Planet formation is a 'race against time,' according to new U of A research
Original article can be found at https://news.arizona.edu/news/planet-formation-race-against-time-according-new-u-research?utm_source=trellis&utm_medium=email&utm_campaign=Plus:%20President%20Garimella%20op-ed,%20Nancy%20Roman%20Space%20Telescope,%20PCORI,%20protoplanetary%20disks,%20MESA
By Mikayla Mace Kelley, University Communcations
The solar system today – at a mature 4.5 billion years of age – is mostly empty space sprinkled with a few planets, asteroids and comets. But in the first few million years of the solar system's life, it was a thick, swirling mass that contained 100 times more gas than dust.
To better understand how and when such vast amounts of gas vanished from our solar system disk and others like it, University of Arizona Lunar and Planetary Laboratory doctoral student Naman Bajaj dug into archival data from NASA's James Webb Space Telescope.
In one of the largest studies in planet formation conducted using JWST, this work supports past research that shows the existence of two physical mechanisms working in succession in the first 10 million years of a solar system's life. This study also constrains when the transition between dominating mechanisms occurs.
A real image of disk winds carrying out molecular hydrogen gas from a planet-forming disk located about 450 light-years from us. The white line is the plane of the planet-forming disk, while the bright yellow, orange, pink and purple represent ejected gas.
Naman Bajaj/JWST/MIRI-IFU
Bajaj and his team published their findings today in the Astronomical Journal.
It starts with a newborn star ringed by a dense protoplanetary disk, which hosts its own huge magnetic field. Gas from the disk hitches a ride along magnetic field lines. They are funneled up and out from the solar system's navel at 10-100 miles per second. This outflowing gas is called the "magnetic winds," and it is so massive that it blocks all the X-ray photons, which is high-energy light, from reaching the disk.
But, after a few million years, the magnetic winds weaken. At this point, what's known as 'photoevaporative winds' likely become the dominating force, this study suggests. Composed of energetic X-rays and ultraviolet light from the sun, the photoevaporative wind penetrates the magnetic winds, excites the gas in the disk and expels it from the system. Bajaj likes to compare this to how the energy from the sun heats water on Earth and evaporates it.
"After a few million years, the jets disappear and the molecular winds fade, leaving behind only gentler atomic winds that quietly erode what remains," Bajaj said. "This means that every planetary system with a sun-like star, including our own, likely underwent a vigorous phase of magnetic wind-driven mass loss early in its history, before transitioning to a calmer dispersal phase.
"Planet formation is therefore a race against time," he 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."
The results were based on 72 images of young sun-like stars and their surrounding disks. Each image captured a system at a different stage of life, so when the images were stitched together like stills from a movie, it created a timeline of how planetary systems lose the raw material to build planets.
Data from each system allowed the team to trace the movement of two specific gases: molecular hydrogen, which is two bonded hydrogen atoms, and neon gas that has been ionized, or electrically charged.
They focused on molecular hydrogen because it makes up most of the mass in the universe, and past U of A-led research showed it traces molecular winds. Neon, on the other hand, can only be observed by Webb's midinfrared detectors when it's ionized, meaning it's excited by high-energy radiation in the photoevaporative wind.
"Neon initially traces the fast-moving jets while molecular hydrogen is tracing wider winds. Later, we see neon in the slower, broader motion of the photoevaporative wind when the magnetic jets and winds weaken, and the X-ray photons can excite neon," said Bajaj. "During this phase, molecular hydrogen seems to trace weaker winds or nothing at all."
In 2020, LPL professor Ilaria Pascucci, second author of the paper and Bajaj's advisor, led a team that conducted a similar study to understand how jets and winds evolve. At that time, pre-JWST, they couldn't observe molecular hydrogen directly, but they predicted the existence of molecular winds and that these winds could be massive enough to block X-ray photons at earlier ages. With this new study, by tracing molecular hydrogen directly, Bajaj's team also confirmed these predictions with the JWST images.
Reflecting on the images, what surprised Bajaj most was just how beautiful they were.
"I mean, we expected the images to be nice, we didn't expect them to be absolutely stunning," he said.
Next, the team hopes to more thoroughly constrain how much gas mass is lost over time. Bajaj also wants to pin down exactly at what distance from the star the gas launches from the disk, which is important for understanding at what distance certain planets can form.
Additional co-authors include Sylvie Cabrit of Centre National de la Recherche Scientifique, Smith College's Suzan Edwards, University of Zurich's Gabriele Cugno, Leiden University's Andrew Sellek, NSF NOIRLab's Joan Najita, University of Wisconsin-Madison's Ke Zhang, University of Leicester's Richard Alexander, Peking University's Gregory Herczeg, Uma Gorti of SETI Institute and NASA, U of A's Sophie Clark and Space Telescope Science Institute's Tracy Beck.
A mosaic of the James Webb Space Telescope images used in this study. Each of these images is about 750 astronomical units, or AU, wide and tall. For context, one AU is the average Earth-Sun distance.
Bajaj et al. 2026