Abstract We present the combined JWST/NIRSpec/G395H and NIRISS/SOSS transmission spectrum of a warm mini-Neptune, TOI-1130 b (3.66 R ⊕ , 19.8 M ⊕ , and T eq ∼ 825 K). It is part of a rare and unique multiplanet 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-1130 b we detect multiple molecules—H 2 O (7.5 σ ), CO 2 (3.3 σ ), and SO 2 (3.6 σ ), as well as a tentative detection of CH 4 (∼2 σ ). 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 ( log Z / Z ⊙ = 1 . 8 − 0.3 + 0.4 ) and C/O ratio ( − 0.8 + 1.3 amu. These constraints are consistent with self-consistent forward model grids. We detect no significant He I 1.083 μ m absorption signal and find a mass-loss rate upper limit of 10 11 g s −1 . The volatile-rich high mean molecular weight atmosphere of TOI-1130 b 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-1130 b at the edge of the “radius cliff.” This result hints that the mini-Neptune population may not have a homogeneous formation history; rather, volatile-rich ex situ formation also contributes to its population.
Barat et al. (Tue,) studied this question.