Scientists Detect Gargantuan 'Pimple' That Has Plagued A Star For At Least 7 Years

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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.

Most transit signatures in a star's light curve are similar, comprising a single-step drop. But the planet described in the new study is unusual. Known as TOI-3884 b , it is a Neptune-like world 33 times the mass of Earth. When transiting its star — the tiny M-dwarf star TOI-3884 — the planet creates a two-bump drop in the star's light curve.

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.)

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.