DETECTING EXOMOONS AROUND SELF-LUMINOUS GIANT EXOPLANETS THROUGH POLARIZATION

Abstract

Many of the directly imaged self-luminous gas giant exoplanets have been found to have cloudy atmospheres. Scattering of the emergent thermal radiation from these planets by the dust grains in their atmospheres should locally give rise to significant linear polarization of the emitted radiation. However, the observable disk averaged polarization should be zero if the planet is spherically symmetric. Rotation-induced oblateness may yield a net non-zero disk averaged polarization if the planets have sufficiently high spin rotation velocity. On the other hand, when a large natural satellite or exomoon transits a planet with cloudy atmosphere along the line of sight, the asymmetry induced during the transit should give rise to a net non-zero, time resolved linear polarization signal. The peak amplitude of such time dependent polarization may be detectable even for slowly rotating exoplanets. Therefore, we suggest that large exomoons around directly imaged self-luminous exoplanets may be detectable through time resolved imaging polarimetry. Adopting detailed atmospheric models for several values of effective temperature and surface gravity which are appropriate for self-luminous exoplanets, we present the polarization profiles of these objects in the infrared during transit phase and estimate the peak amplitude of polarization that occurs during the inner contacts of the transit ingress/egress phase. The peak polarization is predicted to range between 0.1 and 0.3 % in the infrared.

Keywords: planets and satellites: atmosphere; planets and satellites: detection; polarization; scattering.

Figure 1

Disk integrated linear polarization at different wavelength bands of a self-luminous spherical exoplanet partially eclipsed by a moon of radius 0.046 times the radius of the planet.

Figure 2

J-band disk integrated polarization of self-luminous, spherical exoplanets partially eclipsed by an exomoon. Thin and thick lines represent the percentage polarization for an exomoon transit with an orbital inclination angle of i = 90° and 88° respectively. Solid lines, dashed-dot lines and dashed lines represents eclipse polarization profile with R_m/R_P =0.046, 0.07 and 0.1 repectively. The orbital period of the exomoon is set to 7 days for all cases. From top to bottom, the horizontal lines represent linear polarization integrated over the disk of a rotation-induced oblate exoplanet (with no eclipse) with spin period 6.1, 6.6, and 15 hours respectively.

Figure 3

J-band polarization of exoplanets with different effective temperature and surface gravity. Solid lines represent exoplanetary models with g = 56ms⁻²; from top to bottom the solid lines represents models with T_eff =1000, 1200, 800 and 600 K. Dashed lines represent the polarization with g = 100ms⁻² and from top to bottom dashed lines represent models with T_eff =1200, 1000, 800 and 600 K. Similarly, dot-dashed line represents model with g = 30ms⁻² and from top to bottom, the dot-dashed lines represent exoplanet models with T_eff =800, 1000, 1200 and 600 K.