The Compositions of the HR 8799 Planets Reflect Accretion of Both Solids and Metal-enriched Gas

Abstract With four giant planets ( m ∼ 5 M Jup −10 M Jup , T eff ∼ 900–1200 K) orbiting between 15 and 70 au, HR 8799 provides an unparalleled test bed for studying giant planet formation and probing compositional trends across the protoplanetary disk. We present new JWST/NIRSpec integral field unit observations (2.85–5.3 μ m, R ≈ 2700) that now include the spectrum of HR 8799 b and higher signal-to-noise ratio spectra for HR 8799 c, d, and e compared to those in J.-B. Ruffio 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 we 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× and 5× stellar, we observe a tentative trend of increasing S/H—a tracer of refractory solids—from 2× to 5× stellar with increasing orbital distance. From HR 8799 b’s NH 3 abundance, we estimate N / H = 21 . 2 − 8.8 + 16.2 × stellar, suggesting that 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× 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 ± 200 M ⊕ . 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× minimum-mass solar nebula) paradigms.