In a discovery that’s fit for a movie, Northwestern University astronomers have directly imaged a Tatooine-like exoplanet, orbiting two suns.
While obtaining an image of a planet beyond our solar system is already rare, finding one that circles two suns is even rarer. But this new world is extra exceptional. It hugs its twin stars more tightly than any other directly imaged planet in a binary system. In fact, it is six times closer to its suns than other previously discovered exoplanets.
The discovery provides an unprecedented look at how planets move and form around multiple stars. It also offers a rare glimpse into how stars and planets orbit together, enabling astrophysicists to test theories of planet formation in complex systems.
The study will be published on Thursday (Dec. 11) in The Astrophysical Journal Letters. Also published in the journal Astronomy and Astrophysics, European astronomers at the University of Exeter report the same discovery.
“Of the 6,000 exoplanets that we know of, only a very small fraction of them orbit binaries,” said Northwestern’s Jason Wang, a senior author of the study. “Of those, we only have a direct image of a handful of them, meaning we can have an image of the binary and the planet itself. Imaging both the planet and the binary is interesting because it’s the only type of planetary system where we can trace both the orbit of the binary star and the planet in the sky at the same time. We’re excited to keep watching it in the future as they move, so we can see how the three bodies move across the sky.”
An expert in exoplanet imaging, Wang is an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences. He also is a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Nathalie Jones, the CIERA Board of Visitors Graduate Fellow at Weinberg and member of Wang’s research group, is the study’s lead author.
A discovery years in the making
The Northwestern team found the new exoplanet hidden within years-old data. When Wang was a Ph.D. student, he helped commission the Gemini Planet Imager (GPI), a specialized instrument designed to capture images of distant worlds by blocking out the overwhelming glare of their stars. Originally operated at the Gemini South telescope in Chile, GPI used adaptive optics and a coronagraph to sharpen images of faint planets orbiting bright stars.
“We undertook this big survey, and I traveled to Chile several times,” Wang said. “I spent most of my time during my Ph.D. just looking for planets. During the instrument’s lifetime, we observed more than 500 stars and found only one new planet. It would have been nice to have seen more, but it did tell us something about just how rare exoplanets are.”
Nearly a decade later, Wang asked Jones to revisit the data. Scientists and engineers currently are upgrading GPI. Next year, it will move to Hawaii, where it will be installed on the Gemini North telescope atop Mauna Kea. As GPI completed its first chapter in Chile, Wang decided it was time to wrap up his original search.
“I didn’t think we’d find any new planets,” Wang said. “But I thought we should do our due diligence and check carefully anyway.”
Tracking a suspicious object
Jones analyzed GPI data taken between 2016 and 2019 and cross-referenced it with data from the W.M. Keck Observatory, using Northwestern’s institutional access. Then, this past summer, she noticed something suspicious. A faint object appeared to be consistently following the motion of a star as it moved across the sky.
“Stars don’t stand still in a galaxy, they move around,” Wang said. “We look for objects and then revisit them later to see if they have moved elsewhere. If a planet is bound to a star, then it will move with the star. Sometimes, when we revisit a ‘planet,’ we find it’s not moving with its star. Then, we know it was just a photobombing star passing through. If they are both moving together, then that’s a sign that it’s an orbiting planet.”
“We also look at the light coming off an object,” Jones added. “We know what light from a star looks like versus what light from a planet looks like. We compared them and decided it better matched what we expect to see from a planet.”
To the team’s surprise, Jones verified the suspicious object was a planet, which GPI captured in 2016, but it had gone unnoticed in earlier analysis. Also this summer, a European team led by University of Exeter astronomers independently found the same planet in its own reanalysis of the data, confirming Jones’ discovery.
Born after dinosaurs walked the Earth
The confirmed planet is huge — six times the size of Jupiter. While hotter than any planet in our solar system, it’s relatively cool compared to other directly imaged exoplanets. It’s located about 446 light-years away from Earth, which Wang describes as “not within our local solar neighborhood but like the next town over.”
Having formed just about 13 million years ago, the new exoplanet also is quite youthful.
“That sounds like a long time ago, but it’s 50 million years after dinosaurs went extinct,” Wang said. “That’s relatively young in universe speak, so it still retains some of the heat from when it formed.”
The team also was struck by how close the exoplanet orbited around its host stars. The stars themselves tightly revolve around one another — taking just 18 Earth days to complete one revolution. The planet, however, takes 300 years to orbit the pair. That’s a little longer than Pluto takes to orbit our sun.
“You have this really tight binary, where stars are dancing around each other really fast,” Wang said. “Then there is this really slow planet, orbiting around them from far away.”
What’s next
Relatively speaking, though, the planet is much closer to its stars than other directly imaged exoplanets bound to binary systems. Although the Northwestern team does not know how this system formed, they posit the binary stars formed first. Then the planet formed around them.
“Exactly how it works is still uncertain,” Wang said. “Because we have only detected a few dozen planets like this, we don’t have enough data yet to put the picture together.”
The team plans to continue studying the system, so they can learn more about how it formed and how it works. Jones currently is writing proposals to obtain more data.
“I’m asking for more telescope time, so we can continue looking at this planet,” Jones said. “We want to track the planet and monitor its orbit, as well as the orbit of the binary stars, so we can learn more about the interactions between binary stars and planets.”
Showing that surprises can hide in plain sight, the discovery underscores the continuing scientific value of archival telescope data. Jones also continues to reanalyze the years-old data to see if previous astronomers missed anything else.
“There are a couple suspicious objects,” she said, “but what they are, exactly, remains to be seen.”
The study, “HD 143811 AB b: A directly imaged planet orbiting a spectroscopic binary in Sco-Cen,” was supported by the Alfred P. Sloan Foundation, U.S. Department of Energy, the National Science Foundation and the Gordon and Betty Moore Foundation. The Gemini North and Gemini South telescopes make up the International Gemini Observatory, which is funded in part by the U.S. National Science Foundation (NSF) and operated by NSF NOIRLab.
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