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Register a new userExploring A Photospheric Radius Correction to Model Secondary Eclipse Spectra for Transiting Exoplanets
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Jonathan J. Fortney
Publication date
2019
fields
Physics
and research's language is
English
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We highlight a physical effect that is often not considered that impacts the calculation of model spectra of planets at secondary eclipse, affecting both emission and reflection spectra. The radius of the emitting surface of the planet is not merely one value measured from a transit light curve, but is itself a function of wavelength, yet it is not directly measurable. At high precision, a similar effect is well-known in transit transmission spectroscopy but this related effect also impacts emission and reflection. As is well-appreciated, the photospheric radius can vary across $sim$4-8 atmospheric scale heights, depending on atmospheric opacity and spectral resolution. This effect leads to a decreased weighting in model calculations at wavelengths where atmospheric opacity is low, and one sees more deeply into the atmosphere, to a smaller radius. The overall effect serves to mute emission spectra features for atmospheres with no thermal inversion but to enhance features for atmospheres with a thermal inversion. While this effect can be ignored for current emph{Hubble} observations, it can lead to wavelength-dependent 10-20% changes in planet-to-star flux ratios in the infrared at $Rsim~200-1000$ (readily achievable for JWST) for low-gravity hot Jupiters, although values of 5% are more typical for the population. The effect is mostly controlled by the ratio of the atmospheric scale height to the planet radius, and can be important at any planetary temperature. Of known planets, the effect is largest for the cool super-puffs at very low surface gravity, where it can alter calculated flux ratios by over 100%. We discuss complexities of including this photospheric radius effect in 1D and 3D atmosphere models.
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A planets emission spectrum contains information about atmospheric composition and structure. We compare the Bayesian Information Criterion (BIC) of blackbody fits and idealized spectral retrieval fits for the 44 planets with published eclipse measurements in multiple thermal wavebands, mostly obtained with the Spitzer Space Telescope. The evidence for spectral features depends on eclipse depth uncertainties. Spitzer has proven capable of eclipse precisions better than 1E-4 when multiple eclipses are analyzed simultaneously, but this feat has only been performed four times. It is harder to self-calibrate photometry when a single occultation is reduced and analyzed in isolation; we find that such measurements have not passed the test of repeatability. Single-eclipse measurements either have an uncertainty floor of 5E-4, or their uncertainties have been underestimated by a factor of 3. If one adopts these empirical uncertainties for single-eclipse measurements, then the evidence for molecular features all but disappears: blackbodies have better BIC than spectral retrieval for all planets, save HD 189733b, and the few planets poorly fit by blackbodies are also poorly fit by self-consistent radiative transfer models. This suggests that the features in extant broadband emission spectra are due to astrophysical and instrumental noise rather than molecular bands. Claims of stratospheric
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