Polarimetry of Exoplanets

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

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 of brown dwarfs (e.g. Marley & Sengupta 2011; Miles-Páez et al. 2017). In these objects, a polarization signal immediately indicates the presence of atmospheric inhomogeneities (i.e. oblateness or patchy clouds). Programs with both the Gemini Planet Imager (GPI) and VLT/SPHERE are attempting to detect the first polarization of a directly imaged planet (e.g. Jensen-Clem et al. 2016; van Holstein et al. 2017) using the instruments’ NIR broadband polarimetry modes (Perrin et al. 2015; Langlois et al. 2014). In order to achieve the sensitivities required to make these measurements, advanced system modeling and calibrations are now being used to achieve polarimetric precisions of ~10- 3—the typical limit for the slow (i.e. ~1 Hz) modulation schemes implemented in these instruments (Wiktorowicz et al. 2014; Millar-Blanchaer et al. 2016; van Holstein et al. 2017 SPIE). While these measurements represent critical first-steps in the polarimetric characterization of directly-imaged exoplanets, the information content of a single broadband polarimetric measurement is limited due to degeneracies between inhomogeneity type, cloud height and atmospheric scatterer properties. Further information can be gained with time-series monitoring, which may reveal a time-varying polarization produced by a rotating cloud patch, or through spectropolarimetry, where the level of polarization in molecular absorption bands (e.g. H2O, CH4) relative to the continuum level can provide important information on the height of the clouds (de Kok et al. 2011). Indeed, a spectropolarimetric upgrade is being considered for GPI within the next few years. Spectropolarimetric observations of exoplanets with GPI will be complemented by the library of brown dwarf polarized spectra being generated by the WIRC+Pol brown dwarf survey at Palomar. These efforts will set the stage for future spectropolarimetric observations with the next generation of direct imaging instruments on new telescopes that will probe lower mass planets at smaller inner working angles. 4.2 Reflected-light Planets Detecting planets in reflected light via direct imaging will require overcoming flux ratios on the order of 108 and higher, at separations of up to only a few λ/D, even for thirty- meter class telescopes. In this regime polarimetry can act as both an effective speckle suppressor as well as a tool for characterizing planetary atmospheres and surfaces. Solar system observations and a range of atmospheric models have suggested that planets seen in reflected light can have peak degrees of linear polarization ranging from several percent to a few tens of percent (e.g. the Earth is up to ~30% polarized at 0.5 μm; Coffeen 1979). For planets at very small angular separations and these levels of polarization, polarimetric differential imaging (i.e. rejecting unpolarized stellar speckles while detecting the planet in polarized light; PDI) has the potential to significantly enhance other standard PSF-subtraction techniques (e.g. ADI, Marois et al. 2006; SDI, Marois et al. 2000) that are limited by a lack of PSF diversity at small angular separations, whereas PDI suffers from no analogous effect. For example, an instrument able to suppress unpolarized speckles at the 103 level (similar to current GPI and SPHERE/IRDIS levels) will gain a factor of 50 in detection limits relative to the raw contrast for a 5% polarized planet. From the ground, higher levels of unpolarized speckle-rejection will require moving to kHz modulation frequencies, as is done for unresolved planets. The SPHERE/ZIMPOL instrument (Thalmann et al. 2008) is the first instrument to combine fast-switching modulation with a coronagraph system, and has demonstrated the contrasts necessary to detect reflected-light from Jupiter-like planets around α Cen A and B (Roelfsema et al. 2016 SPIE). Several new instruments and telescopes are currently under consideration that have the goal of detecting and characterizing directly-imaged planets in reflected-light from both the ground (e.g. TMT-PSI) and space (WFIRST-CGI, HabEx, LUVOIR). Unfortunately, differential polarimetric aberrations are likely to inhibit the application of PDI with the WFIRST-CGI instrument. Polarimetric imaging systems are also being considered for TMT-PSI, HabEx and LUVOIR. For all these systems, in order to fully leverage the true potential of PDI, careful thought must be given to how different optical architectures ultimately affect the polarization (e.g. Breckinridge et al. 2015). A single polarimetric detection may readily rule out a point source as a background star, since planets are likely to be at least an order of magnitude more polarized than known stars (that have a median polarization of ~0.5%; Heiles 2000). Once detected in polarized light, a planet can be monitored throughout its orbit to sample the polarized scattering phase function at a range of scattering angles, breaking many of the degeneracies experienced for single measurement. The shape of the polarized scattering phase function can then be used to distinguish between different surface and atmospheric configurations. 5. Looking to the future While many previous studies have suggested that polarimetry has the capacity for high- impact measurements of exoplanet atmospheres, relatively little attention has been given to polarimetry within the exoplanet community to date. This is due in part to the difficulty of obtaining high signal-to-noise ratio polarimetric measurements with current facilities, and to the lack of existing polarimetric detections. As we start planning for the next generation of instruments and telescopes, it is important to re-examine polarimetry as a tool for exoplanet detection and characterization. In this respect we have laid out a series of recommendations for the community as we move forward: ● A thorough study is required to determine how polarimetry (e.g. ideal wavelengths and spectral resolutions) can best answer questions such as those related to the properties of atmospheric hazes and the prevalence of liquid water on exo-Earths. In particular, synergies with other observational methods should be explored (e.g. multi-wavelength photometry and/or spectroscopy). ● PDI has the potential to detect planets that will be invisible to other differential imaging techniques. For future direct imaging instruments, systematic studies of the expected polarization of reflected-light planets should be combined with end- to-end instrument simulations to determine the true efficacy with which PDI can detect planets, building off lessons learned from current instruments. The results of such a study should in turn feedback into instrument and mission design. ● Attention should be given to the potential of using PDI in concert with high dispersion coronagraphy (Snellen et al. 2015, Mawet et al. 2017, Wang et al. 2017), which can also be used to overcome high contrasts, but requires significant detector real estate when used in an integral field unit to search for planets. Scenarios where PDI is used to find planets and HDC is used to further characterize them should be considered. ● The maturation of fast-modulating coronagraphic imaging polarimetry technologies is a high priority for the detection of directly-imaged reflected-light planets and should be prioritized in the next decade. Such technology could be potentially be used for unresolved planet polarimetry as well.

References (as hyperlinks) Berdyugina et al. 2008 Karalidi and Stam 2011 Miles-Páez et al. 2017 Stam et al. 2004 Berdyugina et al. 2011 de Kok et al. 2011 Moutou et al. 2007 Stam 2008 Bott et al. 2016 Kreidberg et al. 2014 Munoz et al. 2018, arXiv Stolker et al. 2017 Breckinridge et al. 2015 Kuhn et al. 2001 Perrin et al. 2015 Thalmann et al. 2008 Coffeen et al. 1979 Langlois et al. 2014 Rajan et al. 2017 Tremblin et al. 2015 Hansen and Hovenier 1974 Marley and Robinson 2015 Robinson 2017 Wiktorowicz et al. 2014 Iyer et al. 2016 Marley and Sengupta 2011 Seager et al. 2000 Wiktorowicz et al. 2015 Lucas et al. 2009 Marois et al. 2000 Sengupta and Marley 2009 Zugger et al. 2010 Karalidi et al. 2011 Marois et al. 2006 Sengupta and Marley 2010 Karalidi et al. 2012 Martins et al. 2015 Sing et al. 2016 Karalidi et al. 2013 Millar-Blanchaer et al. 2016 Snellen et al. 2015

Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.