Three-Star System Seen Tearing Apart Its Planet-Forming Disk

Observations of the cosmos revealed that GW Orionis – a three-star system located 1,300 light-years from Earth – has a warped planet-forming disk with a tilted ring, displayed on the left panel through an artist’s 3-D model. (Credit: ESO / L. Calçada, Exeter / Kraus et al.)

(CN) — Astronomers have observed the first evidence that a triple star system deformed its surrounding dust rings where planets are formed, leaving the rings contorted and on a tilt. But opinions vary on what force ultimately warped the disks, according to a study released Thursday.

Unlike our solar system where planets orbit on the same plane as our sun, planets in multi-star systems have trajectories that are off-course with host stars, resulting in misaligned rings.

Using the Atacama Large Millimeter/Submillimeter Array project’s radio telescopes and the European Southern Observatory’s Very Large Telescope, a team of astronomers observed the three-star GW Orionis system for eleven years beginning in 2008.

The system’s inner stars, GW Ori A and B, orbit each other and are separated by 1 au (the astronomical unit equal to the distance between Earth and the sun, or 93 million miles) and the third star, GW Ori C, orbits its siblings at a distance of 8 au.

The observations allowed scientists to study the gravitational force of the three stars and map out the orbits of all objects in the system, said researcher Alison Young of the Universities of Exeter and Leicester.

“We found that the three stars do not orbit in the same plane, but their orbits are misaligned with respect to each other and with respect to the disc,” Young said in the team’s statement on their study.

Using computer simulations, researchers modeled the “disc-tearing effect” of the stars’ gravitational pull, which broke the disk into distinct rings, according to the study published in the journal Science.

“The ring casts shadows on a strongly warped intermediate region of the disk,” the study said. “If planets can form within the warped disk, disk tearing could provide a mechanism for forming wide-separation planets on oblique orbits.”

ALMA allowed researchers to measure the shape of the large ring that casts a shadow on the outer disk, leading to a 3D reconstruction of the misaligned inner ring, said University of Exeter researcher Stefan Kraus.

“Our images reveal an extreme case where the disc is not flat at all, but is warped and has a misaligned ring that has broken away from the disc,” said Kraus, who led the study.

An artist’s depiction of the disc structure and stellar orbit of the GW Orionis triple star system, based on observation from ALMA radio telescope project and the European Southern Observatory’s Very Large Telescope. (Credit: Kraus et al., 2020; NRAO/AUI/NSF)

The inner ring of the star system, located 1,300 light-years away from Earth and found in the Orion constellation, also contains enough cosmic dust to form exoplanets.

More than half the stars in the universe are born with one or more companion stars unlike the solitary star at the center of our solar system, which study co-author Alexander Kreplin said could mean more systems contain exoplanets with distant, crooked orbits.

“Any planets formed within the misaligned ring will orbit the star on highly oblique orbits and we predict that many planets on oblique, wide-separation orbits will be discovered in future planet imaging campaigns, for instance with the ELT,” said Kreplin, researcher at the University of Exeter.

A separate team of researchers observing the same set of rings around GW Orionis said in their own study a key element of the disc-tearing effect is missing from the equation. 

“We think that the presence of a planet between these rings is needed to explain why the disc tore apart,” said researcher Nienke van der Marel of the University of Victoria in Canada. “This planet has likely carved a dust gap and broken the disk at the location of the current inner and outer rings.”

The researchers said further studies are needed to determine whether such a planet exists in the dust rings.

If it is detected, or if it is still forming, it would be the first planet ever observed to orbit three stars, according to the researchers.

Van der Marel was part of a team led by Jiaqing Bi of the University of Victoria that published their study in May in The Astrophysical Journal Letters.

Bi’s team found that the outermost ring is the largest ever observed in planet-forming disks and is located 340 au from the star systems’ center. Neptune is about 30 au from the sun.

Researchers did not immediately respond to a request for further comment. 

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