Kavli Affiliate: Andrew Vanderburg
| Summary:
We present the combined JWST/NIRSpec G395H and NIRISS SOSS transmission spectrum of a warm mini-Neptune, TOI-1130b (3.66 R$_oplus$, 19.8 M$_oplus$, $T_eqsim825$ 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$σ$), CO$_2$ (3.3$σ$), and SO$_2$ (3.6$σ$), as well as a tentative detection of CH$_4$ ($sim$2$σ$). We find a strong optical slope in the NIRISS/SOSS spectrum, which is consistent with TESS and CHEOPS transit depth measurements. From equilibrium chemistry retrievals we measure the atmospheric metallicity ($logZ/Z_odot=1.8^+0.4_-0.3$) and C/O ratio ($<$0.75 at 3$σ$ level confidence) and constrain the atmospheric mean molecular weight, $μ$ = 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$μ$m absorption signal and put a mass-loss rate upper limit of $10^11$gs$^-1$. 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.
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