*Blurred Lines: Degeneracies in Modeling Exoplanet Atmospheres*Astrobites | 2018 May 23

Vatsal Panwar wrote:

Transmission spectroscopy is the most commonly used method to characterize the atmospheres of transiting exoplanets that have been discovered to date. The method is seemingly straightforward: photons from the host star passing through the limb of its transiting exoplanet are absorbed or transmitted by the planetary atmosphere to a varying extent in different wavelengths, which should appear as a variation in observed radius of the exoplanet with respect to wavelength. This variation in transit radius with wavelength, known as transmission spectrum, can then be interpreted by adopting either a forward or inverse modeling approach. This means that you can either start with a set of assumptions for the atmospheric properties (like chemical abundances, cloud properties etc.) and construct a physical model that can be fit to the data, or try solving the inverse problem (also known as retrieval) and infer the atmospheric properties from the measured transmission spectra itself.

However, there is a good deal of subtlety behind both approaches of constructing theoretical models for transiting exoplanet atmospheres. Today’s paper traces the assumptions and caveats involved in constructing models for observations taken by the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. In addition to deriving a validated semi-analytical approach from first principles for calculating the transmission spectra in the context of WFC3 observations, authors of today’s paper uncover a crucial and unresolved degeneracy involved in fitting a model to the transmission spectra. ...

**- Kevin Heng, Daniel Kitzmann**

*The theory of transmission spectra revisited: a semi-analytical method for*

interpreting WFC3 data and an unresolved challengeinterpreting WFC3 data and an unresolved challenge

*Monthly Notices of the RAS*470(3):2972 (2017 Sep) DOI: 10.1093/mnras/stx1453

arXiv.org > astro-ph > arXiv:1702.02051 > 07 Feb 2017 (v1), 08 Jun 2017 (v2)