# TOI-3884 b: A rare 6-R$_{\oplus}$ planet that transits a low-mass star with a giant and likely polar spot

**Authors:** J. M. Almenara, X. Bonfils, T. Forveille, N. Astudillo-Defru, D. R. Ciardi, R. P. Schwarz, K. A. Collins, M. Cointepas, M. B. Lund, F. Bouchy, D. Charbonneau, R. F. Díaz, X. Delfosse, R. C. Kidwell, M. Kunimoto, D. W. Latham, J. J. Lissauer, F. Murgas, G. Ricker, S. Seager  
**arXiv:** [2210.10909](https://arxiv.org/abs/2210.10909)  
**Date:** 2022-10-19  
**Summary page:** https://www.summarizepaper.com/en/paper/2210.10909v1/

## Key points

- TESS mission identifies unique exoplanet, TOI-3884 b, orbiting mid-M dwarf star with 4.5 day periodicity
- Follow-up observations by ExTrA and LCOGT confirm presence of planet with chromatic transit
- Host star has large polar spot being transited by sub-Saturn planet with radius of $6.00 \pm 0.18$ R$_{\oplus}$
- First sub-Saturn planet discovered transiting mid-M dwarf
- Transit data analyzed to model starspot using MAP model and juliet to normalize TESS, ExTrA, and LCOGT transits
- Brightness and small size of TOI-3884 make it ideal target for atmospheric characterization using ground-based telescopes and space-based observatories like JWST
- Large transmission spectroscopy metrics for its size make it one of top targets for atmospheric characterization among all known planets
- Discovery provides valuable insights into planetary formation around low-mass stars and highlights how advanced observational techniques can help us better understand our universe's diversity

## Summary

The Transiting Exoplanet Survey Satellite (TESS) mission has identified a unique exoplanet, TOI-3884 b, which orbits a mid-M dwarf star with a periodicity of 4.5 days. Follow-up observations by the ExTrA facility and Las Cumbres Observatory Global Telescope (LCOGT) have confirmed the presence of this planet, revealing that its transit is chromatic in nature. The light curves obtained from these observations suggest that the host star has a large polar spot that is being transited by a sub-Saturn planet with a radius of $6.00 \pm 0.18$ R$_{\oplus}$. This makes TOI-3884 b the first sub-Saturn planet to be discovered transiting a mid-M dwarf. Further analysis of the transit data was performed to model the starspot using the MAP model of the analysis with juliet to normalize TESS, ExTrA, and LCOGT transits. The folded transits were then modeled using the starry code with a quadratic limb-darkening law and top hat function in angular separation representing a circular spot with uniform contrast. The inclination of the spin axis and sky-projected spin-orbit angle are free parameters of this model. The brightness and small size of TOI-3884 make it an ideal target for atmospheric characterization using both ground-based telescopes and space-based observatories such as James Webb Space Telescope (JWST). Additionally, owing to its large transmission spectroscopy metrics for its size, it becomes one of the top targets for atmospheric characterization among all known planets. This discovery provides valuable insights into planetary formation around low-mass stars and highlights how advanced observational techniques can help us better understand our universe's diversity and uncover new mysteries about our cosmic home.