Triple star system features mis-aligned planet-forming dust rings

The ALMA radio telescope array captured this image of a protoplanetary disc around the star GW Orionis. Three dust rings are clearly visible with the innermost ring face on to the observer. The outer rings appear oval because they are tilted with respect to the inner ring. Image: ALMA (ESO/NAOJ/NRAO), Bi et al., NRAO/AUI/NSF, S. Dagnello

Planets orbiting single stars like the Sun are commonplace, and dozens of planetary systems have been found around double stars. But no confirmed planets have been seen to date orbiting triple stars.

To study that possibility, astronomers using the Atacama Large Millimeter/submillimeter Array examined a young triple star system known as GW Orionis featuring two stars just an astronomical unit apart – the distance from Earth to the Sun – and a third companion eight times farther out.

The observations revealed three huge dust rings 46, 188 and 336 au from the central stars (or comparison, Neptune orbits the Sun at a distance of 30 au). Estimates of the amount of dust in the rings are 75, 170 and 245 Earth masses, enough to form the “seeds” of giant planets.

As it turns out, the innermost ring is steeply tilted with respect to the two outer rings.

ESO’s SPHERE instrument on the Very Large Telescope imaged a warped protoplanetary disc around a triple star system known as GW Orionis (right panel) featuring three vast dust rings. The shadow cast by the misaligned innermost dust ring, extending to lower left, allowed researchers to understand the three dimensional structure of the system, shown in an artist’s impression at left. Image: ESO/L. Calçada, Exeter/Kraus et al.

“We were surprised to see the strong misalignment of the inner ring,” said Jiaqing Bi of the University of Victoria in Canada, leader of a team that published their results in The Astrophysical Journal Letters. “But the strange warp in the disk is confirmed by a twisted pattern that ALMA measured in the gas of the disk.”

Computer simulations suggest gravitational interactions between the three stars making up GW Orionis cannot explain the misaligned rings.

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

Independent observations over 11 years by a team led by Stefan Kraus from the University of Exeter, using the European Southern Observatory’s Very Large Telescope, show the innermost ring casts a shadow on the outer rings. That helped them map out the three-dimensional structure of the system, suggesting the three stars could be responsible for the misalignment on their own, without the need for an unseen planet.

A composite image of GW Orionis shows dust rings observed by ALMA (blue) superimposed on a near-infrared image captured by ESO’s Very Large Telescope (shown in orange). Image: ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.

Either way, GW Orionis on going observations of GW Orionis will shed light on planetary formation in complex gravitational environments.

“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,” said team member Alexander Kreplin of the University of Exeter.