The findings, led by University of Arizona researcher Naman Bajaj and co-authored by SETI Institute scientist Uma Gorti, mark one of the largest planet-formation studies conducted with JWST to date. Published in The Astronomical Journal, the work shows that no single mechanism clears a protoplanetary disk. Instead, systems transition from magnetically driven jets and winds in their youth to atomic and photoevaporative winds later on, setting a hard deadline for building gas giants like Jupiter and Saturn.
A Clock That Cannot Be Reset
Planets assemble from disks of gas and dust swirling around young stars, but that gas is finite. Once it disperses, the window for forming thick atmospheres closes for good. Understanding how and when this happens is central to explaining why planetary systems, including our own, look the way they do.
The sun’s own protoplanetary disk once held roughly 100 times more gas than dust. Most of that material vanished within the first few million years of the solar system’s life. Today, 4.5 billion years later, the inner system is mostly empty space. The new JWST data helps explain why.
“Disk dispersal sets a fundamental clock for planet formation: once the gas is gone, the opportunity to build gas-rich planets is essentially over,” Gorti said. The study confirms that this clock is governed by multiple winds operating at different stages, not a single dominant process.
Tracing Molecular Hydrogen and Neon
Bajaj’s team used archival data from JWST’s Mid-Infrared Instrument, MIRI, to examine 72 systems at varying points in their early development. Each system functions like a single frame in a chronological movie, allowing researchers to reconstruct how disk winds evolve over millions of years.
The team focused on two telltale signatures of escaping gas: molecular hydrogen, the most abundant molecule in protoplanetary disks, and ionized neon. JWST’s sensitivity and resolution allowed the researchers to distinguish broad molecular hydrogen winds from the narrower jets and winds traced by neon.
The results were striking. Extended emissions from molecular hydrogen or ionized neon appeared in 66 of the 72 disks studied. Conical molecular hydrogen winds turned up in 46 systems, while fast-moving neon jets appeared in 40. Every system showing a neon jet also displayed signs of a wind traced by molecular hydrogen or oxygen, suggesting these outflows are deeply interconnected.
From Magnetic Winds to Photoevaporation
In the youngest systems, where material is still actively falling onto the central star, JWST detected strong jets and broad winds composed of both molecular and atomic gas. These outflows match predictions for winds driven by magnetic fields threading through the disk. Gas travels along these field lines, carrying mass and angular momentum away from the disk.
As systems age and the inflow of material slows, the jets weaken and the winds shift toward an atomic composition. At this stage, high-energy radiation from the young star penetrates the thinning disk, heating the gas until it escapes. That process, known as photoevaporation, becomes the dominant dispersal mechanism in older disks.
Gorti has spent decades modeling how ultraviolet and X-ray radiation from young stars drives photoevaporative winds. The new observations bridge her theoretical work with direct astronomical evidence, showing photoevaporation gaining ground as magnetically driven winds fade across dozens of systems.
Confirming a 2020 Prediction
The study also validates work from 2020 by Lunar and Planetary Laboratory professor Ilaria Pascucci, the paper’s second author and Bajaj’s adviser. Before JWST, Pascucci’s team could not observe molecular hydrogen directly, but they predicted that molecular winds existed and were massive enough to block X-ray photons at earlier ages. By tracing molecular hydrogen directly with JWST, Bajaj’s team confirmed those predictions with observational data.
“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.”
Building on the T Cha Discovery
The new research extends a 2024 study in which Bajaj, Gorti, and colleagues used JWST to image gas escaping from the protoplanetary disk around the young star T Cha. That earlier work proved JWST could study disk dispersal in a single system. The current paper scales that approach to 72 stars, revealing how the balance of jets, molecular winds, and atomic winds shifts as planetary systems mature.
Together, the two studies frame a broader narrative: planet formation is not just about accumulation of material, but also about its steady removal. The gas that could become a gas giant’s atmosphere is simultaneously being heated, ionized, and blown into interstellar space.
What Happens Next
The immediate priority for Bajaj and his colleagues is quantifying how much gas these winds actually remove over time, and pinpointing where in the disk the escaping material originates. Those measurements could determine not only how quickly the planet-forming window closes, but also which orbital regions remain gas-rich long enough to produce giant planets versus smaller rocky worlds.
If photoevaporation dominates in the later stages, it may preferentially strip gas from the inner disk, potentially explaining why terrestrial planets in our solar system are rocky while gas giants formed farther out. Conversely, if molecular winds remove material more uniformly, the architecture of planetary systems may depend more on timing than location.
Future JWST programs are likely to target disks at intermediate ages, where the transition from magnetic to photoevaporative winds is actively underway. Observing that handoff in real time could refine models of planet formation and help astronomers predict which nearby young stars are still within their gas-rich, planet-building phase. The race, it turns out, is observable — and the clock is already running.
— Priya Nair, science desk, AXO News