ALMA spots exoplanet and possible sibling sharing same orbit

The Atacama Large Millimeter/submillimeter Array zooms in on a young star surrounded by a circumstellar disc in which planets are forming. Closer in, a Jupiter-class planet, PDS 70b (solid circle), orbits the star alongside a cloud of debris (dotted circle) that could be a new planet forming in the same orbit or the remnants of one already formed. Another large planet, PDS 70C, is visible just inside the circumstellar disk at the 3 o’clock position. Image: ALMA (ESO/NAOJ/NRAO) /Balsalobre-Ruza et al

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have been studying a solar system 400 light years from Earth that features two Jupiter-class exoplanets and a circumstellar ring of debris where new planets are in the process of forming.

One of the two large planets, known as PDS 70b, appears to share its orbit with a cloud of material that could be the building blocks of a new planet or the remnants of one already formed. It’s the strongest evidence yet of so-called Trojan planets sharing the same orbit.

“Two decades ago it was predicted in theory that pairs of planets of similar mass may share the same orbit around their star, the so-called Trojan or co-orbital planets,” said lga Balsalobre-Ruza, a student at the Centre for Astrobiology in Madrid who led the paper published today in Astronomy & Astrophysics. “For the first time, we have found evidence in favour of that idea.”

In Earth’s solar system, about 12,000 Trojan asteroids share Jupiter’s orbit in two Lagrangian zones where the gravity of the Sun and the planet combine to trap material. Studying PDS 70b, astronomers detected a faint signal from one of the zones in that planet’s orbit where a cloud of debris with up to twice the mass of Earth’s Moon might reside.

The cloud could indicate the presence of an existing Trojan world or one in the process of forming.

“Our work is the first evidence that this kind of world could exist,” said Balsalobre-Ruza. “We can imagine that a planet can share its orbit with thousands of asteroids as in the case of Jupiter, but it is mind blowing to me that planets could share the same orbit.”

To fully confirm the detection, the team plans to use ALMA again after several years to see if PDS 70b and its presumed sibling move together around their star. If so, said Balsalobre-Ruza, “this would be a breakthrough in the exoplanetary field.”