservoir of life-bearing materials where it’s easy to exchange some of that information and reseed life in the system,” said Jason Steffen of the University of Nevada at Las Vegas. Steffen and colleague Gongjie Li, of the Harvard-Smithsonian Center for Astrophysics, studied the implications close systems could have on the development for life. In systems where neighboring planets both maintain conditions suitable for life have a higher potential not only to share biological seeds but also support travel and communication between worlds.
“It wouldn’t take very much to imagine communication between the two systems, or even for intelligent life on one to colonize the other,” Steffen said.
While Earth and Mars may have participated in a similar exchange, the scientists looked at pairings where distances are around five times smaller, not much larger than those between Earth and its moon. Less than five out of the 1,041 confirmed planetary systems identified by Kepler have such pairings, but that could be enough. Although only about a third of the Kepler worlds exist in multiplanet systems, Steffen expects there are many hidden companion worlds unseen by Kepler, which can only spot planets that pass or transit in the plane between their world and Earth.
“My suspicion is that the majority, and probably the overwhelming majority, of the single planet systems that have been seen by Kepler are actually multiplanet systems where the other planets don’t transit,” he said.
Of those, many could host pairs with both worlds capable of conditions where life could evolve. When asteroids or comets collide with one world, material thrown into space could fall onto its neighbor in as little as a single orbit, though most would average about a thousand years. Not only would this require gentler impacts to send the ingredients for life packing, it would also mean that material would need to survive for far shorter timescales than required in the solar system, where material could take millions of years to travel between the Earth and Mars.
“If it can escape [the planet], it can make it there,” Stefan said.
Of course, not all worlds that lie in the habitable zone, the region around their star where liquid water could theoretically remain on the surface, will necessarily be habitable. Mars is a perfect example, too small to hold onto its water, and Venus sits just at the edge of the sun’s region. Still, if only one out of 100 survive, millions of habitable neighbors could exist.
Steffen said the inspiration for investigating these options came from a press release image for the Kepler 36 system, where two massive worlds lie only about five times the Earth-moon distance apart. The image showed a realistic size of a planet over the city of Seattle, Washington, which prompted him to wonder how seeing such a world up close might inspire its residents.
It wouldn’t take much to imagine residents traveling from one world to another. A close neighbor could easily serve as an outpost to the rest of the system, as well. Had the moon been habitable, humans might have been more likely to seize it as a jumping-off point.
“We sent people to [the moon],” Steffan said. “The main reason that they didn’t stay was because there was nothing to eat or breathe.”
The research was published in the Astrophysical Journal.