Polarimetric Detection of Exoplanets Transiting T and L Brown Dwarfs

Abstract

While scattering of light by atoms and molecules yields large amount of polarization at the B-band of both T and L dwarfs, scattering by dust grains in cloudy atmosphere of L dwarfs gives rise to significant polarization at the far-optical and infra-red wavelengths where these objects are much brighter. However, the observable disk averaged polarization should be zero if the clouds are uniformly distributed and the object is spherically symmetric. Therefore, in order to explain the observed large polarization of several L dwarfs, rotation-induced oblateness or horizontally inhomogeneous cloud distribution in the atmosphere is invoked. On the other hand, when an extra-solar planet of Earth-size or larger transits the brown dwarf along the line of sight, the asymmetry induced during the transit gives rise to a net non-zero, time dependent polarization. Employing atmospheric models for a range of effective temperature and surface gravity appropriate for T and L dwarfs, I derive the time dependent polarization profiles of these objects during transit phase and estimate the peak amplitude of polarization that occurs during the inner contact points of the transit ingress/egress phase. It is found that peak polarization in the range of 0.2-1.0% at I-and J-band may arise of cloudy L dwarfs occulted by Earth-size or larger exoplanets. Such an amount of polarization is higher than that can be produced by rotation-induced oblateness of even the rapidly rotating L dwarfs. Hence, I suggest that time resolved imaging polarization should be a potential technique to detect transiting exoplanets around L dwarfs.

Time-resolved image polarimetry of TRAPPIST-1 during planetary transits

“…They clearly deviate from the null polarization at the 3 σ confidence level. This result provides a solid confirmation of the linearly polarized atmosphere of Trappist-1 and supports the theory of Sengupta (2016). The transits of Trappist-1e and b were not optimally observed for this project: partial transit coverages and low number of data points acquired during transit, which strongly affects the 3 σ confidence levels of the in-transit mean polarimetric values.…”

“…Another likely asymmetry is the presence of a planet transiting the dwarf's dusty disk (Sengupta 2016(Sengupta , 2018. According to the theory, the net non-zero, time-dependent polarization is maximum at the inner contacts of the planetary ingress and egress phases.…”

Testing the existence of optical linear polarization in young brown dwarfs

“…• Presence of a transiting companion. During the transit, the symmetry of the dusty atmosphere of the young brown dwarf or giant exoplanet is broken, and the linear polarimetric signal should be enhanced (Sengupta 2016). The peak polarization is predicted to range between 0.1-0.3% in the near infrared.…”

Traces of exomoons in computed flux and polarization phase curves of starlight reflected by exoplanets

“…Despite not yet having been exploited, great hope is placed in the polarimetry capabilities of current and future telescopes as powerful tools for detecting and characterizing exoplanets (see Seager et al 2000;Saar & Seager 2003;Stam 2003Stam , 2008Stam et al 2004;Snik & Keller 2013). Previous works in this field involve the modeling of stellar polarization during planetary transits (Carciofi & Magalhães 2005;Kostogryz et al 2011Kostogryz et al , 2015Wiktorowicz & Laughlin 2014;Sengupta 2016), the modeling of light curves and polarization of starlight reflected signals in the visible range of Earth-like planets (Stam 2008;Karalidi et al 2012;Rossi & Stam 2017) and giant Jupiter-like planets (Seager et al 2000; A&A 618, A162 (2018) Stam et al 2004), as well as the modeling of exoplanetary atmospheres in the infrared (De Kok et al 2011;Marley & Sengupta 2011), which demonstrated the usefulness of direct observations for exoplanet characterization. More recently, Bott et al (2016) reported linear polarization observations of the hot Jupiter system HD 189733, and Ginski et al (2018) announced the detection of planetary thermal radiation that is polarized upon reflection by circumstellar dust.…”