Every planet has to be built in a hurry.

New observations from NASA's James Webb Space Telescope (JWST) suggest that the young stars nurturing baby planets are also blowing away the ingredients those planets need. Once the gas is gone, the chance to build a giant world goes with it.

The study, published in The Astronomical Journal, examined 72 young, Sun-like stars. It was led by Naman Bajaj of the University of Arizona. His co-authors include his advisor Ilaria Pascucci of the university's Lunar and Planetary Laboratory and Uma Gorti of the SETI Institute. According to the SETI Institute, it is among the largest studies of planet formation carried out with JWST.

A nursery with a time limit

Planets take shape inside protoplanetary discs, which are swirling pancakes of gas and dust around newborn stars. Think of them as the building site.

Our own Sun once had one. In its first few million years, it was surrounded by a disc holding roughly 100 times more gas than dust. Nearly all of that gas has since vanished, leaving a 4.5-billion-year-old solar system that is mostly empty space.

That matters because gas is what giant planets such as Jupiter and Saturn are made from. If a disc loses its gas too quickly, growing planets may not have time to gather the enormous atmospheres that make them gas giants.

"Planet formation is therefore a race against time," Bajaj said. "Gas giants like Jupiter must assemble their massive atmospheres while the disk is still substantial enough to supply them, before winds and jets carry that raw material away into space."

A family album of young stars

The team used archival data from JWST's Mid-Infrared Instrument. The 72 systems are at different stages of development, so together they work a bit like a family album. Each is a snapshot of a different age, and lining them up lets researchers reconstruct how a disc empties over time.

They looked for two tell-tale signs of escaping material. The first was molecular hydrogen, the most abundant molecule in these discs. The second was ionised neon. JWST's sharp vision let them tell broad molecular hydrogen winds apart from the jets traced by neon.

They found extended emission from these gases in 66 of the 72 discs. Cone-shaped molecular hydrogen winds appeared in 46 systems, and fast-moving neon jets in 40. Every system with a neon jet also showed a wind, traced by either molecular hydrogen or oxygen.

Two ways to clear a disc

The pattern changes with age. In the youngest systems, material is still falling onto the central star. There the team saw powerful jets alongside broad winds of both molecular and atomic gas. These are consistent with winds driven by magnetic fields threading through the disc, which guide gas outward and carry material away.

As a system matures and less material feeds the star, the jets weaken and the escaping gas becomes more atomic. Now high-energy radiation from the young star can penetrate the thinning disc and heat the gas until it escapes, a process called photoevaporation. It is a little like the Sun drying up a puddle, only the puddle is a planetary nursery.

So, according to the researchers, no single mechanism clears a disc. Systems appear to begin with magnetically driven jets and winds. Later they shift towards a phase in which atomic winds, including those from photoevaporation, matter more.

Gorti has spent decades studying how discs disperse. "What is exciting about this study is that we can now see, across a large sample of young systems, how the mechanisms that remove gas from planet-forming disks change with time," she said. "Disk dispersal sets a fundamental clock for planet formation: once the gas is gone, the opportunity to build gas-rich planets is essentially over."

Building on earlier work

Before JWST, astronomers could not directly observe molecular hydrogen in these discs. A 2020 study led by Pascucci predicted that molecular winds should exist. The new observations trace that gas directly and, the team says, confirm the prediction. The survey also scales up a 2024 study in which Bajaj, Gorti and colleagues used JWST to image gas escaping the disc around a young star called T Cha.

What comes next

The study shows how the gas-clearing mechanisms change, but not yet how much gas they remove. The researchers now want to measure that, and to pinpoint where in a disc the escaping material comes from.

Those measurements could show how quickly the window for planet-building closes. They could also show which regions of a disc can produce which kinds of planets before the gas runs out. For a solar system that got its Jupiter in time, it is a reminder of how close the race can be.

The study: Bajaj et al., "JWST/MIRI Reveals the Evolution from Molecular to Atomic Disk Winds", The Astronomical Journal 172(3):161, 2026. DOI: 10.3847/1538-3881/ae9089.