A precise metallicity and carbon-to-oxygen ratio for a warm giant exoplanet from its panchromatic JWST emission spectrum
Lindsey S. Wiser, Taylor J. Bell, Michael R. Line, Everett Schlawin, Thomas G. Beatty, Luis Welbanks, Thomas P. Greene, Vivien Parmentier, Matthew M. Murphy, Jonathan J. Fortney, Kenny Arnold, Nishil Mehta, Kazumasa Ohno, Sagnick Mukherjee
Abstract
WASP-80 b, a warm sub-Jovian (equilibrium temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo>∼</mml:mo> </mml:math> 820 K, 0.5 Jupiter masses), presents an opportunity to characterize a rare gas giant exoplanet around a low-mass star. In addition, its moderate temperature enables its atmosphere to host a range of carbon and oxygen species (H 2 O, CH 4 , CO, CO 2 , NH 3 ). In this paper, we present a panchromatic emission spectrum of WASP-80 b, the first gas giant around a late K/early M-dwarf star and the coolest planet for which the James Webb Space Telescope has obtained a complete emission spectrum spanning 2.4 to 12 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> </mml:math> m, including NIRCam F322W2 (2.4 to 4 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> </mml:math> m) and F444W (4 to 5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> </mml:math> m), and MIRI LRS (5 to 12 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="normal">μ</mml:mi> </mml:math> m). We report confident detections of H 2 O, CH 4 , CO, and CO 2 , and a tentative detection of NH 3 . We estimate WASP-80 b’s atmospheric metallicity and carbon-to-oxygen ratio and compare them with estimates for other gas giants. Despite the relative rarity of giant planets around low-mass stars, we find that WASP-80 b’s composition is consistent with other hot gas giants, suggesting that the formation pathway of WASP-80 b may not be dissimilar from hot gas giants around higher-mass stars.