# Time-resolved image polarimetry of Trappist-1 during planetary transits

## TL;DR

This study used time-resolved polarimetry during planetary transits of Trappist-1 to detect atmospheric dust, revealing enhanced polarization signals that suggest a dusty atmosphere of the host star, with implications for exoplanet characterization.

## Contribution

First detection of increased linear polarization during transits of Trappist-1, indicating a dusty atmosphere and demonstrating polarimetry as a tool for characterizing ultra-cool dwarf atmospheres.

## Key findings

- Enhanced polarization during transits confirms dusty atmosphere
- Polarization signals are low but detectable with current methods
- Potential for polarization to validate transiting planets around ultra-cool dwarfs

## Abstract

We obtained linear polarization photometry ($J$-band) and low-resolution spectroscopy ($ZJ$-bands) of Trappist-1, which is a planetary system formed by an M8-type low-mass star and seven temperate, Earth-sized planets. The photopolarimetric monitoring campaign covered 6.5 h of continuous observations including one full transit of planet Trappist-1d and partial transits of Trappist-1b and e. The spectrophotometric data and the photometric light curve obtained over epochs with no planetary transits indicate that the low-mass star has very low level of linear polarization compatible with a null value. However, the "in transit" observations reveal an enhanced linear polarization signal with peak values of $p^* = 0.1\,\%$ with a confidence level of 3 $\sigma$, particularly for the full transit of Trappist-1d, thus confirming that the atmosphere of the M8-type star is very likely dusty. Additional observations probing different atmospheric states of Trappist-1 are needed to confirm our findings, as the polarimetric signals involved are low. If confirmed, polarization observations of transiting planetary systems with central ultra-cool dwarfs can become a powerful tool for the characterization of the atmospheres of the host dwarfs and the validation of transiting planet candidates that cannot be corroborated by any other method.