Exploring Biases of Atmospheric Retrievals in Simulated JWST Transmission Spectra of Hot Jupiters

Abstract

With a scheduled launch in 2018 October, the James Webb Space Telescope ( JWST) is expected to revolutionize the field of atmospheric characterization of exoplanets. The broad wavelength coverage and high sensitivity of its instruments will allow us to extract far more information from exoplanet spectra than what has been possible with current observations. In this paper, we investigate whether current retrieval methods will still be valid in the era of JWST, exploring common approximations used when retrieving transmission spectra of hot Jupiters. To assess biases, we use 1D photochemical models to simulate typical hot Jupiter cloud-free atmospheres and generate synthetic observations for a range of carbon-to-oxygen ratios. Then, we retrieve these spectra using TauREx, a Bayesian retrieval tool, using two methodologies: one assuming an isothermal atmosphere, and one assuming a parameterized temperature profile. Both methods assume constant-with-altitude abundances. We found that the isothermal approximation biases the retrieved parameters considerably, overestimating the abundances by about one order of magnitude. The retrieved abundances using the parameterized profile are usually within 1σ of the true state, and we found the retrieved uncertainties to be generally larger compared to the isothermal approximation. Interestingly, we found that by using the parameterized temperature profile we could place tight constraints on the temperature structure. This opens the possibility of characterizing the temperature profile of the terminator region of hot Jupiters. Lastly, we found that assuming a constant-with-altitude mixing ratio profile is a good approximation for most of the atmospheres under study.

The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets

“…We assume isothermal profiles (T iso in Table 1) because our aim is only to study the effect of mass uncertainty on retrieved atmospheric profiles and our focus is only on transmission spectroscopy, which is less sensitive to the full PT profile. Even though isothermal profiles have been shown to bias retrieved molecular abundances (Rocchetto et al 2016), this assumption will not impact our results. To ensure this, we check our results by using the 5-parameter PT profile parameterization from Guillot (2010).…”

Observing transiting planets with JWST

“…The noise floor for NIRSpec is expected to be below 100 ppm (Ferruit et al 2014), and we adopt a value of 75 ppm here. Following Rocchetto et al (2016) one may assume a noise floor value of 20 ppm for NIRISS. Further, we set the MIRI noise floor value to 40 ppm, because the values adopted in the existing literature range from 30 to 50 (see Beichman et al 2014;Greene et al 2016, respectively).…”

Observing Exoplanets with the James Webb Space Telescope

“…Several studies have recently simulated JW ST spectra of a variety of planet types and have assessed what can be learned from these data from direct examination of spectra (Deming et al 2009;Mollière et al 2017) or statistical Bayesian retrieval techniques (Barstow et al 2015;Greene et al 2016;Rocchetto et al 2016;Batalha and Line 2017;Howe et al 2017). We now draw from these works to discuss the expected data quality and molecular features that may be detectable in JW ST spectra of several different types of exoplanet atmospheres.…”

“…JW ST spectra will have higher spectral resolution over much larger spectral range (see Table 2. Rocchetto et al (2016) find that the large 1 − 10 µm bandwidth of JW ST spectra will require improving retrievals to include allowing temperature variation with altitude in transmission spectra. This may seem unusual given that transmission spectra are generally thought to be sensitive to absorbers only at the highest altitudes.…”

“…Feature strengths of the common CH 4 , CO, CO 2 , H 2 O, and NH 3 (not yet detected) molecules infer a planet's C/O ratio and overall metal abundance, constraining its formation history. JWST will be able to obtain high quality transmission and emission spectra of numerous warm-to-hot planets down to ∼ 10 Earth masses or less (Greene et al 2016;Rocchetto et al 2016;Mollière et al 2017), advancing exoplanet characterization into a new era.…”