[Paper Review] Uses of Linear Polarization as a Probe of Extrasolar Planet Atmospheres

Steven H. Saar, Sara Seager|arXiv (Cornell University)|May 22, 2003

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper proposes using linear polarization (LP) to detect and characterize extrasolar giant planets by exploiting their high fractional LP (up to ~40%) compared to their host stars, which exhibit very low LP (~10⁻⁴). By measuring LP as a function of orbital phase, the method enhances star-planet contrast by a factor of ~200 and provides detailed diagnostics on atmospheric scatterers' composition, size, and shape, offering a powerful alternative to intensity-based detection.

ABSTRACT

We point out some advantages of making observations of extrasolar planets in linearly polarized (LP) light. Older cool stars have quite low levels (~ 10^-4 to 10^-5) of fractional LP, while extrasolar planets can have relatively high fractional LP (~0.1). Observations in LP light can therefore significantly enhance contrast between the planet and its parent star. Data on LP as a function of planetary orbital phase can be used to diagnose the properties (e.g., composition, size, and shape) of the scatterers in the planetary atmosphere. We discuss the feasibility of LP observations of extrasolar planets.

Motivation & Objective

• To investigate the feasibility of using linear polarization (LP) as a detection method for extrasolar planets.
• To assess how LP measurements improve contrast between planets and their host stars compared to intensity-based observations.
• To determine whether LP data can diagnose atmospheric properties such as composition, particle size, and shape of scatterers in exoplanet atmospheres.
• To evaluate the potential of LP polarimetry and spectroscopy for detecting reflected light from close-in extrasolar giant planets (CEGPs).
• To compare the diagnostic power of LP curves versus intensity light curves in distinguishing atmospheric particle properties.

Proposed method

• Modeling the broadband linear polarization (BLP) of host stars using magnetic intensification effects in optically thick lines, based on Saar & Huovelin (1993).
• Simulating planetary LP using Monte Carlo scattering techniques with a fiducial model of a 1.34R_J planet at 0.05 AU with MgSiO₃ clouds and a log-normal particle size distribution (mean 5 μm, σ = 1.5 μm).
• Comparing LP and intensity phase curves for different atmospheric compositions (MgSiO₃, Fe, Al₂O₃) and scattering mechanisms (Rayleigh scattering) to assess diagnostic sensitivity.
• Calculating the star-planet contrast in LP versus intensity, showing a contrast improvement from ~5×10⁻⁵ to ~10⁻² in the V band.
• Proposing two observational strategies: (1) BLP polarimetry to detect a phased LP signal at a few percent level, and (2) LP spectroscopy to distinguish polarized planetary reflection from magnetically polarized stellar lines.
• Using the distinctive spectral signatures of Stokes I (unpolarized stellar spectrum) and Stokes Q/U (polarized stellar lines) to identify planetary reflected light.

Experimental results

Research questions

• RQ1 Can linear polarization measurements significantly improve the detectability of extrasolar planets by enhancing the contrast between the planet and its host star?
• RQ2 How does the phase-dependent linear polarization curve of a planet reveal information about the composition, size, and shape of atmospheric scatterers?
• RQ3 To what extent does LP outperform intensity-based light curves in diagnosing atmospheric particle properties?
• RQ4 What observational strategies—polarimetry or spectro-polarimetry—can make LP detection of exoplanets feasible with current or near-future instrumentation?
• RQ5 How do the LP characteristics of host stars (e.g., ~10⁻⁴ fractional polarization) compare to those of planets (e.g., ~0.1), and what does this imply for detection?

Key findings

• The host stars of known close-in extrasolar giant planets (CEGPs) exhibit very low fractional linear polarization, typically ~10⁻⁴, due to magnetic intensification in spectral lines.
• The fiducial model planet with MgSiO₃ clouds produces up to ~40% fractional linear polarization of its reflected light, significantly higher than the host star.
• Linear polarization measurements improve the star-planet contrast in the V band from ~5×10⁻⁵ (intensity) to ~10⁻² (LP), a factor of ~200 enhancement.
• LP curves preserve information about single-scattering properties and particle characteristics better than intensity light curves, which are smoothed by multiple scattering.
• The LP curve is more sensitive than the intensity curve to differences in particle composition (e.g., MgSiO₃ vs. Fe vs. Al₂O₃) and size distribution, enabling better atmospheric diagnostics.
• LP spectroscopy offers a promising method to distinguish planetary reflected light (Stokes I, linearly polarized) from stellar lines (Stokes Q/U, magnetically polarized), aiding detection in multi-line observations.