Looking for the rainbow on exoplanets covered by liquid and icy water clouds

Karalidi, Theodora; Stam, Daphné; Hovenier, J. W.

doi:10.1051/0004-6361/201220245

Cited by 48 publications

(67 citation statements)

References 56 publications

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“…This may indicate that other scattering components, for example aerosols, thin liquid water clouds, and ice clouds contribute significantly to the Earth polarization in the I band (see Karalidi et al 2012).…”

Section: Summary and Discussionmentioning

confidence: 99%

“…In the models, only very thick, liquid water clouds were included, but no thin liquid water clouds and no ice clouds. Karalidi et al (2012) shows that with more realistic cloud properties for Earth, the degree of polarization can vary strongly depending on the cloud's optical thickness. Our data could now be used to test and to improve model calculations for the Earth polarization.…”

Section: Comparison With the Models From Stam (2008)mentioning

confidence: 99%

“…Model calculations have been made for the fractional polarization of the reflected light from Earth-like planets (Stam 2008), as well as the polarization produced by reflecting clouds (e.g., Karalidi et al 2011Karalidi et al , 2012Bailey 2007) or glint from ocean water surfaces (Williams & Gaidos 2008). The models provide an adequate description of the dominating scattering processes and the signatures of different surface types.…”

Section: Introductionmentioning

confidence: 99%

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Measurement of the earthshine polarization in theB, V, R, andIbands as function of phase

Bazzon

Schmid

Gisler

2013

A&A

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Context. Earth-like, extrasolar planets may soon become observable with upcoming high contrast polarimeters. Therefore, the characterization of the polarimetric properties of the planet Earth is important for interpreting expected observations and planning of future instruments. Aims. Benchmark values for the polarization signal of integrated light from the planet Earth in broad band filters are derived from new polarimetric observations of the earthshine backscattered from the Moon's dark side. Methods. The fractional polarization of the earthshine p es is measured in the B, V, R, and I filters for Earth-phase angles α between 30• and 110• with a new, specially designed wide field polarimeter. In the observations, the light from the bright lunar crescent is blocked with focal plane masks. Because the entire Moon is imaged, the earthshine observations can be corrected for the stray light from the bright lunar crescent and twilight. The phase dependence of p es is fitted by a function p es = q max sin 2 α. Depending on wavelength λ and the lunar surface albedo a, the polarization of the backscattered earthshine is significantly reduced. To determine the polarization of the planet Earth, we correct our earthshine measurements by a polarization efficiency function for the lunar surface (λ, a) derived from measurements of lunar samples from the literature. Results. The polarization of the earthshine decreases toward longer wavelengths and is about a factor 1.3 lower for the higher albedo highlands. For mare regions the measured maximum polarization is about q max,B = 13% for α = 90• (half moon) in the B band. The resulting fractional polarizations for the planet Earth derived from our earthshine measurements and corrected by (λ, a) are 24.6% for the B band, 19.1% for the V band, 13.5% for the R band, and 8.3% for the I band. Together with the literature values for the spectral reflectivity, we obtain a contrast C p between the polarized flux of the planet Earth and the (total) flux of the Sun with an uncertainty of less than 20%, and we find that the best phase for detecting an Earth twin is around α = 65• . Conclusions. The obtained results provide a multiwavelength and multiphase set of benchmark values that are useful for assessing different instrument and observing strategies for the future high contrast polarimetry of extrasolar planetary systems. Polarimetric models of Earth-like planets are in qualitative agreement with our results, but there are also significant differences that might guide more detailed computations.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Comparison With the Models From Stam (2008)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Measurement of the earthshine polarization in theB, V, R, andIbands as function of phase

Bazzon

Schmid

Gisler

2013

A&A

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“…Phase curves of Earth covering phases near full illumination have not been obtained (Mallama 2009), but high-lying ice clouds (Karalidi et al 2012) and photometric variability due to weather patterns (Karalidi et al 2012;Livengood et al 2011) will probably mask a virtual glory. For Titan, the optically thick haze and ice condensates above the sparse, tropospheric liquid methane clouds will almost certainly prevent potential glories from being viewed from afar.…”

Section: Glories In the Solar System Venus As An Exoplanetmentioning

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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“…While the rainbow peak could be visible in the phase light curve, it is better defined, particularly for small cloud particles, in the polarisation phase curve (Bailey 2007;Karalidi et al 2012). The size of the disk integrated rainbow polarisation signal for the Earth has not been measured, but in principle could be obtained from lunar earthshine polarisation measurements.…”

Section: Signatures Of Habitabilitymentioning

confidence: 99%

The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

Bailey

2014

Publ. Astron. Soc. Aust.

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The last few years has seen a dramatic increase in the number of exoplanets known and in the range of methods for characterising their atmospheric properties. At the same time, new discoveries of increasingly cooler brown dwarfs have pushed down their temperature range which now extends down to Y-dwarfs of < 300 K. Modelling of these atmospheres has required the development of new techniques to deal with the molecular chemistry and clouds in these objects. The atmospheres of brown dwarfs are relatively well understood, but some problems remain, in particular the behavior of clouds at the L/T transition. Observational data for exoplanet atmosphere characterisation is largely limited to giant exoplanets that are hot because they are near to their star (hot Jupiters) or because they are young and still cooling. For these planets there is good evidence for the presence of CO and H2O absorptions in the IR. Sodium absorption is observed in a number of objects. Reflected light measurements show that some giant exoplanets are very dark, indicating a cloud free atmosphere. However, there is also good evidence for clouds and haze in some other planets. It is also well established that some highly irradiated planets have inflated radii, though the mechanism for this inflation is not yet clear. Some other issues in the composition and structure of giant exoplanet atmospheres such as the occurrence of inverted temperature structures, the presence or absence of CO2and CH4, and the occurrence of high C/O ratios are still the subject of investigation and debate.

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Looking for the rainbow on exoplanets covered by liquid and icy water clouds

Cited by 48 publications

References 56 publications

Measurement of the earthshine polarization in theB, V, R, andIbands as function of phase

Measurement of the earthshine polarization in theB, V, R, andIbands as function of phase

Glory revealed in disk-integrated photometry of Venus

The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

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