The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d

Abstract Kepler-51 is a 500 Myr G dwarf hosting three “super-puffs” and one low-mass nontransiting planet. Kepler-51d, the coolest ( T eq ∼ 350 K) transiting planet in this system is also one of the lowest-density super-puffs known to date ( ρ p = 0.038 ± 0.009 g cm −3 ). With a planetary mass of M p = 5.6 ± 1.2 M ⊕ and a radius of R p = 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 μ m 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 μ bar. 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.

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Journal
The Astronomical Journal
Published
2026-03-16
DOI
https://doi.org/10.3847/1538-3881/ae33c0
Primary Topic
Stellar, planetary, and galactic studies
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article
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article

The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d

Zachory K. Berta-Thompson, Kevin B. Stevenson, Caleb I. Cañas, Guangwei Fu et al.
The Astronomical Journal
Stellar, planetary, and galactic studies
article

The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d

Zachory K. Berta-Thompson, Kevin B. Stevenson, Caleb I. Cañas, Guangwei Fu, Jessica E. Libby-Roberts, Renyu Hu, Peter Gao, Aaron Bello-Arufe, Yui Kawashima, Leslie Hebb, Yayaati Chachan, Kento Masuda, Catriona Murray, Suvrath Mahadevan, Kazumasa Ohno, Armen Tokadjian, Caroline Morley
article en

Abstract

Abstract Kepler-51 is a 500 Myr G dwarf hosting three “super-puffs” and one low-mass nontransiting planet. Kepler-51d, the coolest ( T eq ∼ 350 K) transiting planet in this system is also one of the lowest-density super-puffs known to date ( ρ p = 0.038 ± 0.009 g cm −3 ). With a planetary mass of M p = 5.6 ± 1.2 M ⊕ and a radius of R p = 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 μ m 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 μ bar. 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 Astronomical JournalVol. 171(4)
California Institute of Technology (US), Goddard Space Flight Center (US), Pennsylvania State University (US), Jet Propulsion Laboratory (US), Hobart and William Smith Colleges (US), Johns Hopkins University (US), University of California, Santa Cruz (US), University of Colorado Boulder (US), Carnegie Institution for Science (US), Cornell University (US), Kyoto University (JP), Johns Hopkins University Applied Physics Laboratory (US), Osaka Health Science University (JP), National Astronomical Observatory of Japan (JP), The University of Texas at Austin (US), University of Tampa (US), The University of Osaka (JP)
Openalex Percentile: Top 18%
Stellar, planetary, and galactic studies
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