JWST unveils a high mean molecular weight atmosphere for mini-Neptune TOI-1130b: Evidence for formation beyond the water ice line

We present the combined JWST/NIRSpec G395H and NIRISS SOSS transmission spectrum of a warm mini-Neptune, TOI-1130b (3.66 R, 19.8 M, T eq about825 K). It is part of a rare and unique multi-planet system TOI-1130, which hosts an inner mini-Neptune and an outer hot Jupiter locked in a 2:1 mean motion resonance. From the transmission spectrum of TOI-1130b we detect multiple molecules -- H 2 O (7.5 sigma), CO 2 (3.3 sigma), and SO 2 (3.6 sigma), as well as a tentative detection of CH 4 (about 2 sigma). We find a strong optical slope in the NIRISS/SOSS spectrum, which is consistent with TESS and CHEOPS transit depth measurements. From equilibrium chemistry retrievals we measure the atmospheric metallicity (=1.8+0.4-0.3) and C/O ratio (< 0.75 at 3 sigma level confidence) and constrain the atmospheric mean molecular weight, mu = 5.5+1.3-0.8 amu. These constraints are consistent with self-consistent forward model grids. We detect no significant He I 1.083 mu m absorption signal and put a mass-loss rate upper limit of 10 11 g-1. The volatile-rich high mean molecular weight atmosphere of TOI-1130b along with the `pebble-filtering' effect of the outer hot Jupiter supports the ex-situ formation scenario beyond the water ice line and subsequent migration, coherent with its present orbital architecture. A volatile-rich formation scenario could also potentially explain the location of TOI-1130b at the edge of the `radius cliff'. This result hints that the mini-Neptune population may not a homogeneous formation history; rather, volatile-rich ex-situ formation also contributes to its population.