Exoplanet PDS 70b, a gas giant seven times the mass of Jupiter, appears to share an orbit with a ball of dust around the mass of Earth’s moon, which could be forming a new planet
By Alex Wilkins
19 July 2023
The planetary system PDS features a star at its centre (in large white circle), orbited by the planet PDS 70b (in solid white circle) and a cloud of debris (in dotted white circle). Further out, the large yellow ring is a planet-forming circumstellar disc and there is another planet, PDS 70c, on its inner edge
ALMA (ESO/NAOJ/NRAO) /Balsalobre-Ruza et al.
When a young sibling is born, older children must learn to share – and something similar could be happening in a star system around 400 light years away, where a still-forming planet seems to be muscling in on the orbit of a gas giant that is already there.
If confirmed, this would be the first time we have ever seen two planets sharing an orbit. While we know of many small asteroids that co-orbit with Jupiter, called trojans, and even some around Earth, these were probably captured fully-formed by the planets’ gravitational fields, rather than forming in place.
Now, Álvaro Ribas at the University of Cambridge and his colleagues have glimpsed a dust cloud, which looks either like a planet forming or the remnants of a planet, that appears to be in the same orbit as the exoplanet PDS 70b, a gas giant seven times the mass of Jupiter, which itself is still in the early stages of formation. “If this is true, and if this ends up leading to the formation of asteroids, moons and potentially terrestrial planets, then it opens up the possibility to these sorts of trojans being born in situ,” says Ribas.
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To find the second planet, Ribas and his colleagues used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the world’s second largest telescope, to look at PDS 70b’s Lagrangian points, gravitationally stable points where trojan bodies might be likely to exist. They found a ball of dust around the mass of Earth’s moon made up of centimetre-sized rocks.
“This is an extremely young system,” says Matija Cuk at the SETI Institute in California. “This orbital material is primordial, it formed with the planet, this dust and maybe gas have been gathered into the trojan point as the planet was growing. That’s not something we have in our own solar system.”