Polarimetry of Exoplanets
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POLARIMETRY OF EXOPLANETS Max Millar -Blanchaer*,a, Suniti Sanghavia, Sloane Wiktorowiczb, Rebecca Jensen-Clemc Vanessa Baileya, Kimberly Bottd, James BreckinriDgee,f, Jeffrey Chilcoteg,h, Nicolas Cowani, Michael FitzgeralDj, Paul Kalasc, TheoDora KaraliDik, Tiffany Katariaa, John Krista, MereDith Kupinskil, Franck Marchisµ, Mark Marleyn, Stan Metchevo, Rebecca Oppenheimerp, Marshall Perrinq, Laurent Pueyoq, Tyler Robinsonr, Sara Seagers, William Sparksq, Robert Stencelt, Gautam Vasishta, Ji Wangf, Jason Wangc, Robert Westa, Schuyler Wolffu, Robert T. Zellema aJet Propulsion Laboratory, California Institute of Technology; bAerospace Corporation; cUniversity of California, Berkeley; dUniversity of Washington, Virtual Planetary Lab; eUniversity of Arizona; fCalifornia Institute of Technology, gStanforD University; hUniversity of Notre Dame; iMcGill University; jUniversity of California, Los Angeles; kUniversity of California, Santa Cruz; lCollege of Optical Sciences, University of Arizona; µSETI Institute; nNASA Ames Research Center; oUniversity of Western Ontario; pAmerican Museum of Natural History; qSpace Telescope Science Institute; rNorthern Arizona University; sMassachusetts Institute of Technology; tUniversity of Denver; uLeiden Observatory *[email protected] | 1 (626) 840 9193 © 2018 California Institute of Technology Executive Summary Polarimetry is an extremely useful tool for the characterization of exoplanets. Polarimetric observations with future telescopes have the potential to revolutionize our understanding of scattering processes in the atmospheres and on the surfaces of planets. In particular, time-series and spectropolarimetric measurements can distinguish between different cloud and surface types. Notably, polarimetry has the ability to constrain planetary albedos and may ultimately be able to reveal the presence of a liquid water surface. To maximize the gain from polarimetric measurements careful attention must be paid to the design and implementation of future instrument designs. 1.0 Introduction The past decade has seen the rise of broadband photometric and spectroscopic characterization of exoplanets, revealing a wide variety of different atmospheric properties. However, many of the systems observed via transit spectroscopy have revealed high-altitude hazes that hamper detailed atmospheric characterization at sub- micron wavelengths (e.g. Kreidberg et al. 2014; Sing et al. 2016). While longer wavelength observations (such as NIR observations from HST, Spitzer and in the near future, JWST) can probe deeper atmospheric levels, model degeneracies make it difficult to differentiate hazes from clouds, and to determine their chemical composition. Additionally, discrepancies between directly-imaged planet spectra and atmospheric models have highlighted the need for more sophisticated treatments of clouds and energy transport mechanisms (e.g. Marley & Robinson 2015; Tremblin et al. 2015; Rajan et al. 2017). Thus, for both unresolved and directly imaged planets a more thorough understanding of the atmospheric scattering components is required. Polarimetry provides an avenue to rectify this problem and will be able to address critical questions as to the nature of the reflected-light planets that will be detected with future telescopes. Polarimetry is highly complementary to standard photometric and spectroscopic techniques because it leverages the full vectorial nature of light. Molecules, hazes, clouds and different surfaces all produce unique polarization signatures, while their total intensity flux densities can often be degenerate (see Section 2). In this respect, polarimetric measurements of exoplanet atmospheres have the potential to provide critical empirical constraints on atmospheric properties inaccessible through other observational means. This potential is exemplified by previous solar system studies, where polarimetry has played an important role in the understanding of planet atmospheres1. Notably, polarimetric measurements of Venus’ atmosphere at a range of scattering angles suggest that the upper atmosphere of Venus is dominated by a haze of liquid sulfuric acid droplets (Hansen & Hovenier 1974). In this white paper, we will summarize the current models of exoplanet polarimetry (Section 2), and discuss efforts to measure polarization of both unresolved exoplanets (Section 3) and directly imaged exoplanets (Section 4). Finally, in Section 5 we make recommendations for the coming decade on how to maximize the unique diagnostic power of polarimetry with the next generation of telescopes. 1 See Chapters 17 and 19 of “Polarimetry of Stars and Planetary Systems”, Cambridge University Press, 2015. Figure 1: The flux F and polarization P of starlight reflected by three exoplanet models, model 1 bearing only molecules, model 2 featuring a tropospheric cloud layer in addition to molecules, and model 3 bearing stratospheric haze (Figure reproduced from Stam et al. 2004). Models 2 and 3 are nearly identical in total Flux, but show significantly different polarization signatures. 2.0 Modeling Polarized Planets The degree of polarization of starlight that has been reflected by a planet depends strongly on the composition and structure of the planetary atmosphere. Seager et al. (2000) and Stam et al. (2004) were the first to use numerical simulations of polarized spectra of starlight reflected by Jupiter-like exoplanets to show that polarimetry could be used both to detect these planets and to characterize their atmospheres. Their work revealed that a planet’s degree of polarization as a function of wavelength (Figure 1) and orbital phase (Figure 2) can act as critical discriminators between different atmospheric models whose total intensity photometry and spectra are nearly identical. Further modelling studies have demonstrated that polarimetry can provide information not only on the presence of clouds or hazes in a planet’s atmosphere, but also on the characteristics of clouds/hazes (Karalidi et al. 2011, 2012; de Kok et al. 2011), cloud patchiness (Karalidi & Stam 2012; Karalidi et al. 2013), surface features and/or composition (Stam 2008), the shape of the planet (Marley and Sengupta 2011; de Kok et al. 2011; Stolker et al. 2017) and its line of sight inclination angle (Sanghavi et al. 2018, in prep). Of particular relevance to the search for habitability, reflection off liquid water oceans may provide a unique polarimetric signature that differs from other surface types (Zugger et al. 2010; Robinson 2017). 3.0 Unresolved Planet Polarimetry Many short-period, transiting hot Jupiter exoplanets appear to harbor haze particles in their atmospheres (Iyer et al. 2016; Sing et al. 2016). Monitoring the integrated broadband polarized light (i.e. star+planet) throughout a planet’s orbit (rather than only during transit or occultation) probes the polarized scattering phase function of the atmosphere across a large range of phase angles, which can provide strong constraints on cloud and haze particle size distribution and composition (via index of refraction). Furthermore, polarimetry is inherently a differential technique, as it measures a ratio instead of a scalar (such as when measuring a radiance via photometry or spectroscopy), which dramatically reduces instrumental systematics. Nonetheless, detecting reflected-light polarization from planets spatially unresolvable from their host stars requires modulation at kHz frequencies to overcome instrumental systematics in order to achieve the necessary instrumental polarimetric precisions of ~10-6. While early polarimetry of close-in hot Jupiters has been ambiguous (Berdyugina et al. 2008, 2011; Lucas et al. 2009; Wiktorowicz 2009; Wiktorowicz et al. 2015; Bott et al. 2016), recent Figure 2:The flux F and polarization P of reflected starlight averaged over 650--950 nm for an inclination angle of 90º for the three model atmospheres from Figure 1 (Figure reproduced from Stam et al. 2004). observations of an exoplanet have revealed the existence of 0.035-0.11 μm corundum (Al2O3) haze particles in its upper atmosphere (Wiktorowicz et al., 2018, in prep). Moutou et al. (2007) have also suggested that high resolution spectropolarimetry may provide another means of characterizing unresolved planets, using similar cross correlation techniques currently being applied to detect planets in total intensity (Martins et al. 2015). Simulations by Munoz (2018) suggest that current telescopes may be able to detect a WASP 7b-like planet around a 51 Peg-like star (V=5.5). Both broadband and spectropolarimetric techniques will typically be photon-limited, so characterizing smaller planets around fainter stars will require moving to 30-40m telescopes. 4.0 Directly Imaged Planet Polarimetry Measuring the polarization of directly-imaged planets that are resolved separately from their host stars has the advantage that the intensity of the uncorrected star light left after the AO and coronagraph systems is orders of magnitude smaller than the stellar light in the case of unresolved planets, lowering the required polarimetric precision. This fact makes measuring polarimetry an extremely powerful tool for characterizing the atmospheric scatterers in both the currently-accessible self-luminous planets, as well as the reflected-light planets that will be detected with future generations of telescopes. 4.1 Self-luminous Planets The thermal emission of young self-luminous giant planets is expected to have linear polarizations at the ~1% level and below, similar to the polarization