The Polarization of the Planet-Hosting WASP-18 System

Abstract

We report observations of the linear polarization of the WASP-18 system, which harbors a very massive (∼ 10 M J ) planet orbiting very close to its star with an orbital period of 0.94 days. We find the WASP-18 system is polarized at ∼200 parts-per-million (ppm), likely from the interstellar medium predominantly, with no strong evidence for phase dependent modulation from reflected light from the planet. We set an upper limit of 40 ppm (99% confidence level) on the amplitude of a reflected polarized light planetary signal. We compare the results with models for a number of processes that may produce polarized light in a planetary system to determine if we can rule out any phenomena with this limit. Models of reflected light from thick clouds can approach or exceed this limit, but such clouds are unlikely at the high temperature of the WASP-18b atmosphere. Additionally, we model the expected polarization resulting from the transit of the planet across the star and find this has an amplitude of ∼1.6 ppm, which is well below our detection limits. We also model the polarization due to the tidal distortion of the star by the massive planet and find this is also too small to be measured currently.

TESS Full Orbital Phase Curve of the WASP-18b System

“…Since the difference between the measured secondary eclipse depth and the expected thermal emission in the TESS bandpass is only at a 0.8σ significance, we cannot claim a detection of reflected light, and therefore place a 2σ upper limit of 35 ppm on the relative contribution of reflected light to the atmospheric brightness. This upper limit is consistent with the upper limit on reflected polarized light from the planet measured by Bott et al (2018).…”

HIPPI-2: A versatile high-precision polarimeter

“…The vertical grey lines show when the primary mirror was realuminised. The HIPPI data and some of the HIPPI-2 data shown here and/or reported in table 8 has been previously reported (Bailey et al, 2015;Cotton et al, 2016a;Marshall et al, 2016;Bott et al, 2016;Cotton et al, 2017a,b;Bott et al, 2018;Cotton et al, 2019a,b;Bailey et al, 2019), but the data have been reprocessed to benefit from refinements in the software. 2.8 ± 0.5 2018JUL/AUG 425SP B 11.9 407.3 3 2 2 0 1 0 0 3 19.8 ± 6.2 49.7 ± 9.4 2018JUL/AUG 500SP B 11.9 445.0 2 2 2 2 1 0 0 2 18.6 ± 1.4 41.2 ± 2.2 2018JUL/AUG g B 11.9 470.4 3 3 2 2 1 0 0 2 13.6 ± 1.1 80.9 ± 2.2 2018JUL/AUG Clear B 11.9 489.4 3 3 2 2 2 0 0 2 10.6 ± 0.9 79.6 ± 2.6 2018JUL/AUG V B 11.9 537.9 1 1 2 0 1 0 0 2 20.7 ± 1.5 87.0 ± 2.1 2018JUL/AUG r B 11.9 605.4 1 2 2 0 1 0 0 3 18.6 ± 1.4 81.3 ± 2.8 2018JUL/AUG r R 11.9 625.6 4 2 0 2 2 0 0 1 12.5 ± 1.2 88.5 ± 2.7 2018JUL/AUG 650LP R 11.9 725.7 3 2 0 2 2 0 0 1 8.1 ± 1.…”

The rotationally modulated polarization of ξ Boo A

“…Searching for polarization from Rayleigh scattering particles in a hot Jupiter atmosphere in the combined light of the star and planet is a difficult prospect Bott et al 2018). In the best case, an orbital period-modulated signal will have an amplitude of up to a few 10s of ppm (Seager et al 2000;Bailey et al 2018).…”

“…This is the usual configuration of the instrument for observation of exoplanet systems, (e.g. the (inactive) WASP-18 system, Bott et al 2018). For the G7 spectral type of ξ Boo A the effective wavelength is 486.1 nm and the modulation efficiency 0.840.…”

“…Wade et al 1996;Rosén et al 2015), but also because it is a potential source of noise in studying other polarimetric phenomena. In particular, a number of groups have been searching for the polarized light that is scattered from the atmosphere of a close hot Jupiter planet, in the combined light of the star and the planet (Berdyugina et al 2011;Lucas et al 2009;Bott et al 2018). If identified, such a signal can reveal details of the planet's atmosphere: its albedo and cloud properties.…”