Astronomers observing TOI-3884b found its orbit misaligned by 62° from its star’s rotation, uncovered through multicolour starspot-crossing signals.
The super-Neptune TOI-3884b passing in front of the red dwarf star TOI-3884.
Photo Credit: Mayuko Mori, Astrobiology Center
Astronomers looking into the dwarf-star system TOI-3884b have stumbled upon something pretty intriguing: the planet's orbit doesn't quite match up with the rotation of its host star. They've been using a bunch of ground-based telescopes to spot these unusual "starspot-crossing" events during the planet's transits, which have revealed some odd configurations in the system. This finding challenges the usual ideas about how such systems develop.
As per the paper, using the multicolour instruments MuSCAT3 and MuSCAT4 on 2-metre telescopes of the Las Cumbres Observatory, researchers observed three separate transits in early 2024. Each time the planet passed in front of its red-dwarf star, it crossed cooler, darker starspots — analogous to sunspots — which are roughly 200 K cooler than the star's average surface temperature. These spot-crossing signals, varying in colour, helped to determine the starspot's temperature and extent across the stellar surface.
Extended brightness monitoring between December 2024 and March 2025 revealed the star rotates every 11.05 days. Combining this rotation period with transit data allowed astronomers to reconstruct the three-dimensional alignment of the system. The result: the orbital axis of TOI-3884b is tilted by about 62° relative to the star's spin axis.
Such a large misalignment is often taken as a sign of dynamic past disturbances — for instance, interactions with massive planets or companion stars — yet no such objects have been identified in this system, leaving this cosmic misfit a mystery.
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