Northwestern University astronomers have directly imaged a distant planet that orbits two stars like the fictional Tatooine in the Star Wars universe.
Although the data used to identify the planet, located approximately 446 light-years away from Earth, was captured nearly a decade ago, its existence was not confirmed until this year.
The team behind the discovery said that the direct imaging of an exoplanet orbiting two stars marks a historic first for imaging exoplanets in binary systems with this tight an orbit and offers scientists the unprecedented opportunity to test theories of planet formation in complex stellar systems.
The Northwestern team also said they plan to scan more previously captured images to see whether other planets have been overlooked in the initial data analysis.
“There are a couple suspicious objects,” explained Nathalie Jones, the CIERA Board of Visitors Graduate Fellow at Weinberg and the lead author of a paper detailing the discovery in a statement, “but what they are, exactly, remains to be seen.”
Search for ‘Rare’ Star Wars Planet Began with Decade-Old Treasure Trove
According to Jason Wang, a researcher from Northwestern and the study’s senior author, only a “very small fraction” of the 6,000 exoplanets discovered in the last three decades orbit binary star systems like Tatooine in Star Wars. The researcher also noted that even fewer of that small group of exoplanets have been imaged directly, “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,” Wang explained.
The Northwestern scientist was part of a group searching for exoplanets when he was a Ph.D. student in 2016. At the time, he and other Northwestern astronomers had been using the recently commissioned Gemini Planet Imager (GPI), a specialized instrument designed to capture images of distant worlds by blocking out the significantly brighter glare of their host stars. That effort yielded only one new planet among 500 observed 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.”
Although the results were frustrating, the researcher also noted that spending so much time examining such a large sample set of stars with only a single planetary discovery “did tell us something about just how rare exoplanets are.”
In the latest study, Jones, Wang, and colleagues decided to take a fresh look at the original effort’s data to see whether new approaches could identify planets missed during the first analysis. Still, given the anti-climactic result of the previous attempt, Wang admitted he “didn’t think we’d find any new planets.”
“But I thought we should do our due diligence and check carefully anyway,” he added.
Mystery Object’s Light and Motion Reveal Previously Undiscovered Planet
The study’s first step involved an in-depth analysis of data captured between 2016 and 2019. Next, Jones cross-referenced the original GPI data set with separate observations made by the W.M. Keck Observatory. According to the team’s statement, Jones “noticed something suspicious” this past summer when scanning the data.
A closer look revealed an object that wasn’t behaving correctly. Instead, the mysterious object appeared to be following the star’s motion as it moved across the sky. Wang noted that stars “don’t stand still,” but instead move around. Still, the data Jones was observing led her to question whether it was a star or a planet orbiting a star.
“We look for objects and then revisit them later to see if they have moved elsewhere,” Wang explained. “If a planet is bound to a star, then it will move with the star.”
The researcher said that later analysis will sometimes reveal that a suspect light was not moving with the star but was instead just “a photobombing star” passing through the image. Conversely, if the second looks at the data, it shows the newly discovered signal is moving in concert with the star, Wang said, “that’s a sign that it’s an orbiting planet.”
Another method used to determine if they have indeed found a planet orbiting two suns involved measuring the object’s light. Wang said scientists already know the difference between light from a star and light reflected by a planet, so analyzing the light should also offer clues to its nature.
After analyzing the light from the new signal, Wang said the team decided the discovery “better matched” a planetary signature than a stellar one.
A Mysterious Planet’s Tight Orbit Remains Unexplained
As rare as it is to image such a planet directly, the researchers said they were “struck” by its unique orbit. Unlike the 18 days it takes the stars to orbit each other, the team said the planet requires about 300 years to complete a full orbit around both stars. For comparison, they said this is just “a little longer” than Pluto needs to orbit the 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.”
Although a 300-year orbit is long in Earth terms, the Northwestern team noted that the planet is much closer to its host stars than the handful of directly imaged exoplanets in binary star systems.
When speculating on the formation of such an unusual system, which appears relatively young, having formed around 13 million years ago (for comparison, Earth formed around 4.54 billion years ago), the researchers suggest the twin stars formed first, with the planet probably forming later. Wang admits how the process works “is still uncertain” and suggests more observations of similar systems will need to be made before any definitive conclusions on formation can be drawn.
“Because we have only detected a few dozen planets like this, we don’t have enough data yet to put the picture together,” he explained.
Searching for More Unexplained Signals
Next, Jones, Wang, and colleagues are already preparing proposals requesting more telescope time to continue analyzing this star system, in hopes of understanding how it formed and how the overall three-body system works.
“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,” Wang explained. “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.”
Seemingly confirming the team’s study, a European team led by researchers from the University of Exeter “independently found the same planet” after a reanalysis of their own data. Encouraged by these types of results, Jones said she is continuing to reanalyze the “years-old” data to see if previous studies had missed anything else.
“Showing that surprises can hide in plain sight, the discovery underscores the continuing scientific value of archival telescope data,” the team explained.
The study “HD 143811 AB b: A directly imaged planet orbiting a spectroscopic binary in Sco-Cen” was published in The Astrophysical Journal Letters.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.