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Parameter degeneracies associated with interpreting HST WFC3 transmission spectra of exoplanetary atmospheres

Novais, Aline LU orcid ; Fisher, Chloe ; Ghezzi, Luan ; Kitzmann, Daniel ; Thorsbro, Brian LU orcid and Heng, Kevin (2025) In Monthly Notices of the Royal Astronomical Society 538(4). p.2521-2547
Abstract

The Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope has provided an abundance of exoplanet spectra over the years. These spectra have enabled analysis studies using atmospheric retrievals to constrain the properties of these objects. However, follow-up observations from the JWST have called into question some of the results from these older datasets, and highlighted the need to properly understand the degeneracies associated with retrievals of WFC3 spectra. In this study, we perform atmospheric retrievals of 38 transmission spectra from WFC3 and use model comparison to determine the complexity required to fit the data. We explore the effect of retrieving system parameters such as the stellar radius and planet’s... (More)

The Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope has provided an abundance of exoplanet spectra over the years. These spectra have enabled analysis studies using atmospheric retrievals to constrain the properties of these objects. However, follow-up observations from the JWST have called into question some of the results from these older datasets, and highlighted the need to properly understand the degeneracies associated with retrievals of WFC3 spectra. In this study, we perform atmospheric retrievals of 38 transmission spectra from WFC3 and use model comparison to determine the complexity required to fit the data. We explore the effect of retrieving system parameters such as the stellar radius and planet’s surface gravity, and thoroughly investigate the degeneracies between individual model parameters – specifically the temperature, abundance of water, and cloud-top level. We focus on three case studies (HD 209458b, WASP-12b, and WASP-39b) in an attempt to diagnose some of the issues with these retrievals, in particular the low retrieved temperatures when compared to the equilibrium values. Our study advocates for the careful consideration of parameter degeneracies when interpreting retrieval results, as well as the importance of wider wavelength coverage to break these degeneracies, in agreement with previous studies. The combination of data from multiple instruments, as well as analysis from multiple data reductions and retrieval codes, will allow us to robustly characterize the atmosphere of these exoplanets.

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author
; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
exoplanets, planets and satellites: atmospheres
in
Monthly Notices of the Royal Astronomical Society
volume
538
issue
4
pages
27 pages
publisher
Oxford University Press
external identifiers
  • scopus:105001558230
ISSN
0035-8711
DOI
10.1093/mnras/staf397
language
English
LU publication?
yes
id
ecca9084-3058-48d2-a294-e762a34336d3
date added to LUP
2025-08-25 13:27:25
date last changed
2025-08-25 13:27:59
@article{ecca9084-3058-48d2-a294-e762a34336d3,
  abstract     = {{<p>The Wide Field Camera 3 (WFC3) instrument on the Hubble Space Telescope has provided an abundance of exoplanet spectra over the years. These spectra have enabled analysis studies using atmospheric retrievals to constrain the properties of these objects. However, follow-up observations from the JWST have called into question some of the results from these older datasets, and highlighted the need to properly understand the degeneracies associated with retrievals of WFC3 spectra. In this study, we perform atmospheric retrievals of 38 transmission spectra from WFC3 and use model comparison to determine the complexity required to fit the data. We explore the effect of retrieving system parameters such as the stellar radius and planet’s surface gravity, and thoroughly investigate the degeneracies between individual model parameters – specifically the temperature, abundance of water, and cloud-top level. We focus on three case studies (HD 209458b, WASP-12b, and WASP-39b) in an attempt to diagnose some of the issues with these retrievals, in particular the low retrieved temperatures when compared to the equilibrium values. Our study advocates for the careful consideration of parameter degeneracies when interpreting retrieval results, as well as the importance of wider wavelength coverage to break these degeneracies, in agreement with previous studies. The combination of data from multiple instruments, as well as analysis from multiple data reductions and retrieval codes, will allow us to robustly characterize the atmosphere of these exoplanets.</p>}},
  author       = {{Novais, Aline and Fisher, Chloe and Ghezzi, Luan and Kitzmann, Daniel and Thorsbro, Brian and Heng, Kevin}},
  issn         = {{0035-8711}},
  keywords     = {{exoplanets; planets and satellites: atmospheres}},
  language     = {{eng}},
  number       = {{4}},
  pages        = {{2521--2547}},
  publisher    = {{Oxford University Press}},
  series       = {{Monthly Notices of the Royal Astronomical Society}},
  title        = {{Parameter degeneracies associated with interpreting HST WFC3 transmission spectra of exoplanetary atmospheres}},
  url          = {{http://dx.doi.org/10.1093/mnras/staf397}},
  doi          = {{10.1093/mnras/staf397}},
  volume       = {{538}},
  year         = {{2025}},
}