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_⊕, Teq∼825 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 — H₂O (7.5σ), CO₂ (3.3σ), and SO₂ (3.6σ), as well as a tentative detection of CH₄ (∼2σ). 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_⊙=1.8+0.4−0.3) and C/O ratio (<0.75 at 3σ 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 1.083μm absorption signal and put a mass-loss rate upper limit of 1011gs−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.

Saugata Barat, Tyler Fairnington, Shelby Courreges, Chelsea Huang, Andrew Vanderburg, Caroline V. Morley, Judith Korth, Hannu Parviainen, Alexis Brandeker, George Zhou, Thomas M. Evans-Soma, Lizhou Sha, Douglas N. C. Lin, Duncan Wright, Ava Morrissey, Emma Nabbie, Karen A. Collins Phil Evans, Tristan Guillot, Keith Horne, Don J. Radford, Richard P. Schwarz, Avi Shporer, Gregorg Srdoc, Olga Suarez

Comments: Accepted ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.02036 [astro-ph.EP] (or arXiv:2605.02036v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.02036
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Related DOI:
https://doi.org/10.3847/2041-8213/ae5f8b
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Submission history
From: Saugata Barat
[v1] Sun, 3 May 2026 20:04:12 UTC (11,441 KB)
https://arxiv.org/abs/2605.02036

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