How many planets can form a conga line around a star?

Our Earth has siblings, the other seven planets in our solar system, but it doesn’t have a twin with which to share its ring of space. The Earth sails alone through its orbit. However, other solar systems can have crazier families chasing each other around a sun: twins, triplets, or even quattuorvigintuplets (That’s 24 Earth-sized planets in a single orbit!)

Computer simulations by an international team of astronomers illustrated how two dozen planets can share the same orbit, in research published this spring in the Monthly Notices of the Royal Astronomical Society. These wacky configurations can be stable for billions of years, even outliving the stars around them. However, it is highly unlikely that nature creates packed planetary orbits, which is why the researchers suggest that the detection of such a system could be a sign of intelligent extraterrestrial life, possibly even an interstellar message that could exist for eons.

“Our paper explores a further branch of possible planetary systems that could exist,” says lead author Sean Raymond, a CNRS researcher at the Laboratoire d’Astrophysique de Bordeaux. “I love that it’s so unexpected and weird, and that so many planets can end up sharing the same orbit.”

Multiple planetary systems, such as our solar system, are often referred to as peas in a pod. But these co-orbiting planets could be “pearls on a necklace,” says University of Kansas astronomer Jonathan Brande, who was not affiliated with the new research.

However, no one had proposed observing two planets in the same orbit until a paper was posted on the arXiv preprint server last week, but most exoplanet astronomers are skeptical, especially since the signal was not seen in the data. from other large exoplanet-hunting telescopes like TESS. This paper was written by a group of amateur astronomers who captured observations with small, commercially available telescopes. “I don’t think it’s the kind of thing you could pull off in your backyard,” Brande says, regarding the alleged detection.

[Related: These 6 exoplanets somehow orbit their star in perfect rhythm]

There are a few known examples of co-orbits involving smaller objects. Our solar system actually has some such strange orbits, known as horseshoe or tadpole orbits, depending on their shapes. Jupiter’s Trojan asteroids: they will soon be visited for the first time by spacecraft Lucy—share the gas giant’s orbital path like tadpoles, oscillating around points before and after Jupiter in its path around the sun. Two of Saturn’s moons, Janus and Epimethus, orbit the horseshoe-shaped ringed planet together, swapping places periodically.

Since objects in our solar system share orbits, it seems reasonable that there could be exoplanets that also share paths. “There are many exoplanet systems in which the planets seem to fill every available niche of stable real estate,” says Raymond. This new research takes this concept to the extreme, seeing how many planets can fit into the same orbit and remain stable.

The research team’s simulations also reveal that such co-orbiting planets would have distinct signals for astronomers here on Earth to observe. The Kepler space telescope and other space observatories can reveal so-called transit time variations (TTVs), where the gravitational pull between nearby planets changes slightly as a planet passes in front of its star. The TTVs of a system of 24 Earth-mass planets sharing an orbit would be large enough for astronomers to see, but it would take months or years of regular monitoring to notice the effect, according to astronomer Rob Zellem of the Propulsion Laboratory. a NASA Jet.

Although academics have not been swayed by the latest observation of putative co-orbiting planets, there is certainly an important role for amateur astronomers in exoplanet science, Zellem adds. “Given the ability of the observers…we could definitely use their expertise,” she says, especially through citizen science projects like NASA’s Exoplanet Watch.

[Related: This alien world could help us find Planet Nine in our own solar system]

However, a robust detection of co-orbiting planets could be truly exciting – not just a glimpse into nature’s extreme diversity, but possibly even a sign of extraterrestrial life. “Something like a designed coorbiting planetarium might not be unambiguously artificial, but it would be weird enough to prompt further study,” says Brande.

The study authors believe that these strange orbits could actually be a perfect technological signature, or a sign of intelligent life beyond Earth. Co-author David Kipping, an astronomer at Columbia University, explains that once an advanced civilization builds an unnatural ring of co-orbiting planets, it won’t require any power to maintain it and will be visible for billions of years—a perfect match for an interstellar message. “The likelihood of this happening really comes down to whether there is someone with the ability and the will to do it,” he says. “We have no idea. But if we don’t look, we’ll never know.”

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