JWST Unveils a High Mean Molecular Weight Atmosphere for Mini-Neptune TOI-1130 b : Evidence for Formation Beyond the Water Ice Line**This study uses CHEOPS data observed as part of the Guaranteed Time Observation (GTO) programme CH_PR149003
(2026) In Astrophysical Journal Letters 1002(2).- 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 Teq ∼ 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—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... (More)
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 Teq ∼ 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—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 ( (Formula presented) logZ/Z⊙=1.8−0.3+0.4 ) and C/O ratio (<0.75 at 3σ level confidence) and constrain the atmospheric mean molecular weight, μ = 5.5 (Formula presented) −0.8+1.3 amu. These constraints are consistent with self-consistent forward model grids. We detect no significant He I 1.083 μm absorption signal and find a mass-loss rate upper limit of 1011 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.
(Less)
- author
- organization
- publishing date
- 2026-05
- type
- Contribution to journal
- publication status
- published
- subject
- keywords
- Exoplanet atmospheric composition (2021), Exoplanet formation (492), Mini Neptunes (1063), Transmission spectroscopy (2133)
- in
- Astrophysical Journal Letters
- volume
- 1002
- issue
- 2
- publisher
- IOP Publishing
- external identifiers
-
- scopus:105037897725
- ISSN
- 2041-8205
- DOI
- 10.3847/2041-8213/ae5f8b
- language
- English
- LU publication?
- yes
- id
- ce87ced1-241a-4feb-838b-e6206cc0d18c
- date added to LUP
- 2026-08-17 11:47:14
- date last changed
- 2026-08-17 11:48:12
@article{ce87ced1-241a-4feb-838b-e6206cc0d18c,
abstract = {{<p>We present the combined JWST/NIRSpec/G395H and NIRISS/SOSS transmission spectrum of a warm mini-Neptune, TOI-1130 b (3.66 R<sub>⊕</sub>, 19.8 M<sub>⊕</sub>, and T<sub>eq</sub> ∼ 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<sub>2</sub>O (7.5σ), CO<sub>2</sub> (3.3σ), and SO<sub>2</sub> (3.6σ), as well as a tentative detection of CH<sub>4</sub> (∼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 ( (Formula presented) logZ/Z⊙=1.8−0.3+0.4 ) and C/O ratio (<0.75 at 3σ level confidence) and constrain the atmospheric mean molecular weight, μ = 5.5 (Formula presented) −0.8+1.3 amu. These constraints are consistent with self-consistent forward model grids. We detect no significant He I 1.083 μm absorption signal and find a mass-loss rate upper limit of 10<sup>11</sup> g s<sup>−1</sup>. 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.</p>}},
author = {{Barat, Saugata and Fairnington, Tyler and Courreges, Shelby and Huang, Chelsea and Vanderburg, Andrew and Morley, Caroline V. and Korth, Judith and Parviainen, Hannu and Brandeker, Alexis and Zhou, George and Evans-Soma, Thomas M. and Sha, Lizhou and Lin, Douglas N.C. and Wright, Duncan and Morrissey, Ava and Nabbie, Emma and Collins, Karen A. and Evans, Phil and Guillot, Tristan and Horne, Keith and Radford, Don J. and Schwarz, Richard P. and Shporer, Avi and Srdoc, Gregorg and Suarez, Olga}},
issn = {{2041-8205}},
keywords = {{Exoplanet atmospheric composition (2021); Exoplanet formation (492); Mini Neptunes (1063); Transmission spectroscopy (2133)}},
language = {{eng}},
number = {{2}},
publisher = {{IOP Publishing}},
series = {{Astrophysical Journal Letters}},
title = {{JWST Unveils a High Mean Molecular Weight Atmosphere for Mini-Neptune TOI-1130 b : Evidence for Formation Beyond the Water Ice Line**This study uses CHEOPS data observed as part of the Guaranteed Time Observation (GTO) programme CH_PR149003}},
url = {{http://dx.doi.org/10.3847/2041-8213/ae5f8b}},
doi = {{10.3847/2041-8213/ae5f8b}},
volume = {{1002}},
year = {{2026}},
}