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

References in corpus (14)

• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• The Transiting Exoplanet Survey Satellite
• The James Webb Space Telescope
• Detection of atmospheric haze on an extrasolar planet: The 0.55 - 1.05 micron transmission spectrum of HD189733b with the Hubble Space Telescope
• Large-scale magnetic topologies of late M dwarfs
• Stellar magnetism: empirical trends with age and rotation
• Large-scale magnetic topologies of early M dwarfs
• A Kepler Study of Starspot Lifetimes with Respect to Light Curve Amplitude and Spectral Type
• WASP-42 b and WASP-49 b: two new transiting sub-Jupiters
• No Sun-like dynamo on the active star $\zeta$ Andromedae from starspot asymmetry
• Investigating the young AU~Mic system with SPIRou: large-scale stellar magnetic field and close-in planet mass
• The global and small-scale magnetic fields of fully convective, rapidly spinning M dwarf pair GJ65A and B
• Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating
• TOI-3884 b: A rare 6-R${}_{\oplus }$ planet that transits a low-mass star with a giant and likely polar spot