Astronomers may have found two exoplanets sharing the same orbit

We may have the first concrete evidence of 'mind-blowing' Trojan exoplanets.
Chris Young
An artist's impression of an exoplanet.
An artist's impression of an exoplanet.

Nazarii Neshcherenskyi / iStock 

An international team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to find what they believe is a 'sibling' alien world to an exoplanet orbiting a distant star.

The team detected a cloud of debris that may be sharing the planet's orbit and could be the building blocks of a new planet or the remnants of one in the process of forming a press statement reveals.

It would be the most compelling evidence that two exoplanets can share the same orbit if confirmed.

Detecting a Trojan exoplanet

The astronomers behind the discovery, who published their findings in a paper in the journal Astronomy & Astrophysics, explain how they may have the first evidence for so-called Trojan exoplanets, or co-orbital planets.

"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.

Astronomers may have found two exoplanets sharing the same orbit
In this ALMA image, PD70b is highlighted by a yellow circle and the cloud of debris by a dotted yellow circle.

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.

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