Hidden circumbinary giant planet emerges from decade old Gemini data
The newly confirmed world, designated HD 143811 AB b, is about six times the mass of Jupiter and lies roughly 446 light years from Earth. Its age is estimated at about 13 million years, meaning it formed around 50 million years after the extinction of the dinosaurs and still retains heat from its formation, which makes it detectable in direct imaging.
In this system, the two host stars orbit each other every 18 Earth days, while the planet requires about 300 years to complete a single orbit around the pair. That orbital period is slightly longer than Pluto's path around the Sun, yet among directly imaged planets in binary systems this planet orbits its stars at the smallest known separation, about six times closer than comparable systems.
Northwestern University's Jason Wang, an assistant professor of physics and astronomy and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), specializes in imaging exoplanets and co-led the study. Graduate researcher Nathalie Jones, the CIERA Board of Visitors Graduate Fellow at Northwestern's Weinberg College of Arts and Sciences, led the analysis that uncovered the planet in archival datasets.
Wang originally helped commission GPI as a Ph.D. student, using its adaptive optics and coronagraph to block starlight and search for faint companions around more than 500 stars, work that yielded only one new planet at the time and underscored the rarity of directly imaged exoplanets. With GPI now being upgraded and prepared for installation on the Gemini North telescope on Mauna Kea in Hawaii, Wang asked Jones to perform a deeper reanalysis of the earlier observations to close out the survey.
Jones examined GPI data from 2016 to 2019 and combined it with observations from the W. M. Keck Observatory obtained through Northwestern's institutional access, looking for faint sources that consistently moved with their parent star across the sky. She identified a dim object that tracked the host star's motion and showed a light signature more consistent with a planet than a star, indicating it was gravitationally bound to the binary rather than a background interloper.
Follow-up comparison with a European team's independent reanalysis, led by University of Exeter astronomers and reported in Astronomy and Astrophysics, confirmed that both groups had detected the same planet in the archival data. The system's configuration, with a tight inner binary and a massive, slowly orbiting outer planet, provides an observational testbed for models of planet formation and orbital dynamics in multiple-star environments.
Although the exact formation pathway for HD 143811 AB b is still uncertain, researchers propose that the two stars formed first and the planet subsequently condensed out of the surrounding material in a circumbinary disk. Only a few dozen planets are known in such arrangements, and very few have been directly imaged alongside both stars and planet, limiting astronomers' ability to fully constrain theories for these systems.
The team plans to seek additional telescope time to monitor the motion of both the binary stars and the planet, refining their orbits and probing interactions between the stellar pair and the circumbinary companion. Continued analysis of archival images may reveal more faint candidates, illustrating how existing datasets can still yield major discoveries when revisited with refined techniques.
Research Report:GPI+SPHERE detection of a 6.1 MJup circumbinary planet around HD 143811
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