Since 2016, an ordinary-looking F-type star called KIC 8462852, nicknamed Tabby’s star, has behaved anything but ordinarily. In a new study published in the Monthly Notices of the Royal Astronomical Society on July 6, researchers revisited this star to find out what is really causing its irregular dimming.
Spooky star
Tabby’s star’s light curve shows a string of irregular, asymmetric dimming events. Some reduce its brightness by more than 20%. The depths and durations of these dimmings also vary wildly from one dip to the next. The star also fades slowly over years, more than any of the hundreds of comparable stars studied. It quickly became one of the most studied mysteries in modern astronomy. There was speculation about alien megastructures before more grounded explanations took hold.
Follow-up observations in infrared and optical wavelengths, as well as from ground-based telescopes, have ruled out several possibilities. These include a dusty disk, a colliding asteroid belt and a single catastrophic impact. What has emerged instead suggests clouds of dust passing in front of the star. This dust likely comes from shattered comets, ring systems or other orbiting debris. Exactly what is producing these dips has remained unclear. Whether a hidden companion object is responsible is still an open question.
Now, this new study, led by Cristina Madurga-Favieres of the University of Warwick, revisits Tabby’s star using fresh data. It uses NASA’s Transiting Exoplanet Survey Satellite (TESS) to test these long-standing theories.
A clear signal
In TESS observations, the team found a symmetric, round-shaped dip in the star’s light, lasting about 21 hours and dimming its brightness by just over 1%. This kind of dip, called a transit, is created when an object orbiting the star blocks some of its starlight as it passes in front.
The transit occurred on Sept. 3, 2019, and was not previously reported. This signal looked much cleaner than the other irregular dips seen in this star’s light. Combining years of TESS data with new radial velocity measurements, the team estimated that the object that produced this transit orbits the star roughly every three to four years, though its exact period remains uncertain. Its mass is likely somewhere between 5 and 14 Jupiter masses, and its radius is around 1.7 times Jupiter’s radius. This mass and size range place it right on the boundary between a giant planet and a brown dwarf.
“This work presents a symmetric round-shaped transit found in the TESS data of KIC 8462852, suggesting the existence of a companion, either a giant planet or a brown dwarf, around the star,” the team writes in the paper.
The researchers tested several alternative explanations, including a background eclipsing binary star, a stray dust cloud or a cluster of exocomets. None fit the data as well as a single, solid companion.
Who is the culprit?
The team says future monitoring around the predicted time windows identified in this study should be able to pin down the exact orbital period and confirm whether the companion is truly there.
“JWST and HST observations could be used to observe and study the primordial atmosphere of the companion and, along with the discovery of the next transit, they would help to disclose more information about this system,” the team concludes.
If confirmed, this would be the first companion ever directly detected transiting Tabby’s star, and it could finally explain the star’s decade-long mystery. While its presence produces a repeating transit pattern in the star’s light, its gravity may be responsible for flinging comets and debris into orbits around the star that produce the irregular dimming that first caught astronomers’ attention.
“The strongest theory is that a group of exocomets or planetesimal fragments are responsible for the irregular and apparently non-periodic dips in the Kepler light curve of Tabby’s star,” the team explains. “The presence of a companion would explain why these bodies are driven to the vicinities of the star, breaking up, as it would orbitally perturb them.”
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Publication details
Cristina Madurga-Favieres et al, Evidence for a giant companion orbiting Tabby’s star, Monthly Notices of the Royal Astronomical Society (2026). DOI: 10.1093/mnras/stag1273
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A 21-hour dip could help explain Tabby’s star’s decade-long dimming mystery (2026, August 3)
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