When a young sibling is born, older children must learn to share â 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 planetsâ gravitational fields, rather than forming in place.
Now, 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. âIf 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,â 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 worldâs second largest telescope, to look at PDS 70bâs Lagrangian points, gravitationally stable points where trojan bodies might be likely to exist. They found a ball of dust around the mass of Earthâs moon made up of centimetre-sized rocks.
âThis is an extremely young system,â says at the SETI Institute in California. âThis 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. Thatâs not something we have in our own solar system.â
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 âclearing the neighbourhoodâ â this is why Pluto was demoted to dwarf planet status, because its moon Charon is large enough to have its own gravitational influence.
âIf 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,â says Ribas. That said, the IAUâs definition only formally applies to planets in the solar system, making the status of this baby planet a grey area.
Astronomy & Astrophysics
