奇妙な惑星系形成起源を解明（Astronomers pin down origins of planetary odd couple）

2026-05-05 マサチューセッツ工科大学（MIT）

The planetary odd couple of a mini-Neptune and hot Jupiter probably formed out beyond its star’s “frostline,” in the colder region of the protoplanetary disk.Image: Jose-Luis Olivares, MIT

＜関連情報＞

• https://news.mit.edu/2026/astronomers-pin-down-origins-planetary-odd-couple-0505
• https://iopscience.iop.org/article/10.3847/2041-8213/ae5f8b

JWSTがミニネプチューンTOI-1130 bの高平均分子量大気を解明：水氷線以遠での形成の証拠 JWST Unveils a High Mean Molecular Weight Atmosphere for Mini-Neptune TOI-1130 b: Evidence for Formation Beyond the Water Ice Line

Saugata Barat, Tyler Fairnington, Shelby Courreges, Chelsea Huang, Andrew Vanderburg, Caroline V. Morley, Judith Korth, Hannu Parviainen, Alexis Brandeker, George Zhou,…
The Astrophysical Journal Letters  Published: 2026 May 5
DOI:10.3847/2041-8213/ae5f8b

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—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 1.083 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.