They are known as the ‘unicorns’ of the astronomy world.
But scientists have now uncovered the strongest evidence yet that two exoplanets could really share the same orbit.
These so-called Trojan or co-orbital planets have been suspected for 20 years but never discovered.
Named after the rocky bodies co-orbiting with a planet that are common in our own solar system — the most famous of which are Jupiter’s Trojan asteroids — confirmation of their existence would be an exciting moment for astronomers.
An international team of researchers has discovered a cloud of debris they say could be the “little brother” of a planet orbiting a distant star 400 light-years away.
Discovery: Scientists have found the strongest evidence yet that two exoplanets may share the same orbit. This image shows the planetary system PDS 70, which is located 400 light-years from Earth. It has a star at its center, around which the planet PDS 70 b orbits (both shown with a solid yellow circle). In the same orbit as PDS 70b is a debris cloud (circled by a dotted line) that could be the building blocks of a new planet or the remnants of an already formed planet
They could be the building blocks of a new planet or the remnants of an already formed planet, the experts say.
If confirmed, the discovery would be the most compelling evidence yet that two exoplanets may share one orbit.
“Twenty years ago, it was theoretically predicted that pairs of planets of similar mass could share the same orbit around their star, the so-called Trojan or co-orbital planets,” says lead researcher Olga Balsalobre-Ruza of the Center for Astrobiology. in Madrid.
“For the first time, we have found evidence for that idea.”
The team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope in the Atacama Desert in northern Chile to make their observations.
Co-author Jorge Lillo-Box, a senior research scientist at the Center for Astrobiology, said: “Exotrojans [Trojan planets outside the Solar System] have been like unicorns until now: they may exist in theory, but no one has ever discovered them.’
Until now maybe.
The researchers say a Trojan exoplanet and its sibling may exist in the PDS 70 system, a young star known to harbor two giant Jupiter-like planets, PDS 70b and PDS 70c.
They discovered the debris cloud in an area in PDS 70b’s orbit where Trojans are expected to exist, known as the Lagrangian zone.
There are two of these regions in a planet’s orbit where the combined gravity of the star and planet can trap material, hence making it the ideal location for hunting “brother” exoplanets.
The team of scientists detected a faint signal in one of the planet’s Lagrangian zones, suggesting a cloud of debris might be hiding there.
They think it is either a Trojan world itself or a planet that is currently forming.
“Who can imagine two worlds sharing the length of the year and the conditions of habitability? Our work is the first proof that this kind of world could exist,” said Balsalobre-Ruza.
“We can imagine that a planet could share its orbit with thousands of asteroids, as in the case of Jupiter, but I’m surprised that planets could share the same orbit.”
Telescope: The team of astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope (pictured) in the Atacama Desert in northern Chile for their research
Itziar De Gregorio-Monsalvo, from the European Southern Observatory, who also contributed to the study, said: “It raises new questions about the formation of Trojans, how they evolve and how common they are in different planetary systems.”
To confirm their detection, the researchers will have to wait until after 2026, when they will use ALMA to see if both PDS 70b and its sister cloud of debris move significantly along their orbit together around the star.
“This would be a breakthrough in the exoplanetary field,” Balsalobre-Ruza said.
De Gregorio-Monsalvo added: “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. ‘
It had been suggested that the reason co-orbital exoplanets had gone undetected was because they may not be detectable with current techniques.
Analysis: Researchers discovered the debris cloud in an area of PDS 70b’s orbit where Trojans are expected to exist, known as the Lagrangian zone
There are two of these regions in a planet’s orbit where the combined gravity of the star and planet can trap material, hence making it the ideal location for hunting “brother” exoplanets. This chart shows the forecast for the positions of the planets and their Lagrangian points in different epochs
There is also a belief that Trojans are removed from their systems quite quickly compared to the relative age of the universe, which means they are harder to spot.
Essentially, they would be thrown out of their path thanks to the gravity of a nearby star and then collide with a star or another planet before they could be detected.
Jupiter’s Trojan asteroids are more than 12,000 rocky bodies orbiting the sun in the same orbit as the gas giant.
In 2021, the so-called Lucy probe blasted into space ahead of its journey to Jupiter.
Once there it will study two groups of asteroids running in swarms ahead and behind the gas giant as part of a 12-year mission.
In total, Lucy will study seven Trojan horses, which NASA says are “the fossils” of the solar system and contain important clues about its early evolution.
The research has been published in the journal Astronomy & Astrophysics.