An exoplanet with Trojan? Exotrojans Breakthrough: Astronomers Find Possible Twin Planets Sharing an Orbit

Twenty years ago, the theory was proposed that pairs of similarly sized planets could occupy the same orbit around their star, a concept known as Trojan or co-orbital planets. Now, for the first time, we have discovered evidence supporting this notion.

Through the power of the Atacama Large Millimeter/submillimeter Array (ALMA), scientists have discovered what could be a twin of a planet circling a remote star. The team pinpointed a debris cloud potentially in the same orbit as this planet, which could either be the precursors of a fresh planet or leftovers of a previously formed one. If verified, this finding would be the most solid proof till now that two exoplanets can share a single 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 favour of that idea,” explains Olga Balsalobre-Ruza, a student at the Centre for Astrobiology in Madrid, Spain who spearheaded the research paper recently published in Astronomy & Astrophysics.

Trojans, celestial bodies in the same orbit as a planet, are a common occurrence in our Solar System. The most widely recognized example is Jupiter’s Trojan asteroids – more than 12,000 rocky objects sharing the Sun’s orbit with the gas giant. Despite predictions of Trojans, particularly Trojan planets, potentially existing around stars other than our Sun, proof remains limited.

“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,” mentions Jorge Lillo-Box, a senior researcher at the Centre for Astrobiology and a co-author.

An international team of scientists has now deployed ALMA, partnered by the European Southern Observatory (ESO), to secure the strongest observational evidence yet for the existence of Trojan planets, specifically in the PDS 70 system. This young star hosts two giant planets, PDS 70b and PDS 70c, akin to Jupiter. Examining stored ALMA observations of this system, the team identified a debris cloud in a spot of PDS 70b’s orbit where Trojans are hypothesized to reside.

Trojans occupy the Lagrangian zones, two widespread areas in a planet’s orbit where the combined gravitational attraction of the star and planet can ensnare material. Investigating these two zones in PDS 70b’s orbit, the astronomers detected a faint signal from one, indicating the potential presence of a debris cloud with a mass approximately double that of our Moon.

This debris cloud could signal a Trojan planet in this system, either already formed or still 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,” comments Balsalobre-Ruza. “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.”

Nuria Huélamo, a senior researcher at the Centre for Astrobiology and another co-author, states, “Our research is a first step to look for co-orbital planets very early in their formation.”

Itziar De Gregorio-Monsalvo, ESO Head of the Office for Science in Chile and another contributor to the research, adds, “It opens up new questions on the formation of Trojans, how they evolve and how frequent they are in different planetary systems.”

The team will require patience till after 2026 for full confirmation, as they plan to utilize ALMA to observe if both PDS 70b and its associated debris cloud noticeably move in their shared orbit around the star.

“This would be a breakthrough in the exoplanetary field,” asserts 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,” De Gregorio-Monsalvo concludes.

Source: 10.1051/0004-6361/202346493

Image Credit: ESO