Distant planet may be the first known to share its orbit with another

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.

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.

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.”

We currently only have a snapshot of the suspected planet, though it is in a position where we would expect it to be if it was co-orbiting. To fully confirm its orbit, we will have to wait for a follow-up observation in February 2026 and, to learn more about its composition, we might have to wait until 2030 for ALMA to be upgraded, says Ribas.

Once we know more, there is still the question of what to call this object. According to the International Astronomical Union (IAU), a planet must be the only dominant mass in its gravitational district, a state of affairs called “clearing the neighbourhood” – this is why Pluto was demoted to dwarf planet status, because its moon Charon is large enough to have its own gravitational influence.

“If something forms at the location of this Lagrangian point and it is the mass of the Earth, say, then the definition of a planet would be fighting against the [IAU] definition,” says Ribas. That said, the IAU’s definition only formally applies to planets in the solar system, making the status of this baby planet a grey area.

Journal reference:

Astronomy & AstrophysicsDOI: 10.1051/0004-6361/202346493