Light Scattering From Exoplanet Oceans and Atmospheres

Zugger, Michael E.; Kasting, James F.; Williams, D. M.; Kane, Timothy J.; Philbrick, C. Russell

doi:10.1088/0004-637x/723/2/1168

Cited by 97 publications

(88 citation statements)

References 27 publications

(34 reference statements)

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“…Madhusudhan & Burrows (2012) provided an analytical solution for scattering models for Lambertian, Rayleigh, isotropic, and asymmetric scattering. Zugger et al (2010) Rayleigh and Lambertian surface models follow the same general S-curve shape between zero flux and full flux. A Lambertian or Rayleigh planet is faint at small α because the illuminated fraction f I is small, hence fewer photons are reflected.…”

Section: Rayleigh Scattering Polarizationmentioning

confidence: 84%

“…Note also that the lower the albedo values, the less likely multiple-scattering becomes and the smaller the errors induced by this approximation. The results were benched-marked against several other models providing photopolarimetric curves as a function of one or several orbital parameters, such as models developed by Buenzli & Schmid (2009), Madhusudhan & Burrows (2012, Fluri & Berdyugina (2010), and Zugger et al (2010). The locations of the polarization peaks and minima were reproduced with very good agreement; they mostly depend on the phase functions and the processing of orbital parameters.…”

Section: Polarized and Unpolarized Albedomentioning

confidence: 94%

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Predicting exoplanet observability in time, contrast, separation, and polarization, in scattered light

Schworer

Tuthill

2015

A&A

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Context. Polarimetry is one of the keys to enhanced direct imaging of exoplanets. Not only does it deliver a differential observable providing extra contrast, but when coupled with spectroscopy, it also reveals valuable information on the exoplanetary atmospheric composition. Nevertheless, angular separation and contrast ratio to the host-star make for extremely challenging observation. Producing detailed predictions for exactly how the expected signals should appear is of critical importance for the designs and observational strategies of tomorrow's telescopes. Aims. We aim at accurately determining the magnitudes and evolution of the main observational signatures for imaging an exoplanet: separation, contrast ratio to the host-star and polarization as a function of the orbital geometry and the reflectance parameters of the exoplanet. Methods. These parameters were used to construct a polarized-reflectance model based on the input of orbital parameters and two albedo values. The model is able to calculate a variety of observational predictions for exoplanets at any orbital time.Results. The inter-dependency of the three main observational criteria -angular separation, contrast ratio, polarization -result in a complex time-evolution of the system. They greatly affect the viability of planet observation by direct imaging. We introduce a new generic display of the main observational criteria, which enables an observer to determine whether an exoplanet is within detection limits: the Separation-POlarization-Contrast (SPOC) diagrams. Conclusions. We explore the complex effect of orbital and albedo parameters on the visibility of an exoplanet. The code we developed is available for public use and collaborative improvement on the python package index, together with its documentation. It is another step towards a full comprehensive simulation tool for predicting and interpreting the results of future observational exoplanetary discovery campaigns.

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Section: Rayleigh Scattering Polarizationmentioning

confidence: 84%

Section: Polarized and Unpolarized Albedomentioning

confidence: 94%

Predicting exoplanet observability in time, contrast, separation, and polarization, in scattered light

Schworer

Tuthill

2015

A&A

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“…Unlike polarisation phase curves, which can be rich in diagnostic features that provide information about the fundamental properties of the scattering particles in the atmosphere (Bailey 2007;Hansen & Arking 1971;Hansen & Hovenier 1974;Zugger et al 2010Zugger et al , 2011, photometry is often less informative (Arking & Potter 1968). Features that appear distinctly in the polarisation phase curves of Venus such as the glory, primary rainbow, or anomalous diffraction (Hansen & Arking 1971;Hansen & Hovenier 1974) were key in the investigation of the Venus clouds.…”

Section: Introductionmentioning

confidence: 99%

Glory revealed in disk-integrated photometry of Venus

Muñoz

Pérez‐Hoyos

Sánchez‐Lavega

2014

A&A

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Context. Reflected light from a spatially unresolved planet yields unique insight into the overall optical properties of the planet cover. Glories are optical phenomena caused by light that is backscattered within spherical droplets following a narrow distribution of sizes; they are well known on Earth as localised features above liquid clouds. Aims. Here we report the first evidence for a glory in the disk-integrated photometry of Venus and, in turn, of any planet. Methods. We used previously published phase curves of the planet that were reproduced over the full range of phase angles with model predictions based on a realistic description of the Venus atmosphere. We assumed that the optical properties of the planet as a whole can be described by a uniform and stable cloud cover, an assumption that agrees well with observational evidence. Results. We specifically show that the measured phase curves mimic the scattering properties of the Venus upper-cloud micron-sized aerosols, also at the small phase angles at which the glory occurs, and that the glory contrast is consistent with what is expected after multiple scattering of photons. In the optical, the planet appears to be brighter at phase angles of ∼11-13 • than at full illumination; it undergoes a maximum dimming of up to ∼10% at phases in between. Conclusions. Glories might potentially indicate spherical droplets and, thus, extant liquid clouds in the atmospheres of exoplanets. A prospective detection will require exquisite photometry at the small planet-star separations of the glory phase angles.

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“…An optical mission could constrain rotation rate (Pallé et al 2008;Oakley & Cash 2009) and obliquity , 2011Fujii & Kawahara 2012). Furthermore, the presence of oceans could be inferred via their colors (Ford et al 2001;Cowan et al 2009Fujii et al 2010), polarization (Zugger et al 2010(Zugger et al , 2011, and/or specular reflection (Williams & Gaidos 2008, but confounding effects include clouds, Robinson et al 2010, andsnow, Cowan et al 2012). These quantities would improve our understanding of planet formation and geophysics, and would make it possible to convert observed thermal phase variations into estimates of intrinsic seasonal cycles.…”

Section: Constraining the Climate Of Hz Exoplanetsmentioning

confidence: 99%

Thermal Phases of Earth-Like Planets: Estimating Thermal Inertia From Eccentricity, Obliquity, and Diurnal Forcing

Cowan

Voigt

Abbot

2012

ApJ

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In order to understand the climate on terrestrial planets orbiting nearby Sun-like stars, one would like to know their thermal inertia. We use a global climate model to simulate the thermal phase variations of Earth analogs and test whether these data could distinguish between planets with different heat storage and heat transport characteristics. In particular, we consider a temperate climate with polar ice caps (like the modern Earth) and a snowball state where the oceans are globally covered in ice. We first quantitatively study the periodic radiative forcing from, and climatic response to, rotation, obliquity, and eccentricity. Orbital eccentricity and seasonal changes in albedo cause variations in the global-mean absorbed flux. The responses of the two climates to these global seasons indicate that the temperate planet has 3× the bulk heat capacity of the snowball planet due to the presence of liquid water oceans. The obliquity seasons in the temperate simulation are weaker than one would expect based on thermal inertia alone; this is due to cross-equatorial oceanic and atmospheric energy transport. Thermal inertia and cross-equatorial heat transport have qualitatively different effects on obliquity seasons, insofar as heat transport tends to reduce seasonal amplitude without inducing a phase lag. For an Earth-like planet, however, this effect is masked by the mixing of signals from low thermal inertia regions (sea ice and land) with that from high thermal inertia regions (oceans), which also produces a damped response with small phase lag. We then simulate thermal light curves as they would appear to a high-contrast imaging mission (TPF-I/Darwin). In order of importance to the present simulations, which use modern-Earth orbital parameters, the three drivers of thermal phase variations are (1) obliquity seasons, (2) diurnal cycle, and (3) global seasons. Obliquity seasons are the dominant source of phase variations for most viewing angles. A pole-on observer would measure peak-to-trough amplitudes of 13% and 47% for the temperate and snowball climates, respectively. Diurnal heating is important for equatorial observers (∼5% phase variations), because the obliquity effects cancel to first order from that vantage. Finally, we compare the prospects of optical versus thermal direct imaging missions for constraining the climate on exoplanets and conclude that while zero-and one-dimensional models are best served by thermal measurements, second-order models accounting for seasons and planetary thermal inertia would require both optical and thermal observations.

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Light Scattering From Exoplanet Oceans and Atmospheres

Cited by 97 publications

References 27 publications

Predicting exoplanet observability in time, contrast, separation, and polarization, in scattered light

Predicting exoplanet observability in time, contrast, separation, and polarization, in scattered light

Glory revealed in disk-integrated photometry of Venus

Thermal Phases of Earth-Like Planets: Estimating Thermal Inertia From Eccentricity, Obliquity, and Diurnal Forcing

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