The changing transit shape of TOI-3884 b

Abstract

TOI-3884 b is a sub-Saturn transiting a fully convective M-dwarf. Observations indicate that the transit shape is chromatic and asymmetric as a result of persistent starspot crossings. This, along with the lack of photometric variability of the host star, indicates that the rotational axis of the star is tilted along our line of sight and the planet-occulted starspot is located close to the stellar pole. We acquired photometric transits over a period of three years with the Swiss 1.2-meter Euler telescope to track changes in the starspot configuration and detect any signs of decay or growth. The shape of the transit changes over time, and so far no two observations match perfectly. We conclude that the observed variability is likely not caused by changes in the temperature and size of the spot, but due to a slight (5.64 $\pm$ 0.64${}^{\circ }$) misalignment between the spot center and the stellar pole, i.e., a small spin-spot angle ($\Theta$). In addition, we were able to obtain precise measurements of the sky-projected spin-orbit angle ($\lambda$) of 37.3 $\pm$ 1.5\degree, and the true spin-orbit angle ($\psi$) of 54.3 $\pm$ 1.4\degree. The precise alignment measurements along with future atmospheric characterisation with the James Webb Space Telescope will be vital for understanding the formation and evolution of close-in, massive planets around fully convective stars.

Accepted for publication in A&A on September 04th, 2025

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