They are known as the ‘unicorns’ of the world of astronomy.
But scientists have now discovered the strongest evidence yet that two exoplanets can actually share the same orbit.
These so-called Trojan or co-orbital planets have been hypothesized for 20 years, but have never been detected.
Named for the rocky bodies in the same orbit as a planet that are common in our own Solar System, the most famous of which are Jupiterian Trojan asteroids, confirmation of their existence would be an exciting moment for astronomers.
An international team of researchers has detected a cloud of debris that they say could be the “brother” of a planet orbiting a distant star 400 light-years away.
Discovery: Scientists have uncovered the strongest evidence yet that two exoplanets can share the same orbit. This image shows the planetary system PDS 70, which is 400 light-years from Earth. It has a star at its center, around which the planet PDS 70 b orbits (both shown as a solid yellow circle). In the same orbit as PDS 70b, there is a cloud of debris (circled by a dotted line) that could be the building blocks of a new planet or the remains of an already formed one.
They may be the building blocks of a new planet or the remains of an already formed one, experts say.
If confirmed, the discovery would be the most convincing evidence yet that two exoplanets can share an orbit.
“Two decades ago it was predicted in theory that pairs of planets of similar mass could share the same orbit around their star, the so-called Trojan or co-orbital planets,” explains Olga Balsalobre-Ruza, principal investigator at the Center for Astrobiology in Madrid.
“For the first time, we have found evidence in favor of 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, principal investigator at the Center for Astrobiology, said: “The exotrojans [Trojan planets outside the Solar System] Until now they have been like unicorns: they are allowed to exist in theory, but no one has ever detected them.
Until now, maybe.
The researchers say that a Trojan exoplanet and its sibling may exist in the PDS 70 system, where a young star is known to host two giant Jupiter-like planets called PDS 70b and PDS 70c.
They detected the debris cloud in an area of PDS 70b’s orbit where Trojans are expected to exist, known as the Lagrange zone.
There are two of these regions in a planet’s orbit where the combined gravitational pull of the star and planet can trap material, making it the ideal place to hunt for ‘sister’ exoplanets.
The team of scientists detected a weak signal in one of the Lagrangian zones of the planet that suggested that a cloud of debris could be hidden there.
They think it’s either a Trojan world itself or a planet currently forming.
‘Who could imagine two worlds that share the length of the year and the conditions of habitability? Our work is the first evidence that this type of world could exist,” said Balsalobre-Ruza.
“We can imagine that a planet can share its orbit with thousands of asteroids as in the case of Jupiter, but it surprises me that planets can 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 research, said: “It opens up new questions about the formation of Trojans, how they evolve and how frequent 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 debris cloud move significantly along in their orbit 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 capabilities of ALMA, planned for 2030, which will dramatically improve the ability of the array to characterize Trojans on many other stars.”
It had been suggested that the reason coorbital exoplanets had not been detected was because they may not be detectable with current techniques.
Analysis: The researchers detected the debris cloud in an area of PDS 70b’s orbit where Trojans are expected to exist, known as the Lagrange zone.
There are two of these regions in a planet’s orbit where the combined gravitational pull of the star and planet can trap material, making it the ideal place to hunt for ‘sister’ exoplanets. This graph shows the predicted positions of the planets and their Lagrangian points at different epochs.
There is also a school of thought that Trojans are removed from your systems quite quickly compared to the relative age of the universe, which means they would be harder to detect.
Essentially, they would be thrown out of their way by the gravitational forces of a nearby star, then collide with a star or another planet before they could be detected.
Jupiter’s Trojan asteroids are more of 12,000 rocky bodies that revolve around the sun in the same orbit as the gas giant.
In 2021, the so-called Lucy probe was launched into space before its trip to Jupiter.
Once there, it will be study two groups of asteroids swarming in front of and behind the gas giant as part of a 12-year mission.
In total, Lucy will study seven Trojans, which according to NASA are ‘the fossils’ of the Solar System and contain important clues to its early evolution.
The research has been published in the journal Astronomy and Astrophysics.
WHAT IS SOUL?
Deep in the Chilean desert, the Atacama Large Millimeter Array, or ALMA, lies in one of the driest places on Earth.
At an altitude of 16,400 feet, about half the cruising height of a jumbo jet and almost four times the height of Ben Nevis, workers had to haul oxygen tanks to complete their construction.
Powered up in March 2013, it is the most powerful ground-based telescope in the world.
It’s also the tallest on the planet and, at almost £1 billion ($1.2 billion), one of the most expensive of its kind.
Deep in the Chilean desert, the Atacama Large Millimeter Array, or ALMA, lies in one of the driest places on Earth. Powered up in March 2013, it is the most powerful ground-based telescope in the world.
