Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

Abstract

Exoplanet emission spectra are often modelled assuming that the hemisphere observed is well represented by a horizontally homogenised atmosphere. However this approximation will likely fail for planets with a large temperature contrast in the James Webb Space Telescope (JWST) era, potentially leading to erroneous interpretations of spectra.We first develop an analytic formulation to quantify the signal-to-noise ratio and wavelength coverage necessary to disentangle temperature inhomogeneities from a hemispherically averaged spectrum. We find that for a given signal-to-noise ratio, observations at shorter wavelengths are better at detecting the presence of inhomogeneities. We then determine why the presence of an inhomogeneous thermal structure can lead to spurious molecular detections when assuming a fully homogenised planet in the retrieval process.Finally, we quantify more precisely the potential biases by modelling a suite of hot Jupiter spectra, varying the spatial contributions of a hot and a cold region, as would be observed by the different instruments of JWST/NIRSpec. We then retrieve the abundances and temperature profiles from the synthetic observations. We find that in most cases, assuming a homogeneous thermal structure when retrieving the atmospheric chemistry leads to biased results, and spurious molecular detection. Explicitly modelling the data using two profiles avoids these biases, and is statistically supported provided the wavelength coverage is wide enough, and crucially also spanning shorter wavelengths. For the high contrast used here, a single profile with a dilution factor performs as well as the two-profile case, with only one additional parameter compared to the 1-D approach.

The Hubble WFC3 Emission Spectrum of the Extremely Hot Jupiter KELT-9b

“…For planets such as KELT-9 b, the irradiation difference between the day-and night-side is expected to induce large day-night temperature contrasts: the difference is expected to be around 2000 K (Wong et al 2019;Mansfield et al 2020). These can lead to three-dimensional biases (e.g., Feng et al 2020;Taylor et al 2020) due to the inhomogeneous day-side emission. In our retrieval, we employ a one-dimensional description of the atmosphere, which might not well represent the actual planet.…”

An Exploration of Model Degeneracies with a Unified Phase Curve Retrieval Analysis: The Light and Dark Sides of WASP-43 b

“…By design, transit and eclipse techniques offer the projection of a three-dimensional atmosphere to a one-dimensional spectrum with a wavelength dependence, from which it is difficult to extract the geometrical repartition of chemical and thermal properties (Feng et al 2016;Line & Parmentier 2016;Caldas et al 2019;Drummond et al 2020;Feng et al 2020;MacDonald et al 2020;Pluriel et al 2020b;Skaf et al 2020;Taylor et al 2020). To overcome these limitations and characterize the longitudinal structure of exoplanets, photometric and spectral phase curves have been used (e.g., Esteves et al 2013;Placek et al 2017;Deming & Knutson 2020;Parmentier & Crossfield 2018;Sing 2018;Barstow & Heng 2020).…”

“…Their paper highlighted the limitations induced when using optimal estimation technique in exoplanet atmospheric studies, where observations have a low signal-to-noise ratio and the prior knowledge on the solution is unknown. Other studies highlighted the importance of using phase curve data to break the degeneracies coming from the three-dimensional aspect of exoplanets (Feng et al 2016(Feng et al , 2020Taylor et al 2020). In particular, Feng et al (2020) performed the first combined retrieval of the WASP-43 b phase curve using a full exploration of the parameter space with the MultiNest sampler (Feroz et al 2009;Buchner et al 2014).…”

ARES. III. Unveiling the Two Faces of KELT-7 b with HST WFC3*

“…In their study of CoRoT-2 b they observed an emission spectrum similar to that of KELT-7 b (i.e., one that is poorly fit by a blackbody but that can be better fit using two blackbodies). Such inhomogeneities will certainly be important in the analysis of emission data from future missions (Taylor et al 2020). Our best fit favors a thermal inversion in the stratosphere of KELT-7 b.…”