Free Astronomy Magazine September-October 2023

39 SEPTEMBER-OCTOBER 2023 ASTRO PUBLISHING is scant. “Exotrojans — Trojan plan- ets outside the Solar System — have so far been like unicorns: they are allowed to exist by theory but no one has ever detected them,” says co-author Jorge Lillo-Box, a senior researcher at the Centre for Astro- biology. Now, an international team of sci- entists have used ALMA, in which ESO is a partner, to find the strongest observational evidence yet that Trojan planets could exist — in the PDS 70 system. This young star is known to host two giant, Jupiter-like planets, PDS 70b and PDS 70c. By analysing archival ALMA observations of this system, the team spotted a cloud of debris at the location in PDS 70b’s orbit where Trojans are expected to exist. Trojans occupy the so-called La- grangian zones, two extended re- gions in a planet’s orbit where the combined gravitational pull of the star and the planet can trap mate- ! U sing the Atacama Large Millime- ter/submillimeter Array (ALMA), astronomers have found the possi- ble ‘sibling’ of a planet orbiting a distant star. This video summarises the discovery. [ESO] co-orbital planets very early in their formation,” says co-author Nuria Huélamo, a senior researcher at the Centre for Astrobiology. “It opens up new questions on the formation of Trojans, how they evolve and how frequent they are in different planetary systems,” adds Itziar De Gregorio-Monsalvo, ESO Head of the Office for Science in Chile, who also contributed to this research. To fully confirm their detection, the team will need to wait until after 2026, when they will aim to use ALMA to see if both PDS 70b and its sibling cloud of debris move signifi- cantly along their orbit together around the star. “This would be a breakthrough in the exoplanetary field,” 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 dra- matically improve the array’s abil- ity to characterise Trojans in many other stars,” concludes De Gregorio- Monsalvo. rial. Studying these two regions of PDS 70b’s orbit, astronomers de- tected a faint signal from one of them, indicating that a cloud of de- bris with a mass up to roughly two times that of our Moon might reside there. The team believes this cloud of de- bris could point to an existing Tro- jan world in this system, or a planet in the process of forming. “Who could imag- ine two worlds that share the duration of the year and the habitability condi- tions? Our work is the first evidence that this kind of world could exist,” says Balsalobre-Ru- za. “We can imag- ine 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.” “Our research is a first step to look for T his colourful image shows the sky around the faint orange dwarf star PDS 70 (in the middle of the image). The bright blue star to the right is χ Centauri. [ESO/Digitized Sky Survey 2. Ack.: Davide De Martin]