Impact of planetary mass uncertainties on exoplanet atmospheric retrievals. (arXiv:1908.06305v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Changeat_Q/0/1/0/all/0/1">Quentin Changeat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Keyte_L/0/1/0/all/0/1">Luke Keyte</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Waldmann_I/0/1/0/all/0/1">Ingo P Waldmann</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tinetti_G/0/1/0/all/0/1">Giovanna Tinetti</a>

In current models used to interpret exoplanet atmospheric observations, the
planetary mass is treated as a prior and is estimated independently with
external methods (e.g: radial velocity or Transit Timing Variation techniques).
This approach is necessary as available spectroscopic data do not have
sufficient wavelength coverage and/or signal to noise to infer the planetary
mass. We examine here whether the planetary mass can be directly retrieved from
higher quality transit spectra as observed by future space observatories. More
in general, we quantify the impact of mass uncertainties on spectral retrieval
analyses. Our approach is analytical and numerical: we first extract
analytically the influence of each parameter to the wavelength-dependent
transit depth. We then adopt a fully Bayesian retrieval model to quantify the
propagation of the mass uncertainty onto other atmospheric parameters. We found
that for clear-sky, gaseous atmospheres the posterior distributions are the
same when the mass is known or retrieved. The retrieved mass is very accurate,
with a precision of more than 10%, provided the wavelength coverage and S/N are
adequate. When opaque clouds are included, the uncertainties in the retrieved
mass increase but atmospheric parameters such as the temperature and trace-gas
abundances are unaffected by the knowledge of the mass. Secondary atmospheres
are more challenging due to the higher degree of freedom for the atmospheric
main component. For broad wavelength range and adequate signal to noise, the
mass can still be retrieved accurately if clouds are not present, and so are
all the other atmospheric parameters. When clouds are added, mass uncertainties
may impact substantially the retrieval of the mean molecular weight: an
independent characterisation of the mass would therefore be helpful to capture
the main atmospheric constituent.

In current models used to interpret exoplanet atmospheric observations, the
planetary mass is treated as a prior and is estimated independently with
external methods (e.g: radial velocity or Transit Timing Variation techniques).
This approach is necessary as available spectroscopic data do not have
sufficient wavelength coverage and/or signal to noise to infer the planetary
mass. We examine here whether the planetary mass can be directly retrieved from
higher quality transit spectra as observed by future space observatories. More
in general, we quantify the impact of mass uncertainties on spectral retrieval
analyses. Our approach is analytical and numerical: we first extract
analytically the influence of each parameter to the wavelength-dependent
transit depth. We then adopt a fully Bayesian retrieval model to quantify the
propagation of the mass uncertainty onto other atmospheric parameters. We found
that for clear-sky, gaseous atmospheres the posterior distributions are the
same when the mass is known or retrieved. The retrieved mass is very accurate,
with a precision of more than 10%, provided the wavelength coverage and S/N are
adequate. When opaque clouds are included, the uncertainties in the retrieved
mass increase but atmospheric parameters such as the temperature and trace-gas
abundances are unaffected by the knowledge of the mass. Secondary atmospheres
are more challenging due to the higher degree of freedom for the atmospheric
main component. For broad wavelength range and adequate signal to noise, the
mass can still be retrieved accurately if clouds are not present, and so are
all the other atmospheric parameters. When clouds are added, mass uncertainties
may impact substantially the retrieval of the mean molecular weight: an
independent characterisation of the mass would therefore be helpful to capture
the main atmospheric constituent.

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