Co-Orbital Planets Discovery: Strongest Evidence Yet of Exoplanets Sharing Orbits

Leveraging the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have potentially identified the 'sibling' of a planet circling a far-off star. They've noticed a cloud of debris possibly sharing the same orbit as the planet. This could either be the foundation for a new planet or the remnants of an already formed one. If validated, this discovery would provide the most compelling evidence to date that two exoplanets can inhabit one orbit.

"Twenty years ago, theory predicted that pairs of similarly-sized planets could coexist in the same orbit around their star. These were termed Trojan or co-orbital planets. For the first time, we have found supporting evidence for this concept," states Olga Balsalobre-Ruza, student at the Centre for Astrobiology in Madrid, Spain, and lead author of the paper published in Astronomy & Astrophysics.

Trojans, rocky celestial bodies cohabiting an orbit with a planet, are familiar in our Solar System, with the most notable example being Jupiter’s Trojan asteroids. Astronomers have theorized that such Trojan planets could exist around stars other than our Sun, but proof has been limited. "Exotrojans (Trojan planets outside the Solar System) have so far been theoretical: they can exist, but no one has detected them," comments co-author Jorge Lillo-Box, a senior researcher at the Centre for Astrobiology.

Currently, an international team of scientists utilized ALMA to find the most robust observational evidence yet of possible Trojan planets in the PDS 70 system. By examining archived ALMA observations of this system, the team identified a debris cloud in PDS 70b’s orbit, where Trojans are predicted to reside.

The team proposes this debris cloud could hint at a Trojan world in this system, or a planet currently forming. “The idea of two worlds sharing the same orbit duration and habitability conditions is mind-boggling,” says Balsalobre-Ruza. "Our work provides the first evidence that such a world could exist."

“This research paves the way to search for co-orbital planets in early formation stages,” adds co-author Nuria Huélamo, a senior researcher at the Centre for Astrobiology. The team is hopeful to fully confirm their findings after 2026, when they plan to use ALMA to observe if both PDS 70b and its sibling debris cloud move considerably along their shared orbit. "Such a discovery would be a game-changer in the field of exoplanetary study," says Balsalobre-Ruza.

"The future of this topic is very exciting and we look forward to the extended ALMA capabilities, planned for 2030, which will dramatically improve the array’s ability to characterise Trojans in many other stars," concludes De Gregorio-Monsalvo.

Research paper