New insights from NASA’s James Webb Space Telescope reveal that the window for gas giant planets to form around young stars may be shorter than previously thought. In a study examining 72 young stars similar to our Sun, scientists discovered that the gas necessary for planet formation is progressively expelled, with the mechanisms responsible evolving as these stellar systems mature.
Planets emerge within dense disks of gas and dust circling newborn stars—an early phase our solar system also experienced. Initially, the young Sun was enveloped by a protoplanetary disk rich in gas, containing approximately 100 times more gas than dust. However, much of this gas was eventually lost.
The gas plays a vital role for planets like Jupiter and Saturn, as it provides the material to build their vast atmospheres. Once this gas dissipates, the opportunity to form gas giants closes. Spearheaded by Naman Bajaj from the University of Arizona with contributions from SETI Institute’s Uma Gorti, the study investigates how this gas escapes and how these escape processes transform over time.
JWST Tracks Gas Escape from Planet-Forming Disks
By analyzing data from JWST’s Mid-Infrared Instrument (MIRI), researchers surveyed disks around 72 youthful, Sun-like stars. Because these stars represent a range of evolutionary stages, the sample offers a comprehensive view of how protoplanetary disks lose gas as they age.
The findings, published in The Astronomical Journal, focus on two key indicators of gas loss: molecular hydrogen, the most prevalent molecule in such disks, and ionized neon. JWST’s sensitivity allows scientists to differentiate between widespread molecular hydrogen winds and neon-highlighted jets and outflows.
The presence of these winds was widespread. Emissions from both molecular hydrogen and ionized neon were detected in 66 out of the 72 disks. Specifically, conical molecular hydrogen winds appeared in 46 systems, while 40 exhibited fast-moving neon jets. The team noted that neon jets were always accompanied by molecular hydrogen or oxygen-traced winds.
Several predictions made before JWST’s launch have now been validated. In 2020, prior to direct molecular hydrogen observations, University of Arizona’s Ilaria Pascucci predicted that young star systems could generate molecular winds dense enough to block X-rays. JWST has since directly detected molecular hydrogen, confirming those earlier models.
Early Stellar Systems Exhibit Strong Magnetically Driven Winds
During the initial phases of a planetary system’s development, energetic activity dominates. Material is actively accreting onto the young star, while JWST observations revealed vigorous jets accompanied by broad outflows of molecular and atomic gas in the youngest stars studied. This pattern aligns with winds propelled by magnetic fields threading through the disk, which channel gas outward along magnetic lines, removing mass and angular momentum.
This dynamic does not remain static. As the system ages, material accretion decreases, causing jet strength to diminish. Atomic gas becomes more significant in the escaping flows. With less obstruction, high-energy radiation from the star penetrates deeper into the disk, heating the gas and ultimately driving it off—a process called photoevaporation.
“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. Disk dispersal sets a fundamental clock for planet formation: once the gas is gone, the opportunity to build gas-rich planets is essentially over,” Uma Gorti said.
The SETI Institute highlights that Gorti has extensively studied how protoplanetary disks evolve and the role of ultraviolet and X-ray radiation in driving photoevaporative winds. JWST now offers a wealth of data spanning multiple star systems to test and expand on this knowledge.
Giant Planet Formation Is a Race Against Time
At early stages, strong magnetic jets and winds dominate, but they fade over time as atomic gas-driven photoevaporative winds take precedence. For forming gas giants, this means that the supply of disk gas is limited and finite. These planets must accumulate their massive atmospheres before their disks shed the necessary gas.
“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.”
Earlier in 2024, Bajaj, Gorti, and colleagues used JWST to observe gas escaping from the protoplanetary disk around the young star T Cha. That study focused on a single star system. Now, the team aims to quantify how much gas these winds remove over time and identify which disk regions contribute the escaping material.
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