A planet-forming disc torn apart

Astronomers say they have identified the first direct evidence that groups of stars can tear apart their planet-forming disc, leaving it warped and with tilted rings.

This suggests that exotic planets may form in inclined rings in bent discs around multiple stars.

Two separate teams have now reported discovering a planet-forming disc with misaligned rings around a triple star system called GW Orionis, which is just over 1300 light-years away in the constellation of Orion.

They suggest it was torn apart either by the gravitational pull from the stars or by a newborn planet. While misaligned discs have been observed previously, it has not been possible to directly link their structure to disc-tearing effects.

In the most recent study, published in the journal Science, a team led by Stefan Kraus from the University of Exeter, UK, observed the system’s disc and monitored the orbital motion of its stars over 11 years using several near-infrared and submillimeter telescopes.

By combining the techniques of interferometry and polarimetry, they identified the misaligned inner ring and imaged the shadow that it casts on the rest of the disc. From this, they developed a three-dimensional model and simulation of the system, which reproduces the eccentric inner ring and a strongly warped intermediate region of the disk.

The research also reveals that this inner ring contains 30 Earth-masses of dust, which could be enough to form planets.

“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…,” says Kraus’s colleague Alexander Kreplin, a co-author.

The international team included researchers from the UK, Belgium, Chile, France and the US.

Their computer simulations show, they say, that the misalignment in the orbits of the three stars could cause the disc around them to break into distinct rings, which is exactly what they see in their observations.

The observed shape of the inner ring also matches predictions from numerical simulations on how the disc would tear.

However, the authors of the second study, published in Astrophysical Journal Letters, hint at another possible explanation for the large misalignment between the inner and middle dust rings.

“We think that the presence of a planet between these rings is needed to explain why the disc tore apart,” says lead author Jiaqing Bi, from the University of Victoria in Canada.

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Atacama Large Millimeter/submillimeter Array (ALMA) images of the planet-forming disc made with data from Bi et al (right) and Kraus et al (left). Credit: ALMA (ESO/NAOJ/NRAO), S Kraus & J Bi; NRAO/AUI/NSF, S Dagnello

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