Construction of an Earth Model: Analysis of Exoplanet Light Curves and Mapping the Next Earth With the New Worlds Observer

2012

A&A

Aims. We present modeled flux and linear polarization signals of starlight that is reflected by spatially unresolved, horizontally inhomogeneous planets and discuss the effects of including horizontal inhomogeneities on the flux and polarization signals of Earth-like exoplanets. Methods. Our code is based on an efficient adding-doubling algorithm, which fully includes multiple scattering by gases and aerosol/cloud particles. We divide a model planet into pixels that are small enough for the local properties of the atmosphere and surface (if present) to be horizontally homogeneous. Given a planetary phase angle, we sum up the reflected total and linearly polarized fluxes across the illuminated and visible part of the planetary disk, taking care to properly rotate the polarized flux vectors towards the same reference plane. Results. We compared flux and polarization signals of simple horizontally inhomogeneous model planets against results of the weighted sum approximation, in which signals of horizontally homogeneous planets are combined. Apart from cases in which the planet has only a minor inhomogeneity, the signals differ significantly. In particular, the shape of the polarization phase function appears to be sensitive to the horizontal inhomogeneities. The same holds true for Earth-like model planets with patchy clouds above an ocean and a sandy continent. Our simulations clearly show that horizontal inhomogeneities leave different traces in flux and polarization signals. Combining flux with polarization measurements would help retrieving the atmospheric and surface patterns on a planet.

Section: Introductionmentioning

confidence: 72%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Modeled flux and polarization signals of horizontally inhomogeneous exoplanets applied to Earth-like planets

2012

A&A

“…Cloud coverage and variability can also influence to a large degree the interpretation of the observations. Oakley & Cash (2009) for example find that mapping the planetary surface is only possible for cloud coverages smaller than the mean Earth one. Even in the case of giant planets or dwarf stars, clouds play a crucial role in defining the atmospheric thermal profile and eventually spectra (Marley et al 2010).…”

Section: Introductionmentioning

confidence: 99%

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

Hovenier

2012

A&A

Aims. Looking for the primary rainbow in starlight that is reflected by exoplanets appears to be a promising method to search for liquid water clouds in exoplanetary atmospheres. Ice water clouds, that consist of water crystals instead of water droplets, could potentially mask the rainbow feature in the planetary signal by covering liquid water clouds. Here, we investigate the strength of the rainbow feature for exoplanets that have liquid and icy water clouds in their atmosphere, and calculate the rainbow feature for a realistic cloud coverage of Earth. Methods. We calculate flux and polarization signals of starlight that is reflected by horizontally and vertically inhomogeneous Earth-like exoplanets, covered by patchy clouds consisting of liquid water droplets or water ice crystals. The planetary surfaces are black. Results. On a planet with a significant coverage of liquid water clouds only, the total flux signal shows a weak rainbow feature. Any coverage of the liquid water clouds by ice clouds, however, dampens the rainbow feature in the total flux, and thus the discovery of liquid water in the atmosphere. On the other hand, detecting the primary rainbow in the polarization signal of exoplanets appears to be a powerful tool for detecting liquid water in exoplanetary atmospheres, even when these clouds are partially covered by ice clouds. In particular, liquid water clouds covering as little as 10-20% of the planetary surface, with more than half of these covered by ice clouds, still create a polarized rainbow feature in the planetary signal. Indeed, calculations of flux and polarization signals of an exoplanet with a realistic Earth-like cloud coverage, show a strong polarized rainbow feature.

“…EPICS is still in its design and optimisation phase, and first light is expected not earlier than 2020. An example of a space telescope concept for exoplanet research that would be ideally suited to observe both the flux and the state of polarisation of starlight that is reflected by exoplanets is the New Worlds Observer (NWO) that has been and will be proposed to NASA (Oakley & Cash 2009;Cash & New Worlds Study Team 2010). An example of a space-telescope with polarimetric capabilities for exoplanet research that has been proposed to the European Space Agency in response to its Cosmic Vision 2015−2020 call for a medium sized mission (M3), is Spectro-Polarimetric Imaging and Characterization of Exo-planetary Systems, or SPICES.…”

Section: Introductionmentioning

confidence: 99%

Flux and polarisation spectra of water clouds on exoplanets

Hovenier

2011

A&A

Context.A crucial factor for a planet's habitability is its climate. Clouds play an important role in planetary climates. Detecting and characterising clouds on an exoplanet is therefore crucial when addressing this planet's habitability. Aims. We present calculated flux and polarisation spectra of starlight that is reflected by planets covered by liquid water clouds with different optical thicknesses, altitudes, and particle sizes, as functions of the phase angle α. We discuss the retrieval of these cloud properties from observed flux and polarisation spectra. Methods. Our model planets have black surfaces and atmospheres with Earth-like temperature and pressure profiles. We calculate the spectra from 0.3 to 1.0 μm, using an adding-doubling radiative transfer code with integration over the planetary disk. The cloud particles' scattering properties are calculated using a Mie-algorithm. Results. Both flux and polarisation spectra are sensitive to the cloud optical thickness, altitude and particle sizes, depending on the wavelength and phase angle α. Conclusions. Reflected fluxes are sensitive to cloud optical thicknesses up to ∼40, and the polarisation to thicknesses up to ∼20. The shapes of polarisation features as functions of α are relatively independent of the cloud optical thickness. Instead, they depend strongly on the cloud particles' size and shape, and can thus be used for particle characterisation. In particular, a rainbow strongly indicates the presence of liquid water droplets. Single scattering features such as rainbows, which can be observed in polarisation, are virtually unobservable in reflected fluxes, and fluxes are thus less useful for cloud particle characterisation. Fluxes are sensitive to cloud top altitudes mostly for α < 60 • and wavelengths <0.4 μm, and the polarisation for α around 90 • and wavelengths between 0.4 and 0.6 μm.