If you are looking for life beyond the solar system, there is strength in numbers.
A new study suggests that systems with multiple planets tend to have rounder orbits than those with only one, indicating a quieter family history. Only child systems and planets with more erratic paths hint at the planetary brothers of the past in clashes violent enough to bring down orbits or even lead to exile. A lasting abundance of sister planets could protect the Earth from destructive chaos and may be part of what made life on Earth possible, says astronomer Uffe Gråe Jørgensen of the Niels Bohr Institute in Copenhagen.
"Is there anything other than the size and position of the Earth around the star that is necessary for life to develop?" Di Jørgensen. "Is it necessary that there be many planets?"
Most of the more than 4,000 exoplanets discovered to date have elongated or eccentric orbits. That makes a noticeable difference to the circular, ordered orbits of the planets in our solar system. Instead of being a rarity, those round orbits are really perfect: for a system with so many planets packaged, Jørgensen and his Niels Bohr partner Nanna Bach-Møller report in an article published online on October 30 in the Monthly Notices of the Royal Astronomical Society.
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Bach-Møller and Jørgensen analyzed the eccentric paths of 1,171 exoplanets orbiting about 895 different stars. The duo found a close correlation between the number of planets and the shape of the orbit. The more planets a system has, the more circular their orbits are, no matter where you look or what kind of star they orbit.
Previously, smaller studies also saw a correlation between the number of planets and the shapes of orbits, says astrophysicist Diego Turrini of the Italian National Institute of Italian Astrophysics in Rome. Those previous studies used only a few hundred planets.
“This is a very important confirmation,” Turrini says. "It gives us an idea of … how likely there is to be no fighting in the family, no destructive events, and your planetary system to remain as it formed … long enough to produce life."
However, there are systems with as many planets as ours. Only one known system is approached: the TRAPPIST-1 system, with approximately seven worlds the size of the Earth (SN: 22/02/17). Astronomers have so far found no solar system, except ours, with eight or more planets. Extrapolating the number of stars expected to have planets in the galaxy, Jørgensen estimates that approximately 1 percent of planetary systems have as many planets as we do.
“It’s not unique, but the solar system belongs to a rare type of planetary system,” he says.
This could help explain why life seems rare in the galaxy, Jørgensen suggests. Exoplanet studies indicate that there are billions of worlds the same size as Earth, whose orbits would make them good places for liquid water. But just being in the so-called “habitable zone” is not enough to make a planet habitable (SN: 10/4/19).
"If there are so many planets where we could live in principle, why aren't we full of UFOs all the time?" Di Jørgensen. "Why don't we get into UFO traffic jams?"
The answer could be in the different histories of planetary systems with eccentric and circular orbits. Theories of the formation of the solar system predict that most planets are born in a disk of gas and dust that surrounds a new star. This means that new planets must have circular orbits and all orbits in the same plane as the disk.
“He wants the planets not to get too close to each other, otherwise their interactions can destabilize the system,” Torrini says. "The more planets you have, the more delicate the balance."
Planets ending in elliptical orbits can arrive through violent encounters with neighboring planets, whether they are direct collisions separating the two planets or almost errors launched by the planets (SN: 27/02/15). It is possible that some of these encounters had completely expelled planets from their solar systems, possibly explaining why planets with eccentric orbits have fewer siblings (SN: 20/03/15).
The survival of the Earth may therefore depend on its neighbors playing well for billions of years (SN: 25/05/05). Nor does it need to escape violence completely, says Jørgensen. A popular theory holds that Jupiter and Saturn moved in their orbits billions of years ago, a remodeling that knocked down the orbits of distant comets and sent them into the inner solar system. Several lines of evidence suggest that comets could bring water to Earth early (SN: 5/6/15).
“It’s not the Earth that’s important,” Jørgensen says. "The whole configuration of the planetary system is important for life to originate on an earth-like planet."