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Institute of Astronomy researchers help confirm double star system with a disc orbiting at right angles


By Paul Brackley


It could give rise to a planet where four Suns would be visible in the sky, and a broad band of dust and gas would rise up from the horizon.

An international team of astronomers have found the first confirmed example of a double star system with a surrounding disc that is at right angles to their orbital plane.

The researchers, including Mark Wyatt and Ben Yelverton from the University of Cambridge’s Institute of Astronomy, secured high-resolution images of the Asteroid belt-sized disc by using the the ALMA (Atacama Large Millimeter/sub millimeter Array) telescope in Chile.

Planets are understood to form in such protoplanetary discs, although it could also be the remnants of an asteroid belt.

Lead author Dr Grant M Kennedy, of the University of Warwick, 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.

“Some of these 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 [ones orbiting a system of two stars].

Artwork of the system HD 98000. A view from the surface of an imagined planer orbiting in the inner edge of the disc. University of Warwick/Mark Garlick
Artwork of the system HD 98000. A view from the surface of an imagined planer orbiting in the inner edge of the disc. University of Warwick/Mark Garlick

“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.”

In this case, the orbit of the two stars around one another was already known from observations quantifying how they moved in relation to one another.

Using ALMA, the researchers confirmed the orientation of the ring of dust and gas, confirming that it had what it is known as a ‘polar’ orbit, perpendicular to the stars’ orbit.

The stars can be thought of as orbiting one another like horses on a carousel, while the surrounding disc is like a giant ferris wheel around it, 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,” added Dr Kennedy. “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.”

If there is a planet or planetoid at the inner edge of the dust ring, the ring would rise almost perpendicularly from the horizon, while the stars would appear to move in and out of the disc plane, giving objects two shadows at times.

A polar circumbinary planet would also have seasons that vary as different latitudes received more or less light throughout the binary orbit.

Co-author Dr Daniel Price, of Monash University’s Centre for Astrophysics (MoCA) and School of Physics and Astronomy, said: “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. But with the new images we see a swirling disc of gas and dust orbiting around two stars.

“It was quite surprising to also find that that disc orbits at right angles to the orbit of the two stars.

“Incredibly, two more stars were seen orbiting that disc. So if planets were born here there would be four suns in the sky!”

He added: “ALMA is just a fantastic telescope, it is teaching us so much about how planets in other solar systems are born.”

The work was published in Nature Astronomy in a paper called “A circumbinary protoplanetary disc in a polar configuration”.

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