PDS 70, a young K7-type star some 370 light-years away from Earth in the constellation of Centaurus, is a unique target for planetary formation studies since it harbors two giant, Jupiter-like protoplanets: PDS 70b and PDS 70c. PDS 70 is surrounded by a highly structured protoplanetary disk with a wide inner cavity presumably carved by its planets. Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have detected a cloud of debris that might be sharing PDS 70b’s orbit and which, they believe, could be the building blocks of a new planet or the remnants of one already formed. If confirmed, this discovery would be the strongest evidence yet that two exoplanets can share one orbit.
Trojan asteroids are common inhabitants of the Solar System.
They are minor bodies populating the L4 and L5 Lagrange regions of a planet, leading and trailing it 60 degrees apart in the same orbital path.
The most famous example is the trojan asteroids of Jupiter — more than 12,000 rocky bodies that are in the same orbit around the Sun as the gas giant.
In 2022, astronomers theoretically demonstrated that trojans as massive as the main planet could be long-term stable, thus inspiring the concept of co-orbital planets.
The fact that the stability condition for co-orbitals is met for pairs of bodies with similar masses opens the possibility of searching for extrasolar trojans using the same methods and instruments as for the currently confirmed exoplanets.
“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,” said Olga Balsalobre-Ruza, a student at the Centre for Astrobiology in Spain.
“For the first time, we have found evidence in favor of that idea.”
Also known as V* V1032 Cen and IRAS 14050-4109, PDS 70 is a located 370 light-years away in the constellation of Centaurus.
This 5.4-million-year-old star hosts two protoplanets and a huge circumstellar disk of dust and gas in which a large region from 20 to 40 AU is cleared of dust.
The inner planet, PDS 70b, is located within the disk gap at a distance of about 22 AU from the star, similar to the orbit of Uranus in our Solar System.
The outer planet, PDS 70c, is located near the outer edge of the disk gap at 34 AU from the star, similar to Neptune’s distance from our Sun.
In their study, Balsalobre-Ruza and her colleagues searched for excess emission compatible with accumulation of dust in the Lagrangian regions of both PDS 70 protoplanets.
They detected a faint signal from L5 Lagrangian region of PDS 70b, indicating that a cloud of debris with a mass up to roughly two times that of our Moon might reside there.
They believe this cloud of debris could point to an existing trojan world in this system, or a planet 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 said.
“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.”
“Our research is a first step to look for co-orbital planets very early in their formation,” said Dr. 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,” said Itziar De Gregorio-Monsalvo, ESO Head of the Office for Science in Chile.
To fully confirm their detection, the astronomers 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 significantly along their orbit together around the star.
“This would be a breakthrough in the exoplanetary field,” Balsalobre-Ruza said.
“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 characterize trojans in many other stars,” Dr. De Gregorio-Monsalvo said.
The team’s paper was published this week in the journal Astronomy & Astrophysics.
O. Balsalobre-Ruza et al. 2023. Tentative co-orbital submillimeter emission within the Lagrangian region L5 of the protoplanet PDS 70b. A&A 675, A172; doi: 10.1051/0004-6361/202346493