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 more rounded orbits than systems with only one, suggesting a quieter family history. Only children’s systems and planets with more irregular paths point to past collisions of planetary siblings that are so violent that they skew their orbits or even lead to exile. A long-lasting abundance of sibling planets could therefore have protected the earth from destructive chaos and 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?” Jørgensen says. “Is it necessary that there be many planets?”
Most of the more than 4,000 exoplanets discovered to date have elongated or eccentric orbits. This is a remarkable difference from the clean, circular orbits of the planets in our solar system. Rather than being a curiosity, these round orbits are actually completely normal – for a system with so many planets packed together, Jørgensen and his Niels Bohr colleague Nanna Bach-Møller report in an article published online on October 30th Monthly announcements from the Royal Astronomical Society.
Bach-Møller and Jørgensen analyzed the eccentric paths of 1,171 exoplanets orbiting 895 different stars. The duo found a close correlation between the number of planets and the orbit shape. The more planets a system has, the more circular their orbits are, no matter where you are looking or what type of star they are orbiting.
Previous studies also found a correlation between the number of planets and orbital shapes, says astrophysicist Diego Turrini of the Italian National Astrophysics Institute in Rome. These earlier studies only used a few hundred planets.
“This is very important confirmation,” says Turrini. “It gives us an idea of how likely it is that there will be no family struggle, no destructive events, and your planetary system will stay as it was formed … long enough to produce life. “
However, systems with as many planets as ours are extremely rare. Only one known system comes close: the TRAPPIST-1 system with seven roughly earth-sized worlds (SN: 02/22/17). Astronomers have not yet found any other planetary systems than ours with eight or more planets. Jørgensen extrapolates the number of stars expected to have planets in the galaxy and estimates that about 1 percent of planetary systems have as many planets as we do.
“It’s not unique, but the solar system is one of a rare type of planetary system,” he says.
That could explain why life in the galaxy seems rare, suggests Jørgensen. Exoplanet studies show that there are billions of worlds as big 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: 04.10.19).
“If there are so many planets we could, in principle, live on, why aren’t UFOs teeming with?” Jørgensen says. “Why don’t we get stuck in traffic jams with UFOs?”
The answer could lie in the various histories of planetary systems with eccentric and circular orbits. Theories about the formation of the solar system predict that most planets will be born in a disk of gas and dust that surrounds a young star. This means that young planets should have circular orbits and all orbits should be in the same plane as the disk.
“You want the planets not to get too close together, otherwise their interactions could destabilize the system,” says Torrini. “The more planets you have, the more delicate the balance.”
Planets landing on elliptical orbits may have got there through violent encounters with neighboring planets, whether through direct collisions that break both planets apart or through near misses that toss the planets (SN: 02/27/15). Some of these encounters may have ejected planets from their planetary systems, which may explain why planets with eccentric orbits have fewer siblings (SN: 03/20/15).
The survival of the earth could therefore depend on its neighbors having played well for billions of years (SN: May 25, 2005). It doesn’t have to have escaped the violence entirely, says Jørgensen. One popular theory is that Jupiter and Saturn shifted their orbits billions of years ago, a reshuffle that skewed the orbits of distant comets and sent them into the inner solar system. Several pieces of evidence suggest that comets may have brought water to early Earth (SN: 06.05.15).
“It’s not the earth that matters,” says Jørgensen. “It is the entire configuration of the planetary system that is important for life to arise on an Earth-like planet.”