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

With four giant planets ($m\sim5-10M_{\rm Jup}$, $T_\rm{eff}\sim900-1200$ 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 ($2.85-5.3μ$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. 2026. 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.

💡 Research Summary

This paper presents a comprehensive atmospheric characterization of the four directly imaged giant planets orbiting the young A‑type star HR 8799, using new JWST/NIRSpec IFU observations that cover the 2.85–5.3 µm wavelength range at a moderate spectral resolution of R≈2700. The dataset includes, for the first time, a high‑signal‑to‑noise spectrum of the outermost planet HR 8799 b (semi‑major axis ≈ 70 au) and improved spectra for the inner planets c, d, and e, achieved through longer integration times and optimized detector readout strategies. The authors employ a sophisticated data reduction pipeline that corrects for correlated 1/f noise, charge transfer from the saturated host star, and residual speckle contamination using a combination of STPSF modeling and spline‑based continuum fitting. Planet detection and spectral extraction are performed with the BREADS framework, which simultaneously fits a planet model (BT‑Settl) and a speckle model directly to the detector images, yielding robust flux measurements across the full bandpass.

Molecular detections include CO, CH₄, H₂O, H₂S, and CO₂ in all four planets, with NH₃ uniquely identified in HR 8799 b. These species provide direct tracers of the elemental abundances of carbon, oxygen, sulfur, and nitrogen. Atmospheric retrievals are carried out with a free‑parameter, disequilibrium chemistry model that allows C/H, O/H, N/H, and S/H to vary independently relative to the host star’s composition, while also accounting for cloud opacity and vertical mixing. The key results are:

1. Carbon and Oxygen Enrichment: All four planets exhibit C/H and O/H ratios 3–5 times the stellar value, while maintaining a C/O ratio close to solar. This indicates that the planets accreted gas strongly enriched in CO relative to the protostellar nebula.

2. Sulfur Trend with Orbital Distance: The sulfur-to-hydrogen ratio (S/H) shows a tentative increase with semi‑major axis, ranging from ~2 × stellar for the innermost planet to ~5 × stellar for the outermost. Since sulfur is predominantly locked in solid form beyond ~1 au in protoplanetary disks, this trend points to progressively larger solid (pebble/planetesimal) accretion at larger orbital radii.

3. Nitrogen Enrichment in the Outer Planet: NH₃ in HR 8799 b yields an N/H ratio of 21.2⁺¹⁶·²_₋₈·₈ × stellar, suggesting that this planet incorporated a substantial amount of nitrogen‑rich gas, consistent with formation between the CO snowline (~30 K) and the more distant N₂ snowline (~25 K).

4. Formation Scenarios: The authors propose a two‑stage accretion picture. The inner planets (c, d, e) likely formed inside the CO snowline, accreting CO‑enriched gas that was already 3 × stellar in metallicity, possibly aided by inward migration within a gas‑rich disk. The outer planet b appears to have formed farther out, between the CO and N₂ snowlines, where it could capture both volatile‑rich gas and a significant flux of solid material drifting inward as pebbles.

5. Pebble Flux Estimate: By accounting for the excess volatile mass inferred from the retrieved abundances, the integrated pebble flux through the disk is estimated at 750 ± 200 M⊕. This value corresponds to roughly twice the solid mass expected in a Minimum Mass Solar Nebula (MMSN), indicating a highly efficient delivery of solids to the forming planets.

6. Consistency with Solid Accretion Models: The observed S/H gradient aligns quantitatively with predictions from both pebble‑accretion and planetesimal‑accretion frameworks that assume a disk surface density of ~2 × MMSN. The simultaneous enrichment in volatile (C, O, N) and refractory (S) elements provides a coherent picture in which both gas and solid reservoirs contributed to the final planetary compositions.

The paper also revisits earlier low‑resolution studies that reported super‑stellar metallicities but suffered from larger uncertainties and limited molecular coverage. The high‑resolution JWST data, combined with a disequilibrium retrieval approach, refine the metallicity estimates downward (still super‑stellar) and, crucially, enable independent constraints on sulfur and nitrogen—elements that are essential for breaking degeneracies inherent in C/O‑only analyses.

In summary, the HR 8799 planetary system exhibits a clear chemical gradient: inner planets are dominated by enriched gas accretion, while the outermost planet bears the signatures of substantial solid accretion and nitrogen‑rich gas capture. These findings substantiate the role of pebble drift and evaporation in delivering volatiles, and they provide empirical support for hybrid formation pathways that combine gas accretion, pebble/planetesimal delivery, and migration. The work demonstrates the power of JWST mid‑infrared spectroscopy to disentangle the complex interplay of gas and solid processes in the birth environments of directly imaged exoplanets.