This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, shows the young planetary system PDS 70, located nearly 400 light-years away from Earth. The system features a star at its centre, around which the planet PDS 70 b (highlighted with a solid yellow circle) is orbiting. On the same orbit as PDS 70b, indicated by a solid yellow ellipse, astronomers have detected a cloud of debris (circled by a yellow dotted line) that could be the building blocks of a new planet or the remnants of one already formed. The ring-like structure that dominates the image is a circumstellar disc of material, out of which planets are forming. There is in fact another planet in this system: PDS 70c, seen at 3 o’clock right next to the inner rim of the disc.
Newswise — By employing the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers made an intriguing discovery: a potential 'sibling' of a planet in orbit around a faraway star. The team identified a debris cloud in proximity to this planet's orbit, raising the possibility that it could either serve as the foundational material for a budding planet or be the vestiges of a planet that has already taken shape. Should this finding be verified, it would constitute the most compelling evidence to date of the coexistence of two exoplanets within a single orbit.
"Approximately twenty years ago, theoretical predictions suggested the existence of pairs of planets with comparable masses, capable of sharing the same orbit around their parent star. These planetary arrangements, known as Trojan or co-orbital planets, have now received their first supporting evidence," reveals Olga Balsalobre-Ruza, a student at the Centre for Astrobiology in Madrid, Spain. Balsalobre-Ruza led the paper published in Astronomy & Astrophysics, marking a significant milestone in our understanding of planetary dynamics.
In our own Solar System, Trojans, which are rocky bodies sharing the same orbit as a planet, are a well-known occurrence [1]. A prime example is the group of Trojan asteroids accompanying Jupiter, consisting of over 12,000 rocky bodies occupying the same orbital path around the Sun as the gas giant. Astronomers have postulated that similar Trojan configurations, including Trojan planets, might also be present around stars beyond our Sun. However, evidence supporting their existence has been scarce until now. Co-author Jorge Lillo-Box, a senior researcher at the Centre for Astrobiology, likens these elusive exotrojans (Trojan planets outside the Solar System) to unicorns—conceptually plausible in theory, yet never directly detected.
In a groundbreaking discovery, an international team of scientists, in collaboration with ESO as a partner, has harnessed the power of ALMA to unveil the most compelling observational evidence supporting the existence of Trojan planets. Their attention was focused on the PDS 70 system, centered around a youthful star known to harbor two massive, Jupiter-like planets named PDS 70b and PDS 70c. By meticulously examining archival ALMA data of this system, the researchers identified a significant cloud of debris precisely situated in PDS 70b's orbit, precisely where Trojans are anticipated to reside. This remarkable finding offers a profound insight into the potential prevalence of Trojan planets within distant star systems.
Trojans are situated within the Lagrangian zones, two expansive areas along a planet's orbit where the combined gravitational forces of the star and the planet can capture celestial material. In-depth examination of these specific regions within PDS 70b's orbit led astronomers to discover a subtle signal emanating from one of them, hinting at the potential presence of a debris cloud with a mass roughly equivalent to twice that of our Moon.
The team posits that this debris cloud holds the potential of revealing either an established Trojan world within the system or a planet undergoing its formation. "The concept of two worlds sharing the same orbital duration and inhabiting conditions is truly remarkable. Our research provides the inaugural evidence for the plausibility of such a world," states Balsalobre-Ruza. While considering that a planet might share its orbit with numerous asteroids, akin to Jupiter's scenario, the notion of planets coexisting within the same orbital path is mind-boggling to her.
"Our investigation marks an initial stride towards identifying co-orbital planets during their early formation," says co-author Nuria Huélamo, a senior researcher at the Centre for Astrobiology. Itziar De Gregorio-Monsalvo, ESO Head of the Office for Science in Chile, who also participated in this study, remarks, "This research paves the way for new inquiries concerning Trojan formation, their evolutionary paths, and their prevalence in diverse planetary systems."
To attain definitive confirmation of their discovery, the team must exercise patience until 2026. At that time, they plan to utilize ALMA again to observe whether both PDS 70b and its associated debris cloud exhibit significant coordinated movement along their orbital path around the star. "Achieving this would represent a groundbreaking advancement in the study of exoplanets," emphasizes Balsalobre-Ruza.
De Gregorio-Monsalvo concludes with enthusiasm, highlighting the thrilling prospects ahead for this field. The team eagerly anticipates the enhanced ALMA capabilities slated for 2030, which promise to significantly bolster the array's capacity to study and characterize Trojans in numerous other stellar systems.
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