“For the first time, we have proof.”

A planet with a “sister” that moves in the same orbit around the star. A team of astronomers believe they have found evidence that it exists. Researchers have detected a cloud of debris that appears to be in the same orbit as exoplanet PDS 70b. And not just any cloud of debris. The debris may contain the building blocks of a new planet.

“Twenty years ago there were already theories predicting that two planets of similar mass could have the same orbit around their star – so-called Trojan or co-orbital planets,” explains Olga Balsalobre-Ruza, a student at Madrid’s Centro de Astrobiologia and head of the the researchwhich was published in June 2023 in Astronomy & Astrophysics. “Now, for the first time, we have found evidence to support this idea.”

To do this, the team used the Atacama Large Millimeter Array (ALMA), one of the most advanced radio telescopes in the world.

In search of the Trojan planets

Trojans, rocky objects that share their orbit with a planet, can be found in many places in our solar system. A well-known example is the Jupiter Trojans, thousands of asteroids that are in the same orbit around the sun as the gas giant.

But is it also possible for two planets to share their orbits? Astronomers have long believed such a thing was possible, but practical evidence is scarce.

“The Trojan planets are like unicorns: in theory, they should exist, but no one has ever detected them”, explains Jorge Lillo-Box, researcher at the Centro de Astrobiologia and co-author of the study.

His team’s discovery is the strongest evidence to date. Using ALMA, the researchers focused on the system around the young star PDS 70, about 370 light-years from Earth. This system was already known to contain two large Jupiter-like planets: PDS 70b and PDS 70c. But now, in the orbit of PDS 70b, what appears to be a cloud of debris twice the size of our moon has been observed.

Breakthrough in research

According to the researchers, this cloud of debris points to a Trojan horse system. In fact, they think the debris may be a planet forming — or the remains of something that once was a planet. “That two planets can share the same orbit is incredible to me,” says Balsalobre-Ruza. “Imagine two worlds where a year is the same length and where living conditions are the same. Our work is the first evidence that these types of worlds exist.

The study’s preliminary results are promising, but to confirm their suspicions, the researchers will have to wait until after 2026. Only then can they use ALMA again to see if PDS 70b and its sister debris cloud are actually moving together in their orbit around the star. Balsalobre-Ruza: “It would be a breakthrough in exoplanet research.”

pds 70 galaxy image created using alma with the newly discovered debris cloud surrounded by a dotted line

Near the planet PDS 70b (circled), researchers have detected a cloud of debris (circled by a dotted line) that appears to share an orbit with the planet.

ALMA (ESO/NAOJ/NRAO) / Balsalobre-Ruza et al.
Portrait of Myrtle Prince

National Geographic Online Editor

Myrthe Prins has worked as a journalist for over ten years – she has written travelogues for Traveler, reported for PZC and interviewed many researchers for her science column in National Geographic Magazine. In addition to her work as an online editor, she writes poetry and prose, learns new languages, and helps her team escape escape rooms.

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