Free Astronomy Magazine May-June 2020

44 MAY-JUNE 2020 SPACE CHRONICLES ments. Astronomers already know that the orbits of binary stars can warp and tilt the disk around them, resulting in a circumbinary disk mis- aligned relative to the orbital plane of its host stars. For example, in a 2019 study led by Grant Kennedy of the University of Warwick, UK, ALMA found a striking circumbinary disk in a polar configuration. “With our study, we wanted to learn more about the typical geometries of circumbinary disks,” said astronomer Ian Czekala of the University of California at Berkeley. Czekala and his team used ALMA data to determine the degree of alignment of nineteen protoplane- tary disks around binary stars. “The high resolution ALMA data was critical for studying some of the smallest and faintest circumbinary disks yet,” said Czekala. The astronomers compared the ALMA data of the circumbinary disks with the dozen ‘Tatooine’ planets that have been found with the Kepler space telescope. To their surprise, the team found that the degree to which binary stars and A rtist impression of a double sunset on a ‘Tatooine’ exo- planet forming in a circumbinary disk that is misaligned with the orbits of its binary stars. [NRAO/AUI/NSF, S. Dagnello] The strange orbits of ‘T A stronomers using the Ata- cama Large Millimeter/sub- millimeter Array (ALMA) have found striking orbital geome- tries in protoplanetary disks around binary stars. While disks orbiting the most compact binary star sys- tems share very nearly the same plane, disks encircling wide binaries have orbital planes that are severely tilted. These systems can teach us about planet formation in complex environments. 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 proto- planetary disks – we now have wonderful observations of these thanks to ALMA – but most of the disks studied so far orbit single stars. ‘Tatooine’ exoplanets form in disks around binary stars, so-called circumbinary disks. Studying the birthplaces of ‘Tatooine’ planets provides a unique opportunity to learn about how planets form in different environ- by ALMA Observatory their circumbinary disks are mis- aligned 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. “We see a clear overlap between the small disks, orbiting compact binaries, and the circumbinary planets found with the Kepler mis- sion,” 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 linger- ing mystery was whether there might be many misaligned planets