Phase Curve Pollution Of Exoplanet Transmission Spectra


Simulated transmission spectra for WASP 12 b and WASP 43 b. Each panel contains a set of spectra for a specific JWST instrument (from left to right: NIRISS, NIRSpec, and MIRI) and atmospheric C/O ratio (upper rows: solar C/O; lower rows: C/O=1.5). The pure transmission spectra (no bias added) are in blue, with a swath denoting the 1σ error bars estimated by using ExoTETHyS.BOATS. The other spectra are biased as described in Section 4.2. The “model−offset” assumes a constant correction that does not account for the wavelength dependence of the bias. A significantly biased spectral slope is observed for WASP 12 b with NIRISS (1σ), and in the bluer part of the NIRSPEC spectra &1σ), and there is a large but nearly constant offset with MIRI. The biased spectra for WASP 43 b are almost entirely within the 1σ error bars. offset and a different slope than the pure transmission ones in the NIRISS passband. The effect on the slope is smaller at the longer wavelengths, and it is negligible in the MIRI passband. The bias for WASP 12 b is much larger than the 1σ error bars in most configurations, while for WASP 43 b it is within about 1σ.

The occurrence of a planet transiting in front of its host star offers the opportunity to observe the planet's atmosphere filtering starlight.

The fraction of occulted stellar flux is roughly proportional to the optically thick area of the planet, the extent of which depends on the opacity of the planet's gaseous envelope at the observed wavelengths. Chemical species, haze, and clouds are now routinely detected in exoplanet atmospheres through rather small features in transmission spectra, i.e., collections of planet-to-star area ratios across multiple spectral bins and/or photometric bands. Technological advances have led to a shrinking of the error bars down to a few tens of parts per million (ppm) per spectral point for the brightest targets.

The upcoming James Webb Space Telescope (JWST) is anticipated to deliver transmission spectra with precision down to 10 ppm. The increasing precision of measurements requires a reassessment of the approximations hitherto adopted in astrophysical models, including transit light curve models.

Recently, it has been shown that neglecting the planet's thermal emission can introduce significant biases in the transit depth measured with the JWST/Mid-InfraRed Instrument, integrated between 5 and 12 μm. In this paper, we take a step forward by analyzing the effects of the approximation on transmission spectra over the 0.6-12 μm wavelength range covered by various JWST instruments. We present open source software to predict the spectral bias, showing that, if not corrected, it may affect the inferred molecular abundances and thermal structure of some exoplanet atmospheres.

G. Morello, T. Zingales, M. Martin-Lagarde, R. Gastaud, P.-O. Lagage

Comments: Published on AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Journal reference: AJ, 161, 174 (2021)
DOI: 10.3847/1538-3881/abe048
Cite as: arXiv:2104.08979 [astro-ph.EP] (or arXiv:2104.08979v1 [astro-ph.EP] for this version)
Submission history
From: Giuseppe Morello
[v1] Sun, 18 Apr 2021 23:49:48 UTC (10,173 KB)
https://arxiv.org/abs/2104.08979
Astrobiology, Astrochemistry

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