Can planets share an orbit?

A planet and its possible Trojan (circled dashed) orbiting the star PDS 70. © ALMA (ESO/NAOJ/NRAO) /Balsalobre-Ruza et al.

So far, the existence of two planets in the same orbit has only been predicted theoretically - but now scientists in a distant star system have uncovered astronomical evidence for such a constellation. Their preliminary results indicate that there is a cloud of matter orbiting the star at the same distance as a known planet. According to the researchers, these could be the building blocks of a "sibling planet" in the process of formation or the remains of a co-orbital celestial body that has already formed.

Mercury orbits the sun most closely, Venus a little further out, then comes the earth... In our solar system, each planet has its own orbit. Only asteroids are known to orbit around the center at the same distance as the large celestial bodies. The most famous example are the so-called Trojans of Jupiter. With regard to the countless planetary systems in the universe, one can ask oneself whether there are several planets orbiting stars at the same distance.

At least theoretically, this already seemed possible: "As early as two decades ago, it was predicted that planets could exist that share an orbit around their star - so-called Trojan or co-orbital planets," says lead author Olga Balsalobre-Ruza from the Center for Astrobiology in Madrid. Concrete examples of such pairs have not yet been found. However, Balsalobre-Ruza and her colleagues now present at least strong evidence for a co-orbital position.

Something else is circling...

The scientists have discovered the possible example in the system of the young star PDS 70, which is about 400 light-years away from us in the constellation Centaurus. It was already known that the two Jupiter-like planets PDS 70 b and PDS 70 c orbited it in different orbits. But as the astronomers report, they have now made another discovery in archived observation data from the Atacama Large Millimeter/submillimeter Array (ALMA): they identified a signal that indicates that a cloud of matter is moving around the star on the same orbit as PDS 70 b .

According to the data, the cluster appears to have a mass about twice that of our moon. The position on the common orbit with the planet also fits into the picture, report Balsalobre-Ruza and her colleagues: The cloud of matter is in one of the so-called Lagrange zones. These are sections of a planet's orbit where the combined gravitational pull of the star and planet can trap material, the scientists explain.

The natal cloud of a "sibling planet"?

Although a cloud of matter is not a planet, the researchers see in their discovery a clear indication of the existence of a co-orbital planetary position. According to them, it seems plausible that the accumulation they observed is material from which a “sister planet” for the protoplanet PDS 70 b is currently forming. However, it could also be the remains of an already formed co-orbital celestial body, say the astronomers.

"We can well imagine a planet sharing its orbit with thousands of asteroids, as in the case of Jupiter, but it seems intriguing that planets could share the same orbit: two worlds sharing the length of the year and the conditions." , says Balsalobre-Ruza. "Our work is a strong indication that these kinds of worlds can actually exist," says the scientist.

The team now wants to stay on the ball: "Our research is a first step to search for co-orbital planets at a very early stage of their formation," says co-author Nuria Huélamo Center for Astrobiology. However, to confirm and expand on their discovery, the researchers will have to wait until another opportunity arises to use ALMA to study the exciting system. In addition, the astronomers hope for future expansions of the possibilities: "This topic is very exciting and we look forward to the expanded ALMA possibilities planned for 2030, which will dramatically improve the array's ability to characterize Trojans at many other stars," says Co-author Itziar De Gregorio-Monsalvo from the ESO Office for Science in Chile.

Discovery presentation video: © ESO

Source: The European Southern Observatory (ESO), Article: Astronomy and Astrophysics, doi: 10.1051/0004-6361/202346493

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