Gas persists inside the dust gaps of three planet-forming disks

Three disks around relatively faint young stars have large central gaps in their millimeter-emitting dust, but gas remains inside those gaps. That is the central finding of a study by A. A. Rota and colleagues, presented in the preprint “CO gaps in cold transition disks around low-mass stars,” arXiv:2611.09449v1. Submitted on 6 November 2026, the preprint is listed as accepted for publication in Astronomy & Astrophysics.

These systems are called transition disks: planet-forming disks with substantial cavities in their dust distribution. One proposed explanation is that a massive companion disturbs the gas and creates a pressure maximum outside its orbit, trapping millimeter-sized dust grains in a ring. But identifying a ring is not the same as detecting its possible planetary architect.

Looking at dust and gas separately

The researchers studied Sz 111, J1633 and J1608, three inclined disks in the Lupus and Ophiuchus star-forming regions. Their host stars have luminosities ranging from about one-fifth to nearly twice the Sun’s. None shows the excess near-infrared emission expected from a substantial amount of hot dust close to the star.

Using the Atacama Large Millimeter/submillimeter Array, or ALMA, the team observed two kinds of emission. The broad emission near a wavelength of 1.3 millimeters traces dust. Specific spectral lines trace carbon monoxide gas in three molecular varieties: ordinary CO, carbon-13 CO and oxygen-18 CO. These varieties differ in their constituent isotopes and their abundance, allowing them to probe different layers and amounts of gas.

The dust images resolve structures a few astronomical units across; one astronomical unit, or au, is the Earth–Sun distance. The gas images have coarser resolution, about 20 au, because the highest-resolution gas observations were too weak to use reliably. The team corrected for the disks’ tilted appearance and measured how emission changes with distance from each star.

Cavities, but not empty interiors

All three disks have bright dust rings peaking roughly 35–40 au from their stars. No separate inner dust disk was detected. Carbon monoxide emission, however, extends inside the dust cavities. The most abundant variety generally peaks closer to the star than the rarer varieties; in J1633, ordinary CO peaks at the stellar position rather than forming a central emission cavity.

These differences do not map directly onto separate physical boundaries. A more abundant molecule can produce detectable, opaque emission where a rarer variety cannot. The apparent sizes of the gas gaps therefore reflect how radiation travels through the disk as well as how much gas is present.

The researchers also converted line brightness into a “brightness temperature”: the temperature a blackbody would need to produce that intensity. For carbon-13 CO, peak values were about 21, 28 and 32 kelvin in Sz 111, J1633 and J1608, respectively. Brightness temperature can approximate the gas temperature when the emission is sufficiently opaque and resolved, but it is not an unconditional thermometer.

Compared with seven previously studied transition disks, the new targets showed no significant overall temperature trend with stellar luminosity. Their values also overlap those reported for disks without large cavities. This does not establish that all these disks have the same temperature structure. The comparison mixes observations with different resolutions and molecular transitions; the paper notes that spatial and spectral resolution can affect inferred brightness temperatures by roughly 10–30%.

Little detected dust, uncertain gas amounts

The absence of inner dust emission places model-dependent upper limits on dust mass: 0.003 Earth masses for Sz 111, 0.02 for J1633 and 0.0006 for J1608. These are limits, not measured masses or proof that no dust remains. They assume, among other things, optically thin emission, an adopted dust opacity and a compact inner disk.

Meanwhile, measured accretion onto the stars indicates that gas continues to reach them. Combining accretion-based gas estimates with dust limits gives very high lower bounds on the inner gas-to-dust ratio, reaching above 100,000 in J1608. This is not a direct measurement at 1 au: it depends on assumptions about temperature, dust distribution and how gas transports angular momentum.

A possible companion is not a detection

The authors tested an existing relation linking dust-ring and gas-gap sizes to companion mass. Applied here, it reaches or exceeds the roughly 15-Jupiter-mass upper boundary of the simulations used to calibrate it. Such extrapolation cannot reliably weigh an unseen object, especially when eccentric orbits could alter the gap structure.

Companions more massive than Jupiter on eccentric orbits remain a possible explanation, not a confirmed result. With only three new targets, the study chiefly establishes their observable gas-and-dust structure. More uniform observations, multiple molecular transitions and models combining chemistry with heating and cooling are needed to turn those structures into firmer physical constraints.