Summary
A combined jwst NIRSpec G395H and NIRISS SOSS transmission spectrum of the warm mini-Neptune TOI-1130b (3.66 R⊕, 19.8 M⊕, Teq ~825 K) reports a high mean molecular weight atmosphere (μ = 5.5 amu) with strong H₂O (7.5σ), CO₂ (3.3σ), and SO₂ (3.6σ) detections plus a tentative CH₄ feature (~2σ). The retrieved atmospheric metallicity (logZ/Z⊙ = 1.8) and constrained C/O ratio (<0.75) are consistent with a volatile-rich, ex-situ formation beyond the water-ice-line followed by inward planetary-migration. The outer hot-Jupiter companion likely “pebble-filtered” inner-disk solids, supporting the migration narrative. The result places toi-1130b at the edge of the “radius cliff” and argues mini-Neptunes are formation-pathway heterogeneous. The paper is not a biosignature claim but informs how jwst interprets sub-Neptune atmospheres adjacent to k2-18b and the hycean debate.
Key Claims
- JWST/NIRSpec G395H + NIRISS SOSS transmission spectrum of toi-1130b detects H₂O at 7.5σ, CO₂ at 3.3σ, SO₂ at 3.6σ, and tentative CH₄ at ~2σ.
- Retrieved atmospheric metallicity logZ/Z⊙ = 1.8 (+0.4/−0.3); C/O < 0.75 at 3σ; mean molecular weight μ = 5.5 (+1.3/−0.8) amu — consistent with a high-μ, volatile-rich envelope rather than a hydrogen-dominated one.
- No significant He I 1.083 μm absorption; mass-loss rate upper limit 10¹¹ g/s.
- toi-1130 is a rare multi-planet system with an inner mini-Neptune and outer hot Jupiter locked in a 2:1 mean motion resonance.
- The high-μ atmosphere plus the outer hot Jupiter’s “pebble-filtering” effect support ex-situ formation beyond the water-ice-line and subsequent migration — coherent with the present 2:1 resonance architecture.
- Volatile-rich formation may explain TOI-1130b’s location at the edge of the planetary “radius cliff.”
- Mini-Neptunes are not a homogeneous population — volatile-rich ex-situ formation contributes alongside in-situ rocky-core scenarios.
Notable Quotes
“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.” — Barat et al.
“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.