Polarization of Trappist-1 by the Transit of Its Planets

Abstract

Being the first, and currently the only, multiple planet hosting dwarf star that is sufficiently cool to form condensate clouds in its atmosphere, Trappist-1 provides a unique opportunity to test the efficiency of image polarimetry as a tool to detect and characterize exoplanets around L- and late M-dwarfs and exomoons around directly imaged self-luminous giant exoplanets. Although scattering of light by atmospheric dust particles should produce a significant amount of linear polarization in the far optical and near infrared, the disk-averaged net detectable polarization of the star must be zero owing to spherical symmetry. However, the transit of its planets would give rise to significant asymmetry and produce phase-dependent polarization with the peak polarization occurring at the inner contact points of planetary transit ingress and egress epochs. Adopting the known stellar and planetary physical parameters and employing a self-consistent cloudy atmosphere model of the M8 dwarf star, the transit polarization profiles and the expected amount of polarization of Trappist-1 during the transit phase of each individual planet, as well as that during the simultaneous transit of two planets, are presented in this paper. It is emphasized that the amount of polarization expected is within the detection limit of a few existing facilities. If polarization is detected confirming the prediction, time-resolved image polarimetry will emerge as a potential tool to detect and characterize small planets around cloudy ultra-cool dwarfs.