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Directly imaged exoplanets in reflected starlight: the importance of knowing the planet radius

Carrión González, Oscar and Garcia Munoz, A. and Cabrera Perez, Juan and Csizmadia, Szilard and Santos, N. C. and Rauer, H (2020) Directly imaged exoplanets in reflected starlight: the importance of knowing the planet radius. Astronomy & Astrophysics, 640, A136. EDP Sciences. doi: 10.1051/0004-6361/202038101. ISSN 0004-6361.

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Official URL: https://www.aanda.org/articles/aa/abs/2020/08/aa38101-20/aa38101-20.html

Abstract

Context. The direct imaging of exoplanets in reflected starlight will represent a major advance in the study of cold and temperate exoplanet atmospheres. Understanding how basic planet and atmospheric properties may affect the measured spectra is key to their interpretation. Aims: We have investigated the information content in reflected-starlight spectra of exoplanets. We apply our analysis to Barnard's Star b candidate super-Earth, for which we assume a radius 0.6 times that of Neptune, an atmosphere dominated by H2-He, and a CH4 volume mixing ratio of 5 × 10-3. The main conclusions of our study are however planet-independent. Methods: We set up a model of the exoplanet described by seven parameters including its radius, atmospheric methane abundance, and basic properties of a cloud layer. We generated synthetic spectra at zero phase (full disc illumination) from 500 to 900 nm and a spectral resolution R ~ 125-225. We simulated a measured spectrum with a simplified, wavelength-independent noise model at a signal-to-noise ratio of 10. With a retrieval methodology based on Markov chain Monte Carlo sampling, we analysed which planet and atmosphere parameters can be inferred from the measured spectrum and the theoretical correlations amongst them. We considered limiting cases in which the planet radius is either known or completely unknown, and intermediate cases in which the planet radius is partly constrained. Results: If the planet radius is known, we can generally discriminate between cloud-free and cloudy atmospheres, and constrain the methane abundance to within two orders of magnitude. If the planet radius is unknown, new correlations between model parameters occur and the accuracy of the retrievals decreases. Without a radius determination, it is challenging to discern whether the planet has clouds, and the estimates on methane abundance degrade. However, we find the planet radius is constrained to within a factor of two for all the cases explored. Having a priori information on the planet radius, even if approximate, helps improve the retrievals. Conclusions: Reflected-starlight measurements will open a new avenue for characterizing long-period exoplanets, a population that remains poorly studied. For this task to be complete, direct-imaging observations should be accompanied by other techniques. We urge exoplanet detection efforts to extend the population of long-period planets with mass and radius determinations.

Item URL in elib:https://elib.dlr.de/137208/
Document Type:Article
Title:Directly imaged exoplanets in reflected starlight: the importance of knowing the planet radius
Authors:
AuthorsInstitution or Email of AuthorsAuthor's ORCID iD
Carrión González, OscarTU BerlinUNSPECIFIED
Garcia Munoz, A.Zentrum fuer Astronomie und Astrophysik, Technische Universitaet Berlin, GermanyUNSPECIFIED
Cabrera Perez, JuanJuan.Cabrera (at) dlr.dehttps://orcid.org/0000-0001-6653-5487
Csizmadia, Szilardszilard.csizmadia (at) dlr.deUNSPECIFIED
Santos, N. C.Instituto de Astrofisica e Ciencias do Espaco, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal; Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua Campo Alegre, 4169-007 Porto, PortugalUNSPECIFIED
Rauer, Hheike.rauer (at) dlr.deUNSPECIFIED
Date:28 August 2020
Journal or Publication Title:Astronomy & Astrophysics
Refereed publication:Yes
Open Access:Yes
Gold Open Access:No
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:640
DOI :10.1051/0004-6361/202038101
Page Range:A136
Publisher:EDP Sciences
ISSN:0004-6361
Status:Published
Keywords:planets and satellites: atmospheres; planets and satellites: gaseous planets; radiative transfer; Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Instrumentation and Methods for Astrophysics
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:Space Exploration
DLR - Research area:Raumfahrt
DLR - Program:R EW - Space Exploration
DLR - Research theme (Project):R - Project PLATO (old)
Location: Berlin-Adlershof
Institutes and Institutions:Institute of Planetary Research > Extrasolar Planets and Atmospheres
Institute of Planetary Research > Leitungsbereich PF
Deposited By: Cabrera Perez, Juan
Deposited On:06 Nov 2020 10:48
Last Modified:06 Nov 2020 10:48

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