Kepler-51 is a 500 Myr G dwarf hosting three “super-puffs” and one low-mass nontransiting planet. Kepler-51d, the coolest (Teq ∼ 350 K) transiting planet in this system is also one of the lowest-density super-puffs known to date (ρp = 0.038 ± 0.009 g cm−3). With a planetary mass of Mp = 5.6 ± 1.2 M⊕ and a radius of Rp = 9.32 ± 0.18 R⊕, the observed properties of this planet are not readily explained by most planet formation theories. Hypotheses explaining Kepler-51d’s low density range from a substantial H/He envelope comprising >30% its mass, a high-altitude haze layer, to a tilted ring system. To test these hypotheses, we present the NIRSpec-PRISM 0.6–5.3 μm transmission spectrum of Kepler-51d observed by the James Webb Space Telescope. We find a spectrum best fit by a sloped line covering the entire wavelength range. Based on forward modeling and atmosphere retrievals, Kepler-51d likely possesses a low-metallicity atmosphere with high-altitude hazes of submicron particle sizes spanning pressures of 1–100 μbar. However, the spectrum could also be explained by a tilted ring with an estimated lifetime on the order of ∼0.1 Myr. We also investigate the stellar activity of this young Sun-like star, extracting a spot temperature significantly hotter than sunspots and spot covering fractions on the order of 0.1%–10% depending on assumed spot parameters.
The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d
Creators
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Libby-Roberts, Jessica E.1, 2
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Bello-Arufe, Aaron3
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Berta-Thompson, Zachory K.4
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Cañas, Caleb I.5
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Chachan, Yayaati6
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Hu, Renyu3, 7
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Kawashima, Yui8
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Murray, Catriona4
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Ohno, Kazumasa9
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Tokadjian, Armen3
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Mahadevan, Suvrath1
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Masuda, Kento10
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Hebb, Leslie11, 12
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Morley, Caroline13
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Fu, Guangwei14
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Gao, Peter15
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Stevenson, Kevin B.14
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1.
Pennsylvania State University
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2.
University of Tampa
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3.
Jet Propulsion Lab
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4.
University of Colorado Boulder
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5.
Goddard Space Flight Center
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6.
University of California, Santa Cruz
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7.
California Institute of Technology
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8.
Kyoto University
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9.
National Astronomical Observatory of Japan
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10.
Osaka University
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11.
Hobart and William Smith Colleges
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12.
Cornell University
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13.
The University of Texas at Austin
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14.
Johns Hopkins University
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15.
Carnegie Institution for Science
Abstract
Copyright and License
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We would like to acknowledge and thank the anonymous reviewer for providing thoughtful comments and suggestions. We are also extremely grateful to Weston Eck, Elena Manjavaca, and the staff at Space Telescope Science Institute for scheduling this observation and accommodating our last-minute instrumental setup requests. We thank Eve Lee for many insightful discussions on super-puff formation, Patrick McCreery for updating Kepler-51 stellar abundances, Andrew Mann for advice on modeling out-of-transit stellar spectra, and Allison Youngblood and David Wilson for sharing their UV stellar constraints for Kepler-51. We also wish to thank Heather Knutson for her thoughtful feedback on the JWST 2571 proposal, which contributed to its selection and the resulting observation.
This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program 2571 and available at MAST: DOI:10.17909/zrjp-rw13.
The authors of this work recognize the Penn State Institute for Computational and Data Sciences (RRID:SCR_025154) for providing access to computational research infrastructure within the Roar Core Facility (RRID: SCR_026424).
The Center for Exoplanets and Habitable Worlds is supported by the Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium.
Y.K. acknowledges support from JSPS KAKENHI grant Nos. 21K13984, 22H05150, and 23H01224. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). C.I.C. acknowledges support by NASA Headquarters through an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by ORAU through a contract with NASA.
Facilities
JWST - James Webb Space Telescope (NIRSpec-PRISM).
Software References
astropy (Astropy Collaboration et al. 2018), chromatic (Z. Berta-Thompson et al. 2025), chromatic_fitting (C. Murray et al. 2025), dynesty (J. S. Speagle 2020), Eureka! (T. Bell et al. 2022), ExoTR ExoTiC-JEDI (L. Alderson et al. 2022), ExoTiC-LD (D. Grant & H. R. Wakeford 2024), jwst (H. Bushouse et al. 2023), matplotlib (J. D. Hunter 2007), numpy (S. van der Walt et al. 2011), pandas (W. McKinney 2010), PLATON (M. Zhang et al. 2019, 2020), POSEIDON (R. J. MacDonald & N. Madhusudhan 2017), scipy (P. Virtanen et al. 2020), spotrod (B. Béky et al. 2014), starry (R. Luger et al. 2021).
Files
Libby-Roberts_2026_AJ_171_221.pdf
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2505.21358 (arXiv)
Funding
- National Aeronautics and Space Administration
- NAS 5-03127
- Space Telescope Science Institute
- 2571
- Pennsylvania State University
- Pennsylvania Space Grant Consortium
- Japan Society for the Promotion of Science
- 21K13984
- Japan Society for the Promotion of Science
- 22H05150
- Japan Society for the Promotion of Science
- 23H01224
- National Aeronautics and Space Administration
- 80NM0018D0004
- National Aeronautics and Space Administration
- NASA Postdoctoral Program -
Dates
- Submitted
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2025-05-22
- Accepted
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2025-12-15
- Available
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2026-03-16Published
Caltech Custom Metadata
- Caltech groups
- Division of Geological and Planetary Sciences (GPS)
- Publication Status
- Published