Astronomers may have found two exoplanets sharing the same orbit

"Two decades ago, it was predicted in theory that pairs of planets of similar mass may share the same orbit around their star, the so-called Trojan or co-orbital planets. For the first time, we have found evidence in favor of that idea," explained Olga Balsalobre-Ruza, a student at the Centre for Astrobiology in Madrid, Spain, who led the paper.

Trojan asteroids are fairly common in our Solar System. The most famous examples are the thousands of Trojan asteroids of Jupiter, though we also know of two Trojan Earth asteroids that share the same orbit around the Sun as our planet.

Astronomers have predicted that Trojan planets could also exist outside our solar system, but they have found little concrete evidence.

"Exotrojans [Trojan planets outside the Solar System] have so far been like unicorns: they are allowed to exist by theory, but no one has ever detected them," explained co-author Jorge Lillo-Box, a senior researcher at the Centre for Astrobiology.

The strongest evidence so far for Trojan worlds

The team behind the new paper has presented their discovery using ALMA, which they argue provides the strongest observational evidence to date that Trojan planets exist.

They analyzed archival ALMA data of a star in the PDS 70 system, which hosts two giant Jupiter-like planets, PDS 70b and PDS 70c, and spotted a cloud of debris at the location in PDS 70b's orbit where Trojans would be expected to exist — specifically in a Lagrangian zone where the combined gravitational pull of the star and the planet create a stable orbit for other objects.

The team found that the debris cloud has a mass of roughly two times that of our Moon. They believe it could point to an existing Trojan world in this system or at least one in the process of forming.

"Who could imagine two worlds that share the duration of the year and the habitability conditions? Our work is the first evidence that this kind of world could exist," Balsalobre-Ruza explained. "We can imagine that a planet can share its orbit with thousands of asteroids as in the case of Jupiter, but it is mind-blowing to me that planets could share the same orbit."

Next, the team will conduct follow-up observations of PDS 70b and the sibling cloud by around 2026 to determine whether they have moved along the same orbit. If they confirm their findings, it "would be a breakthrough in the exoplanetary field,” Balsalobre-Ruza said.