Free Astronomy Magazine September-October 2023

SEPTEMBER-OCTOBER 2023 ready formed. If confirmed, this dis- covery would be the strongest evi- dence yet that two exoplanets can share one 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,” says Olga Balsalobre-Ruza, a student at the Centre for Astrobiology in Madrid, Spain who led the paper published in Astronomy & Astrophysics . Trojans, rocky bodies in the same orbit as a planet, are common in our own Solar System, the most famous example being the Trojan asteroids of Jupiter — more than 12,000 rocky bodies that are in the same orbit around the Sun as the gas giant. As- tronomers have predicted that Tro- jans, in particular Trojan planets, could also exist around a star other than our Sun, but evidence for them by ESO − Bárbara Ferreira U sing the Atacama Large Mil- limeter/submillimeter Array (ALMA), astronomers have found the possible ‘sibling’ of a planet orbiting a distant star. The team has detected a cloud of debris that might be sharing this planet’s orbit and which, they believe, could be the building blocks of a new planet or the remnants of one al- Does this exoplanet have a sibling sharing the same orbit? T his 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, as- tronomers have detected a cloud of debris (circled by a yellow dotted line) that could be the building blocks of a new planet or the rem- nants of one already formed. The ring-like struc- ture 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. [ALMA (ESO/NAOJ/NRAO) /Balsa- lobre-Ruza et al.]