The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5 μ m transmission spectrum of the 17 Myr, Jupiter-size ( R ∼10 R ⊕ ) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneous g -band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20 M ⊕ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0 M ⊕ . This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm −3 , HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H 2 O and CO 2 (≥7 σ ), a modest detection of CO (3.5 σ ), and weak detections of H 2 S and SO 2 (≃2 σ ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO 2 feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03 M ⊕ Myr −1 ), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.

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Publication Details

Journal
The Astronomical Journal
Published
2024-12-01
DOI
https://doi.org/10.3847/1538-3881/ad81d7
Citations
30
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
9.95

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article

The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

Natasha E. Batalha, David J. Wilson, Alexander Brown, Andrew W. Mann et al.
30 citations
The Astronomical Journal
Stellar, planetary, and galactic studies
9.95
article

The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

Natasha E. Batalha, David J. Wilson, Alexander Brown, Andrew W. Mann, Aaron C. Rizzuto, Benjamin M. Tofflemire, C. A. Murray, Zachory K. Berta-Thompson, Isabella Longo, Luis Welbanks, Elisabeth Newton, Madyson G. Barber, Kevin France, Adina D. Feinstein, Cynthia S. Froning, Girish M. Duvvuri, Peter Gao, T. M. Evans, Mackenna L. Wood, Jacob L. Bean, Yamila Miguel, Andrew Vanderburg, J. Sebastian Pineda, Daniel Thorngren, Angeli Sandoval, Shang‐Min Tsai, Eliza M.-R. Kempton, Pa Chia Thao, P. C. Schneider, Adam L. Kraus, Allison Youngblood, Yoav Rotman
article en
30 citations

Abstract

Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5 μ m transmission spectrum of the 17 Myr, Jupiter-size ( R ∼10 R ⊕ ) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneous g -band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20 M ⊕ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0 M ⊕ . This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm −3 , HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H 2 O and CO 2 (≥7 σ ), a modest detection of CO (3.5 σ ), and weak detections of H 2 S and SO 2 (≃2 σ ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO 2 feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03 M ⊕ Myr −1 ), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.

The Astronomical JournalVol. 168(6)
University of California, Riverside (US), Dartmouth College (US), University of North Carolina at Chapel Hill (US), Leiden University (NL), The Graduate Center, CUNY (US), Ames Research Center (US), Southwest Research Institute (US), Goddard Space Flight Center (US), Johns Hopkins University (US), City University of New York (US), University of Colorado Boulder (US), Carnegie Institution for Science (US), Vanderbilt University (US), Dairy Australia (Australia) (AU), University of Chicago (US), Max Planck Institute for Astronomy (DE), Laboratory for Atmospheric and Space Physics (US), Hamburger Fern-Hochschule (DE), European Space Research and Technology Centre (NL), Space Research Organisation Netherlands (NL), Arizona State University (US), Massachusetts Institute of Technology (US), University of Maryland, College Park (US), University of Newcastle Australia (AU), The University of Texas at Austin (US), Michigan State University (US)
Astrophysics Science Division
Life below water
Openalex Percentile: Top 1%
Stellar, planetary, and galactic studies
9.95
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