The Strange Orbits of ‘Tatooine’ Planetary Disks

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA)

 have found striking orbital geometries in protoplanetary disks around binary stars. While disks orbiting the most compact binary star systems 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 protoplanetary 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 environments. Astronomers already know that the 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. 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 protoplanetary 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 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.

“We see a clear overlap between the small disks, orbiting compact binaries, and the circumbinary planets found with the Kepler mission,” 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,” added Czekala.

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. (NASA’s Kepler mission used the transit method, which is one of the ways to find a planet.)

Czekala now wants to find out why there is such a strong correlation between disk (mis)alignment and the binary star orbital period. “We want to use existing and coming facilities like ALMA and the next generation Very Large Array to study disk structures at exquisite levels of precision,” he said, “and try to understand how warped or tilted disks affect the planet formation environment and how this might influence the population of planets that form within these disks.”

“This research is a great example of how new discoveries build on previous observations,” said Joe Pesce, National Science Foundation Program Officer for NRAO and ALMA. “Discerning trends in the circumbinary disk population was only made possible by building on the foundation of archival observational programs undertaken by the ALMA community in previous cycles.”

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

# # #

Media contact:
Iris Nijman
News and Public Information Manager
National Radio Astronomy Observatory (NRAO)
[email protected]
+1 (434) 296-0314

Science contact:
Ian Czekala
University of California at Berkeley
[email protected]
+1 (631) 793 9292

Ian Czekala worked with Eugene Chiang of the University of California at Berkeley; Sean Andrews, Guillermo Torres and David Wilner of the Harvard & Smithsonian Center for Astrophysics; Eric Jensen of Swarthmore College; Keivan Stassun of Vanderbilt University; and Bruce Macintosh of Stanford University.

The astronomers published their results in The Astrophysical Journal. https://iopscience.iop.org/article/10.3847/1538-4357/ab287b

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

NRAO Director Tony Beasley Appointed to New Five-Year Term

 

Dr. Tony Beasley, Director of the National Science Foundation’s National Radio Astronomy Observatory (NRAO), has been appointed to a new five-year term. The Board of Trustees for AUI— which operates NRAO under a cooperative agreement— and the NRAO Director Review Committee conducted a thorough review of Beasley’s leadership and performance earlier this year, and have appointed the Director to the new term through May 2027.

 

“Tony is an outstanding leader and stalwart champion for NRAO, the field of radio astronomy, the beauty of science, and the critical role of big facilities in the R&D ecosystem,” said Adam Cohen, President and CEO of AUI, which operates NRAO under a cooperative agreement. “He continues to support very innovative education and outreach programs to help build the workforce of the future, as well as programs and activities to enhance diversity, equity, and inclusion in our workplaces.” 

 

Over the course of more than two decades, Beasley’s leadership has shaped the present and future of NRAO’s leading-edge radio astronomy facilities, including the Atacama Large Millimeter/submillimeter Array (ALMA), Very Long Baseline Array (VLBA), and Very Large Array (VLA). More recently, he has collaborated on efforts to encourage cooperation between commercial spectrum users and research facilities and has created partnerships to explore the use of Green Bank Observatory’s radar systems in planetary science and defense applications. 

 

Beasley recently has generated significant support for the future of NRAO’s facilities, reaching major milestones in 2021. The observatory’s proposed next generation Very Large Array (ngVLA) received high priority for new ground-based observatories in the U.S. National Academy of Sciences’ Astronomy and Astrophysics Decadal Survey (Astro2020). Late last year, NRAO’s Central Development Laboratory (CDL) received approval and funding through the ambitious ALMA2030 Development Plan to upgrade its Band 6 receivers, which are ALMA’s most productive receivers. 

 

An ardent supporter of diversity, equity, and inclusion in astronomy and astrophysics, Beasley has elevated the efforts of NRAO’s Office of Diversity and Inclusion, Broader Impacts, and community development initiatives, including the National Astronomy Consortium (NAC), RADIAL, National and International Non-traditional Exchange (NINE), Research Experiences for Undergraduate students (REU), and most recently, grants for women in engineering fellowships and the development of a next generation Learning Center (ngLC). 

 

“Being a part of NRAO for more than 20 years has given me the opportunity to observe, contribute to, and lead growth and change in astronomy that positively impacts our facilities and allows us to collaborate with other like-minded institutions,” said Beasley. “I am proud of the work our teams have accomplished in research, engineering, outreach, and equity, and look forward to serving the NRAO community for another five years.”

 

Beasley, who holds a Doctorate in Astrophysics from the University of Sydney, was first appointed as NRAO Director in February 2012, after previously serving the observatory and the radio astronomy community in multiple capacities. He joined NRAO as a Postdoctoral Fellow in 1991 and served as Deputy Assistant Director in 1997 and Assistant Director from 1998 to 2000. He briefly left NRAO that year to become Project Manager for the Combined Array for Research in Millimeter-wave Astronomy (CARMA). In 2004, he returned to NRAO as Assistant Director, as well as Project Manager for the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. From 2008 to 2012, Beasley served as the Chief Operating Officer and Project Manager of NSF’s National Ecological Observatory Network (NEON). In addition to his role as NRAO Director, Beasley presently serves as the AUI Vice President for Radio Astronomy Operations. 

 

In January 2022, Beasley was honored as a Lifetime Fellow of the American Association for the Advancement of Science (AAAS) in recognition of his significant contributions to the field of radio astronomy. 

 

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