Young planet-forming disks lose gas faster than expected

Planet formation begins around young stars, inside swirling disks of gas and dust. A new analysis of data from the Atacama Large Millimeter/submillimeter Array (ALMA) shows that the gas in these disks fades faster than the dust – reshaping ideas about how and when planets take shape.

The new work comes from the ALMA Survey of Gas Evolution of PROtoplanetary Disks (AGE-PRO) – a project led by scientists at the University of Arizona and the University of Wisconsin-Madison.

“Now we have both, the gas and the dust,” said Professor Ilaria Pascucci. “Observing the gas is much more difficult because it takes much more observing time, and that’s why we have to go for a large program like this one to obtain a statistically significant sample.”

A close look at planet-forming disks

AGE-PRO examined 30 disks around Sun-like stars in three star-forming regions that span one to six million years in age.

These disks were studied not only for their dust properties, as in previous research, but also for their gas mass and size – providing the first robust look at how gas content evolves throughout a disk’s lifetime.

The most surprising result of the study was that gas and dust do not vanish together. Large amounts of gas blow away early, then leak more slowly as the disk grows older. Dust, however, tends to persist.

The researchers also found that the gas-to-dust mass ratio evolves over time – and not necessarily in the way earlier models predicted.

Smaller disks weren’t shedding their gas faster, as previously assumed. Instead, the gas-to-dust ratio appeared surprisingly consistent across disks of different sizes, a twist that hints at deeper complexities in disk evolution.

What this means for planet birth

If gas leaves early, giant planets like Jupiter must form rapidly before the disk’s supply runs out. Rocky planets, by contrast, may continue building over a longer timeline, supported by lingering dust.

Ke Zhang of Wisconsin-Madison, principal investigator of AGE-PRO, noted that some older disks still carry more gas than expected. This unexpected retention suggests a wider variety of evolutionary paths – and potentially more diverse planetary systems – than previously assumed.

The disk’s initial mass, size, and angular momentum also influence the types of planets it can form – whether gas giants, icy giants, or mini-Neptunes – as well as their migration patterns after formation.

Finding the faintest signals

ALMA’s sharp vision enabled the team to detect faint “fingerprints” of cold gas in the form of molecular spectral lines.

While carbon monoxide (CO) is the most commonly used gas tracer, AGE-PRO went further – adding diazenylium, formaldehyde, methyl cyanide, and even molecular species containing deuterium, a rare hydrogen isotope. These additional tracers significantly improved the accuracy of gas mass estimates.

This marks the first large-scale chemical survey of planet-forming disks across multiple evolutionary stages, made possible by ALMA’s sensitivity and long integration times.

Eyes on planet-forming regions

The researchers focused on three nearby regions where stars and planets are currently forming: Ophiuchus, Lupus, and Upper Scorpius.

These regions represent different stages in the evolution of protoplanetary disks – with Ophiuchus being the youngest (around 1 million years old).

Lupus is in the middle (1–3 million years), and Upper Scorpius is the oldest (about 5–6 million years). This range allowed the team to observe how gas and dust behave over time as disks age.

Radio signals and planet-forming disks

Dingshan Deng, a graduate student at the University of Arizona’s Lunar and Planetary Laboratory, handled the painstaking data reduction for the Lupus region.

The task involved turning raw radio signals into detailed images of the disks, a step essential for measuring gas mass.

“Thanks to these new and long observations, we now have the ability to estimate and trace the gas masses, not only for the brightest and better studied disks in that region, but also the smaller and fainter ones,” said Deng.

“Thanks to the discovery of gas tracers in many disks where it hadn’t been seen before, we now have a well-studied sample covering a wide range of disk masses in the Lupus star-forming region.”

A legacy for future work

According to Professor Pascucci, it took years to figure out the proper data reduction approach and analysis to produce the images used in this paper for the gas masses and in many other papers of the collaboration.

The AGE-PRO data create a public spectral-line library covering key stages of disk evolution. Astronomers worldwide can now compare young and old disks using the same observational foundation.

The team hopes this resource will fuel new models of planet formation, offering insights into how worlds emerge and migrate inside their dusty cradles.

The full study was published in the journal The Astrophysical Journal.

Image Credit: NSF/AUI/NSF NRAO/S.Dagnello

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