Unexpected Atmosphere Composition of a Warm Mini-Neptune express
Steam over a boiling pot slightly tints the light of a flashlight: it blocks some colors and lets others through. That's roughly how we study the air of distant planets. When a planet passes in front of its star, a tiny fraction of starlight pierces its atmosphere, and molecules leave dark bands in the spectrum: their "fingerprints."
The James Webb Space Telescope JWST did this for the planet TOI-1130b — a warm mini-Neptune (about 3.7 times wider than Earth and 20 times heavier). Its atmosphere turned out to be a real chemical cocktail. Clearly visible are water (H₂O), carbon dioxide (CO₂), and even sulfur dioxide (SO₂)—the latter is rarely found on such planets. There are hints of methane (CH₄) as well, but not as clear yet.
The composition reveals the planet's origin. The atmosphere is rich in heavy elements (metallicity about 60 times solar), and the carbon-to-oxygen ratio is low. Such a mixture is typical of worlds born far from the star, beyond the "snow line"—the boundary where water freezes into ice. Most likely, TOI-1130b formed in the cold outer regions of the system and then moved closer to the star. Possibly under the influence of its more massive neighbor — a hot Jupiter on an outer orbit. That one, like a giant vacuum cleaner, could have "filtered" pebbles and ice, directing some material inward.
This discovery adds an important detail to the portrait of mini-Neptunes — the most common type of planet in the galaxy, which are absent in our solar system. It seems they don't have a single recipe for formation: some grow in place, others come from afar, bringing volatile substances with them. By studying their air, we are essentially peeking into the pot where planetary diversity is simmering.
Original abstract
as written by the authors arXiv:2605.02036
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$$_{\oplus}$$, $$T_{eq}\sim825$$ 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$$\sigma$$), CO$$_2$$ (3.3$$\sigma$$), and SO$$_2$$ (3.6$$\sigma$$), as well as a tentative detection of CH$$_4$$ ($$\sim$$2$$\sigma$$). 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 ($$\log{Z/Z_{\odot}}=1.8^{+0.4}_{-0.3}$$) and C/O ratio ($$<$$0.75 at 3$$\sigma$$ level confidence) and constrain the atmospheric mean molecular weight, $$\mu$$ = 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$$\mu$$m absorption signal and put a mass-loss rate upper limit of $$10^{11}$$g\s$$^{-1}$$. 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.
The equilibrium temperature of TOI-1130b is about 825 kelvins (roughly 550 °C). That's hotter than the surface of Venus but cooler than the flame of a gas stove.