Astronomers May Have Found Two Exoplanets Sharing One Orbit

One planet per orbit—that’s the way all the planets and exoplanets discovered so far operate, but that might not be universal. Scientists analyzing a solar system called PDS 70 say there’s evidence of more than one planet sharing an obit. While theoretically possible, this would be the first observational evidence supporting the models.

The PDS 70 system, which sits 370 light-years from Earth, is known to have two gas giant planets: PDS 70b and PDS 70c. This solar system is of intense scientific interest because it’s one of only a handful of places where we can directly image exoplanets. Usually, the stars are too bright compared to their planets, but PDS 70 is young, and its planets are still forming from the protoplanetary disk.

Astronomers previously detected what may be a moon-forming disk around PDS-70c, but it’s 70b’s turn to confound astronomers. Researchers at Spain’s Centre for Astrobiology pored over observations of PDS 70 from the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, which is how scientists identified PDS 70b and 70c, as well as the moon-forming disk. This time, the team spotted a faint signal along PDS 70b’s orbit. The haze is in a region of gravitational resonance known as a Lagrange point. In our solar system, Lagrange points collect smaller objects dragged along with the planet. We call these objects trojans, so the fuzzy detection in PDS 70 could be the first “exotrojans.”

PDS 70 as seen by ALMA with PDS 70b and its companion indicated.
Credit: ALMA (ESO/NAOJ/NRAO)

The team estimates that the newly detected material is twice as massive as Earth’s Moon. This signal could point to a planet beginning to form in the same orbit as PDS 70b, or it could be a planet that has already formed. As the theory goes, this second planet could remain safely in the Lagrange point, following PDS 70b around the star until the end of its lifespan. “Who could imagine two worlds that share the duration of the year and the habitability conditions? Our work is the first evidence that this kind of world could exist,” says study co-author Olga Balsalobre-Ruza.

The study lays out all the evidence, but that’s not enough to confirm co-orbital planets in PDS 70. The team plans to conduct new solar system observations to see if the debris (or exoplanet) is tracking along with PDS 70b, but that will have to wait until after 2026. ALMA is currently undergoing upgrades, but when done, the data quality will improve by a factor of 200-400.