Astronomers using the Atacama Large Millimeter/sub-millimeter Array (ALMA) have spotted the first example of a binary system that has flipped its protoplanetary disk to a position that leaps over the orbital plane of the two stars. The discovery is reported in the journal Nature Astronomy.
An artist’s impression of a view of the HD 98800BaBb binary star system and the surrounding disk. Image credit: Mark Garlick, University of Warwick.
“Disks 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,” said team leader Dr. Grant M. Kennedy, an astronomer at the University of Warwick, UK.
“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.”
“A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now we had no evidence of misaligned disks in which these planets might form.”
Dr. Kennedy and colleagues used ALMA to pin down the orientation of the protoplanetary ring of gas and dust in a binary star system called HD 98800BaBb.
This stellar system is part of HD 98800, a young quadruple star system about 146.4 light-years away, and a member of the 10-million-year-old TW Hydrae association.
HD 98800 consists of two pairs of binaries — called ‘A’ and ‘B’, or equally ‘AaAb’ and ‘BaBb’ — with semi-major axes of about 1 AU (astronomical unit), which themselves orbit each other with a semi-major axis of 54 AU.
An artist’s impression of the view from a planet in the HD 98800BaBb system. Image credit: Mark Garlick, University of Warwick.
Dr. Kennedy and co-authors were able to establish that the HD 98800BaBb ring was consistent with a perfectly polar orbit.
This means that while HD 98800Ba and Bb orbit each other in one plane, the disk surrounds these stars at right angles to their orbits.
“Perhaps the most exciting thing about this discovery is that the disk shows some of the same signatures that we attribute to dust growth in disks around single stars,” Dr. Kennedy said.
“We take this to mean planet formation can at least get started in these polar circumbinary disks.”
“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 were a planet or planetoid present at the inner edge of the HD 98800BaBb dust ring, the ring itself would appear from the surface as a broad band rising almost perpendicularly from the horizon.
The polar configuration means that the stars would appear to move in and out of the disk plane, giving objects two shadows at times.
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.
“We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single star,” said team member Dr. Daniel Price, a researcher at Monash University, Australia.
“But with the new images we see a swirling disk of gas and dust orbiting around two stars. It was quite surprising to also find that that disk orbits at right angles to the orbit of the two stars.”
“Incredibly, two more stars were seen orbiting that disk. So if planets were born here there would be four suns in the sky!”
_____
Grant M. Kennedy et al. A circumbinary protoplanetary disk in a polar configuration. Nature Astronomy, published online January 14, 2019; doi: 10.1038/s41550-018-0667-x
