The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas
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2026
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| author | Xuan, Jerry W. Ruffio, Jean-Baptiste Chachan, Yayaati Ohno, Kazumasa Kesseli, Aurora Y. Murray-Clay, Ruth A. Lee, Eve J. Moses, Julianne I. Balmer, William O. Baburaj, Aneesh Blake, Geoffrey A. Johnstone, Doug Zhang, Yapeng Knutson, Heather A. Mawet, Dimitri Beichman, Charles Hodapp, Klaus W. Perrin, Marshall D. Konopacky, Quinn M. Meyer, Michael R. Bryden, Geoffrey Greene, Thomas P. Leisenring, Jarron Ygouf, Marie Benneke, Björn Inglis, Julie Wallack, Nicole L. |
| author_facet | Xuan, Jerry W. Ruffio, Jean-Baptiste Chachan, Yayaati Ohno, Kazumasa Kesseli, Aurora Y. Murray-Clay, Ruth A. Lee, Eve J. Moses, Julianne I. Balmer, William O. Baburaj, Aneesh Blake, Geoffrey A. Johnstone, Doug Zhang, Yapeng Knutson, Heather A. Mawet, Dimitri Beichman, Charles Hodapp, Klaus W. Perrin, Marshall D. Konopacky, Quinn M. Meyer, Michael R. Bryden, Geoffrey Greene, Thomas P. Leisenring, Jarron Ygouf, Marie Benneke, Björn Inglis, Julie Wallack, Nicole L. |
| contents | With four giant planets ($m\sim5-10~M_{\rm Jup}$, $T_\rm{eff}\sim900-1200$ K) orbiting between 15-70 au, HR 8799 provides an unparalleled testbed for studying giant planet formation and probing compositional trends across the protoplanetary disk. We present new JWST/NIRSpec IFU observations ($2.85-5.3~μ$m, $R\approx2700$) that now include the spectrum of HR 8799 b, and higher S/N spectra for HR 8799 c, d, and e compared to that in Ruffio & Xuan et al. We detect CO, CH$_4$, H$_2$O, H$_2$S, CO$_2$, and for planet b, NH$_3$. We combine the NIRSpec spectra with $1-5 μ$m photometry to perform atmospheric retrievals that account for disequilibrium chemistry and clouds, and allow C/H, O/H, N/H, and S/H to scale independently. While the four planets are similarly enriched in carbon and oxygen, with C/H and O/H between $3-5\times$ stellar, we observe a tentative trend of increasing S/H - a tracer of refractory solids - from $2-5 \times$ stellar with increasing orbital distance. From HR 8799 b's NH$_3$ abundance, we estimate $\rm N/H=21.2^{+16.2}_{-8.8}\times$ stellar, suggesting the outer planet accreted significant amounts of N-rich gas. Overall, the elemental abundance patterns we observe are consistent with a picture where planet b formed between the CO snowline and the more-distant N$_2$ snowline, while the inner planets accreted $3 \times$ stellar CO-enriched disk gas within the CO snowline. The excess volatile mass from pebble drift and evaporation implies an integrated pebble flux of $750 \pm 200~M_{\oplus}$. The increase in the planets' S/H with orbital distance implies more solid accretion further out, which is quantitatively compatible with expectations from both pebble and planetesimal accretion ($2 \times$ Minimum Mass Solar Nebula) paradigms. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2602_09422 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas Xuan, Jerry W. Ruffio, Jean-Baptiste Chachan, Yayaati Ohno, Kazumasa Kesseli, Aurora Y. Murray-Clay, Ruth A. Lee, Eve J. Moses, Julianne I. Balmer, William O. Baburaj, Aneesh Blake, Geoffrey A. Johnstone, Doug Zhang, Yapeng Knutson, Heather A. Mawet, Dimitri Beichman, Charles Hodapp, Klaus W. Perrin, Marshall D. Konopacky, Quinn M. Meyer, Michael R. Bryden, Geoffrey Greene, Thomas P. Leisenring, Jarron Ygouf, Marie Benneke, Björn Inglis, Julie Wallack, Nicole L. Earth and Planetary Astrophysics Solar and Stellar Astrophysics With four giant planets ($m\sim5-10~M_{\rm Jup}$, $T_\rm{eff}\sim900-1200$ K) orbiting between 15-70 au, HR 8799 provides an unparalleled testbed for studying giant planet formation and probing compositional trends across the protoplanetary disk. We present new JWST/NIRSpec IFU observations ($2.85-5.3~μ$m, $R\approx2700$) that now include the spectrum of HR 8799 b, and higher S/N spectra for HR 8799 c, d, and e compared to that in Ruffio & Xuan et al. We detect CO, CH$_4$, H$_2$O, H$_2$S, CO$_2$, and for planet b, NH$_3$. We combine the NIRSpec spectra with $1-5 μ$m photometry to perform atmospheric retrievals that account for disequilibrium chemistry and clouds, and allow C/H, O/H, N/H, and S/H to scale independently. While the four planets are similarly enriched in carbon and oxygen, with C/H and O/H between $3-5\times$ stellar, we observe a tentative trend of increasing S/H - a tracer of refractory solids - from $2-5 \times$ stellar with increasing orbital distance. From HR 8799 b's NH$_3$ abundance, we estimate $\rm N/H=21.2^{+16.2}_{-8.8}\times$ stellar, suggesting the outer planet accreted significant amounts of N-rich gas. Overall, the elemental abundance patterns we observe are consistent with a picture where planet b formed between the CO snowline and the more-distant N$_2$ snowline, while the inner planets accreted $3 \times$ stellar CO-enriched disk gas within the CO snowline. The excess volatile mass from pebble drift and evaporation implies an integrated pebble flux of $750 \pm 200~M_{\oplus}$. The increase in the planets' S/H with orbital distance implies more solid accretion further out, which is quantitatively compatible with expectations from both pebble and planetesimal accretion ($2 \times$ Minimum Mass Solar Nebula) paradigms. |
| title | The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas |
| topic | Earth and Planetary Astrophysics Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2602.09422 |