A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b

Louis Philippe Coulombe, Björn Benneke, Ryan Challener, Anjali A.A. Piette, Lindsey S. Wiser, Megan Mansfield, Ryan J. MacDonald, Hayley Beltz, Adina D. Feinstein, Michael Radica, Arjun B. Savel, Leonardo A. Dos Santos, Jacob L. Bean, Vivien Parmentier, Ian Wong, Emily Rauscher, Thaddeus D. Komacek, Eliza M.R. Kempton, Xianyu Tan, Mark HammondNeil T. Lewis, Michael R. Line, Elspeth K.H. Lee, Hinna Shivkumar, Ian J.M. Crossfield, Matthew C. Nixon, Benjamin V. Rackham, Hannah R. Wakeford, Luis Welbanks, Xi Zhang, Natalie M. Batalha, Zachory K. Berta-Thompson, Quentin Changeat, Jean Michel Désert, Néstor Espinoza, Jayesh M. Goyal, Joseph Harrington, Heather A. Knutson, Laura Kreidberg, Mercedes López-Morales, Avi Shporer, David K. Sing, Kevin B. Stevenson, Keshav Aggarwal, Eva Maria Ahrer, Munazza K. Alam, Taylor J. Bell, Jasmina Blecic, Claudio Caceres, Aarynn L. Carter, Sarah L. Casewell, Nicolas Crouzet, Patricio E. Cubillos, Leen Decin, Jonathan J. Fortney, Neale P. Gibson, Kevin Heng, Thomas Henning, Nicolas Iro, Sarah Kendrew, Pierre Olivier Lagage, Jérémy Leconte, Monika Lendl, Joshua D. Lothringer, Luigi Mancini, Thomas Mikal-Evans, Karan Molaverdikhani, Nikolay K. Nikolov, Kazumasa Ohno, Enric Palle, Caroline Piaulet, Seth Redfield, Pierre Alexis Roy, Shang Min Tsai, Olivia Venot, Peter J. Wheatley

Research output: Contribution to journalArticlepeer-review

26 Scopus citations


Close-in giant exoplanets with temperatures greater than 2,000 K (‘ultra-hot Jupiters’) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble Space Telescope (HST) and Spitzer Space Telescope 1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis 3–12. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS 13 instrument on the JWST. The data span 0.85 to 2.85 μm in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at >6σ confidence) and evidence for optical opacity, possibly attributable to H, TiO and VO (combined significance of 3.8σ). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy-element abundance (‘metallicity’, M/H=1.03−0.51+1.11 times solar) and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the substellar point that decreases steeply and symmetrically with longitude towards the terminators.

Original languageEnglish (US)
Pages (from-to)292-298
Number of pages7
Issue number7973
StatePublished - Aug 10 2023

ASJC Scopus subject areas

  • General


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