107 research outputs found
Modeled flux and polarisation signals of horizontally inhomogeneous exoplanets, applied to Earth--like planets
We present modeled flux and linear polarisation signals of starlight that is
reflected by spatially unresolved, horizontally inhomogeneous planets and
discuss the effects of including horizontal inhomogeneities on the flux and
polarisation signals of Earth-like exoplanets. 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 polarised 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. We compared flux and
polarisation 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 polarisation 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
polarisation signals. Combining flux with polarisation measurements would help
retrieving the atmospheric and surface patterns on a planet
Looking for the rainbow on exoplanets covered by liquid and icy water clouds
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. 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. 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.Comment: accepted for publication in Astronomy & Astrophysic
Observing the Earth as an exoplanet with LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth
The detections of small, rocky exoplanets have surged in recent years and
will likely continue to do so. To know whether a rocky exoplanet is habitable,
we have to characterise its atmosphere and surface. A promising
characterisation method for rocky exoplanets is direct detection using
spectropolarimetry. This method will be based on single pixel signals, because
spatially resolving exoplanets is impossible with current and near-future
instruments. Well-tested retrieval algorithms are essential to interpret these
single pixel signals in terms of atmospheric composition, cloud and surface
coverage. Observations of Earth itself provide the obvious benchmark data for
testing such algorithms. The observations should provide signals that are
integrated over the Earth's disk, that capture day and night variations, and
all phase angles. The Moon is a unique platform from where the Earth can be
observed as an exoplanet, undisturbed, all of the time. Here, we present LOUPE,
the Lunar Observatory for Unresolved Polarimetry of Earth, a small and robust
spectropolarimeter to observe our Earth as an exoplanet.Comment: 14 pages, 3 figures, submitted in special Issue of Planetary and
Space Science on Scientific Preparations for Lunar Exploratio
The infrared colors of 51 Eridani b: micrometereoid dust or chemical disequilibrium?
We reanalyze near-infrared spectra of the young extrasolar giant planet 51
Eridani b which was originally presented in (Macintosh et al. 2015) and (Rajan
et al. 2017) using modern atmospheric models which include a self-consistent
treatment of disequilibrium chemistry due to turbulent vertical mixing. In
addition, we investigate the possibility that significant opacity from
micrometeors or other impactors in the planet's atmosphere may be responsible
for shaping the observed spectral energy distribution (SED). We find that
disequilibrium chemistry is useful for describing the mid-infrared colors of
the planet's spectra, especially in regards to photometric data at M band
around 4.5 m which is the result of super-equilibrium abundances of carbon
monoxide, while the micrometeors are unlikely to play a pivotal role in shaping
the SED. The best-fitting, micrometeroid-dust-free, disequilibrium chemistry,
patchy cloud model has the following parameters: effective temperature
K with clouds (or without clouds, i.e. the grid
temperature = 900 K), surface gravity = 1000 m/s,
sedimentation efficiency = 10, vertical eddy diffusion
coefficient = 10 cm/s, cloud hole fraction
= 0.2, and planet radius = 1.0
R.Comment: 22 pages, 14 figures, Accepted to A
: Pressure-dependent Changes In Light Curve Phase In Brown Dwarfs From Simultaneous and Observations
We present /IRAC Ch1 and Ch2 monitoring of six brown dwarfs during 8
different epochs over the course of 20 months. For four brown dwarfs, we also
obtained simulataneous /WFC3 G141 Grism spectra during two epochs and
derived light curves in five narrow-band filters. Probing different pressure
levels in the atmospheres, the multi-wavelength light curves of our six targets
all exhibit variations, and the shape of the light curves evolves over the
timescale of a rotation period, ranging from 1.4 h to 13 h. We compare the
shapes of the light curves and estimate the phase shifts between the light
curves observed at different wavelengths by comparing the phase of the primary
Fourier components. We use state-of-the-art atmosphere models to determine the
flux contribution of different pressure layers to the observed flux in each
filter. We find that the light curves that probe higher pressures are similar
and in phase, but are offset and often different from the light curves that
probe lower pressures. The phase differences between the two groups of light
curves suggest that the modulations seen at lower and higher pressures may be
introduced by different cloud layers.Comment: 34 pages, 22 figures, accepted for publication in Ap
Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs
Among the greatest challenges in understanding ultracool brown dwarf and exoplanet atmospheres is the evolution of cloud structure as a function of temperature and gravity. In this study, we present the rotational modulations of GU Psc b—a rare mid-T spectral type planetary-mass companion at the end of the L/T spectral type transition. Based on the Hubble Space Telescope/WFC3 1.1–1.67 μm time-series spectra, we observe a quasi-sinusoidal light curve with a peak-to-trough flux variation of 2.7% and a minimum period of 8 h. The rotation-modulated spectral variations are weakly wavelength-dependent, or largely gray between 1.1 and 1.67 μm. The gray modulations indicate that heterogeneous clouds are present in the photosphere of this low-gravity mid-T dwarf. We place the color and brightness variations of GU Psc b in the context of rotational modulations reported for mid-L to late-T dwarfs. Based on these observations, we report a tentative trend: mid-to-late T dwarfs become slightly redder in J − H color with increasing J-band brightness, while L dwarfs become slightly bluer with increasing brightness. If this trend is verified with more T-dwarf samples, it suggests that in addition to the mostly gray modulations, there is a second-order spectral-type dependence on the nature of rotational modulations
Cloud Atlas: High-precision HST/WFC3/IR Time-resolved Observations of Directly Imaged Exoplanet HD 106906b
HD 106906b is an ~11M_(Jup), ~15 Myr old directly imaged exoplanet orbiting at an extremely large distance from its host star. The wide separation (7 11) between HD 106906b and its host star greatly reduces the difficulty in direct-imaging observations, making it one of the most favorable directly imaged exoplanets for detailed characterization. In this paper, we present HST/WFC3/IR time-resolved observations of HD 106906b in the F127M, F139M, and F153M bands. We have achieved ~1% precision in the lightcurves in all three bands. The F127M lightcurve demonstrates marginally detectable (2.7σ significance) variability with a best-fitting period of 4 hr, while the lightcurves in the other two bands are consistent with flat lines. We construct primary-subtracted deep images and use these images to exclude additional companions to HD 106906 that are more massive than 4 M_(Jup) and locate at projected distances of more than ~500 au. We measure the astrometry of HD 106906b in two HST/WFC3 epochs and achieve precisions better than 2.5 mas. The position angle and separation measurements do not deviate from those in the 2004 HST/ACS/HRC images for more than 1σ uncertainty. We provide the HST/WFC3 astrometric results for 25 background stars that can be used as reference sources in future precision astrometry studies. Our observations also provide the first 1.4 μm water band photometric measurement for HD 106906b. HD 106906b's spectral energy distribution and the best-fitting BT-Settl model have an inconsistency in the 1.4 μm water absorption band, which highlights the challenges in modeling atmospheres of young planetary-mass objects
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