Scientists detect gargantuan 'pimple' that has plagued a star for at least 7 years

a close-up of a sunspot — Sunspots photographed in 2014. Spots that are this small are difficult to detect on distant stars, since they result in brightness fluctuations that are too minuscule to avoid being labeled as noise. However, a novel method may have just uncovered a giant spot on an alien star.

(Image credit: NASA Goddard)

Most exoplanets are discovered as they transit, or move across, their parent stars. But a new study details the opposite scenario: As a giant planet crossed its host star, peculiarities in its transit signature revealed a new discovery about the star itself — in particular, a spot that occupies an enormous 7% of the star's surface and has lasted at least seven years.

Nearly 6,000 exoplanets — planets beyond our solar system — have been confirmed to date. While many methods have helped amass this trove, the most successful has been the transit method. This technique, which has helped to reveal nearly 75% of known exoplanets, measures the transient, tiny decrease in a star's brightness when an orbiting planet passes along the line of sight between the star and an observing telescope.

First recorded by NASA's Transiting Exoplanet Survey Satellite (TESS), these asymmetrical transits piqued astronomers' curiosity. In a 2022 study, they reasoned that the strange transits meant the star's surface wasn't uniformly bright, with the fainter patch possibly being a starspot. (Starspots — like sunspots, but on stars other than the sun — are bunches of tangled magnetic-field lines that are dimmer than the surrounding photosphere.)

To determine features of this starspot, the new study’s authors — astrophysicists from Harvard University and MIT — drew on both the TESS observations along with a computational model of TOI-3884 (starspot included) and its planet. Their analysis revealed that the spot, with a radius of 76,000 miles (122,000 kilometers), occupies "about 7% of the surface area of the star," Patrick Tamburo, a postdoctoral researcher at Harvard University and the new study's first author, told Live Science by email. In contrast, the largest recorded sunspots, measuring 99,000 miles (160,000 km) across, cover a mere 0.3% of Earth's sun.

TOI-3884's starspot also differs from sunspots in its lifespan. "On the Sun, the longest-lived spots last for a few months," Tamburo said. But using observational data collected by the Zwicky Transient Facility in California in previous years, the researchers showed that TOI-3884's "pimple" has been around for at least seven years. Tamburo noted that this isn't surprising, however, as polar spots on rapidly rotating stars have been known to last for decades.

But how could a starspot cause such a peculiar pattern? The 2022 study proposed two possibilities. One is that TOI-3884’s day — the time the star takes to rotate around its axis — is equal to (or a multiple of) the time TOI-3884 b takes to orbit the star once. The second possibility is that the planet orbits over one of the star's poles, which hosts a large, long-lasting, starspot that is slightly off-center.

To test the second hypothesis, the researchers relied on their computational model, searching for values for various parameters that would best explain the observations. The best-fitting explanation, they found, is that TOI-3884 b orbits along a near-perpendicular path around its star, whose spot lies at a latitude of 80 degrees. The modeling also implied that the starspot likely pirouetted around the star's pole, making it partly or fully visible to Earth-based viewers, which would explain the slight variations in TOI 3884-b's transits.

Deepa Jain is a freelance science writer from Bengaluru, India. Her educational background consists of a master's degree in biology from the Indian Institute of Science, Bengaluru, and an almost-completed bachelor's degree in archaeology from the University of Leicester, UK. She enjoys writing about astronomy, the natural world and archaeology.