The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas

Abstract

With four giant planets ($m\sim 5-10M_{\mathrm{Jup}}$, $T_\rm{eff}\sim900-1200$ 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 ($2.85-5.3\hat{I}¼$m, $R\approx 2700$) 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 $1-5\hat{I}¼$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 $N/H=21.2_{-8.8}^{+16.2}\times$ 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 $750\pm 200M_{\oplus }$. 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.

Published in ApJ. The planet spectra are available in Zenodo at https://doi.org/10.5281/zenodo.19355669