Double star solar system flips planet-forming disk into pole position

New research involving a Monash University astronomer has found the first confirmed example of a double-star solar system that has flipped its surrounding disc to a position that leaps over the orbital plane of those stars.

The international team of astronomers, led by the University of Warwick, used the world's most powerful telescope, known as ALMA, to obtain high-resolution images of the Asteroid belt-sized disc.

The overall system presents the unusual sight of a thick hoop of gas and dust circling at right angles to the binary star orbit. Until now, this setup only existed in theorists’ minds, but the ALMA observation proves that polar discs of this type exist, and may even be relatively common.

The new research, published today in Nature Astronomy, was led by Royal Society University Research Fellow Dr Grant M. Kennedy, of the University of Warwick.

Co-author Dr Daniel Price, Monash Centre for Astrophysics (MoCA) said if planets were born in this system, there would be four suns in the sky.

“We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun,” Dr Price said. “But in the new images we see a swirling disc of gas and dust orbiting around two stars. What’s more compelling is that the disc orbits at right angles to the orbit of the two stars.”

“Two more stars were also seen orbiting that disc, also at right angles, so if planets were born here there would be four suns in the sky.”

Dr Kennedy said discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets, but until now we had no evidence of misaligned discs in which these planets might form.

“Some planets end up being misaligned with the spin of the star, so we’ve been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned discs in which these planets might form.”

Dr Kennedy and his fellow researchers used ALMA to pin down the orientation of the ring of gas and dust in the system. The orbit of the binary was previously known, from observations that quantified how the stars move in relation to each other. By combining these two pieces of information they were able to establish that the dust ring was consistent with a perfectly polar orbit.

This means that while the stellar orbits orbit each other in one plane, like two horses going around on a carousel, the disc surrounds these stars at right angles to their orbits, like a giant ferris wheel with the carousel at the centre.

“Perhaps the most exciting thing about this discovery is that the disc shows some of the same signatures that we attribute to dust growth in discs around single stars. We take this to mean planet formation can at least get started in these polar circumbinary discs. If the rest of the planet formation process can happen, there might be a whole population of misaligned circumbinary planets that we have yet to discover, and things like weird seasonal variations to consider.

Seasons on planets in such systems would also be different. On Earth they vary throughout the year as we orbit the Sun. A polar circumbinary planet would have seasons that also vary as different latitudes receive more or less illumination throughout the binary orbit.

The research is supported by the Monash Warwick Alliance. The Monash Warwick Alliance brings together the collective strengths of Monash University and the University of Warwick with a breadth, scale and impact beyond standard practice in the sector.

Image: View of the double star system and surrounding disc.
Credit: University of Warwick/Mark Garlick.