Using data from the Atacama Large Millimeter/submillimeter Array (ALMA), a team of astronomers has found striking orbital geometries in circumbinary protoplanetary disks. Their findings appear in the Astrophysical Journal.
An artist’s impression of a double sunset on a ‘Tatooine’ exoplanet forming in a circumbinary disk that is misaligned with the orbits of its binary stars. Image credit: NRAO / AUI / NSF / S. Dagnello.
In the last two decades, thousands of planets have been found orbiting stars other than our Sun. Some of these planets orbit two stars, just like Luke Skywalker’s home Tatooine.
Planets are born in protoplanetary disks but most of the disks studied so far orbit single stars. ‘Tatooine’ exoplanets form in disks around binary stars, so-called circumbinary disks.
Orbits of binary stars can warp and tilt the disk around them, resulting in a circumbinary disk misaligned relative to the orbital plane of its host stars.
“With our study, we wanted to learn more about the typical geometries of circumbinary disks,” said Dr. Ian Czekala, an astronomer at the University of California, Berkeley.
Dr. Czekala and colleagues used ALMA data to determine the degree of alignment of 19 protoplanetary disks around binary stars.
They compared the ALMA data of the circumbinary disks with the dozen ‘Tatooine’ planets that have been found with NASA’s Kepler space telescope.
To their surprise, the researchers found that the degree to which binary stars and their circumbinary disks are misaligned is strongly dependent on the orbital period of the host stars.
The shorter the orbital period of the binary star, the more likely it is to host a disk in line with its orbit. However, binaries with periods longer than a month typically host misaligned disks.
Two examples of aligned and misaligned circumbinary disks observed with ALMA. Binary star orbits are added for clarity. Left: in star system HD 98800B, the disk is misaligned with inner binary stars. The stars are orbiting each other (in this view, towards and away from us) in 315 days. Right: in star system AK Sco, the disk is in line with the orbit of its binary stars. The stars are orbiting each other in 13.6 days. Image credit: ALMA / ESO / NAOJ / NRAO / I. Czekala & G. Kennedy / AUI / NSF / S. Dagnello.
“We see a clear overlap between the small disks, orbiting compact binaries, and the circumbinary planets found with the Kepler mission,” Dr. Czekala said.
Because the primary Kepler mission lasted 4 years, astronomers were only able to discover planets around binary stars that orbit each other in fewer than 40 days. And all of these planets were aligned with their host star orbits.
A lingering mystery was whether there might be many misaligned planets that Kepler would have a hard time finding.
“With our study, we now know that there likely isn’t a large population of misaligned planets that Kepler missed, since circumbinary disks around tight binary stars are also typically aligned with their stellar hosts,” Dr. Czekala said.
Still, based on this finding, the astronomers conclude that misaligned planets around wide binary stars should be out there and that it would be an exciting population to search for with other exoplanet-finding methods like direct imaging and microlensing.
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Ian Czekala et al. 2020. The Degree of Alignment between Circumbinary Disks and Their Binary Hosts. ApJ 883, 22; doi: 10.3847/1538-4357/ab287b
