A Sulfur-Rich Atmosphere for the Young Jupiter Analog AF Lep b Reveals Significant Solid Accretion

AF Lep b is one of the closest analogs to Jupiter in mass (3-4 M_Jup) and semi-major axis (9 AU) amenable to spectroscopic characterization. This study aimed to characterize its atmospheric composition and use it to constrain the planet's formation history.

The team obtained JWST/NIRSpec high-contrast spectroscopy from 2.85-5.3 µm at R~3000, detecting CO2, H2S, CH4, 12CO (and 13CO), and H2O, plus complementary JWST/NIRCam imaging capturing the planet's continuum flux from 4.0-4.7 µm. They combined these with VLTI/GRAVITY and VLT/SPHERE spectra (1.0-2.5 µm) and performed atmospheric retrievals that included clouds and disequilibrium chemistry while allowing C, O, and S abundances to vary independently.

AF Lep b exhibits metal enrichment across C, O, and S with C/H=2.9±0.5, O/H=3.7±0.6, and S/H=4.7±0.7 times solar (and stellar). The planet's slightly sub-solar C/O and C/S are consistent with formation near its observed location and disfavor formation beyond the CO snowline. The sulfur enrichment implies significant accretion of disk solids during formation, and the authors estimate the planet contains 56±7 M_Earth of solids.

The C, O, and S enrichment levels of AF Lep b are similar to those of Jupiter and other super-Jupiters like HR 8799 bcde. They also show that the degree of atmospheric metal enrichment of these imaged planets is similar to the bulk metal enrichment of transiting gas giants with masses greater than ~1 M_Jup, suggesting that the process and efficiency of metal accretion for gas giants may not be strongly dependent on orbital distance or planet mass.