On ice
James Webb telescope finds ice older than the star it orbits
Radiation and collisions reshape the planetary building materials.
Beta Pictoris is a young star, roughly 20 million years old, and located about 64 light-years away from Earth. It is surrounded by a vast disk of dust and rubble where planets are still forming—astronomers have already found three. In a new Science study, a team led by Sarah Betti, an astronomer at the Space Telescope Science Institute in Baltimore, used the James Webb Space Telescope to look for ice in this disk. Researchers found both water and carbon dioxide ice.
The water ice is glassy rather than crystalline, and it is mixed with carbon dioxide in roughly the same proportions we see in comets of our own Solar System. This suggests ingredients necessary to form water-rich worlds might be common around young stars.
The missing ice
Beta Pictoris is one of the best studied stars in the sky. Its disk was among the first ever imaged, back in 1984, and we see it almost exactly edge-on, which makes it a natural laboratory for studying how planetary systems grow. It’s what astronomers call a debris disk, where the dust is constantly replenished by collisions among comets and planetesimals, small bodies that are the building blocks of planets.
The outer parts of the disk are far enough from the star that water and other volatile molecules should freeze into solid ice. The disk holds a large reservoir of carbon monoxide gas, which ultraviolet light from the star should destroy within 50 to 300 years. For that gas to still be there, something must keep replenishing it—most likely it’s released from ice when volatile-rich comets collide. The properties of the planet Beta Pictoris b also suggest it formed from ice-rich material.
Yet despite plenty of reasons to suspect the system is full of ice, earlier searches have failed to find it. Attempts to detect water vapor found nothing, and a search for the telltale dip that water ice leaves in starlight at a wavelength of 3 micrometers also failed.
According to theoretical models, ultraviolet radiation from the star can destroy small grains of pure ice faster than collisions can produce them. On the other hand, grains of dirty ice, mixed with other molecules, should survive longer.
Reading the reflection
Betti and her colleagues observed Beta Pictoris on December 7, 2023, using Webb’s Near Infrared Spectrograph (NIRSpec). They operated it in a mode that records a full spectrum, from 0.6 to 5.2 micrometers, for every point in the image. After subtracting the overwhelming glare of the star, the team measured how much starlight the disk’s dust reflects at each wavelength to look for the fingerprints of ices.
Water ice absorbs light strongly near 3 micrometers, while carbon dioxide ice absorbs either at 4.27 micrometers or 4.39 micrometers when it contains the heavier carbon-13 isotope. The team first mapped the water ice features across the disk and found they were strikingly uniform. They then extracted detailed spectra from two spots on opposite sides of the star, each roughly 100 astronomical units out.
Both spectra showed water and carbon dioxide ices, the latter in both its common form and the rarer carbon-13 version. But the shape of those features turned out to tell us more than the fact they were there.
Dirty glass
The carbon dioxide features were shifted slightly toward shorter wavelengths and were much broader than those of pure carbon dioxide ice measured in the lab. The team says in the paper that this is how carbon dioxide behaves when its molecules are trapped inside water ice. In other words, the two ices aren’t sitting side by side as separate grains—they are thoroughly mixed, which makes it the kind of dirty ice that models suggested could survive the star’s ultraviolet radiation.
The team calculated that there is slightly more than one carbon dioxide molecule for every 10 water molecules, meaning it’s about 11 percent of the ice. That falls within the 2.5 to 30 percent range measured in Solar System comets and the 7 to 20 percent seen in the ice of disks around other newborn stars. It means the ices of Beta Pictoris may have formed through the same processes as in our own Solar System, building up on dust grains in cold, dense, water-rich clouds during the earliest stages of star formation.
The structure of the water ice also pointed in this direction. When water freezes at moderate temperatures, its molecules settle into an orderly crystal lattice, and crystalline ice produces a sharp peak in the spectrum just above 3 micrometers. That peak was absent at Beta Pictoris. The ice is almost entirely amorphous, a disordered, glass-like solid that forms when water molecules freeze so fast and so cold that they stick wherever they land. The team estimates that less than 7 percent of the water ice in the system is crystalline, which is somewhat surprising.
Models predicted that ice in debris disks should mostly be crystalline. For amorphous ice to stay in that form, it has to remain colder than about 100 Kelvin, or minus 173° Celsius. Otherwise, something has to keep breaking up its crystal structure, such as high-energy ultraviolet light, cosmic rays, or particles streaming from the star. Either way, the researchers concluded the ice most likely formed in the interstellar medium, long before the planets of Beta Pictoris began to take shape.
The remaining question was how the icy grains got spread so evenly across the disk.
Constant collisions
The team argues that this even ice distribution requires frequent collisions among planetesimals. Each collision would release fresh icy dust, including ice that had been buried and protected deep inside the colliding bodies, replenishing what the starlight destroys. We already have evidence that such collisions do happen. Earlier infrared images revealed a curved, lopsided streak of dust in the southwestern arm of the disk, thought to be debris from a violent collision a little less than halfway out from the star.
Betti’s team’s findings support the idea that the disk’s carbon monoxide gas comes from colliding icy bodies and offer an explanation of how ice can survive in an environment constantly exposed to ultraviolet light. What the team could not do is determine exactly how much crystalline ice is present or rule out other molecules hiding in the mixture.
But if a young planetary system 64 light-years away may have started with much the same icy ingredients as ours, then primordial, interstellar ice might be a common ingredient of planetary systems in general. This means the raw materials for water-rich, potentially habitable worlds might be common, too.
Science, 2026. DOI: 10.1126/science.adw7770
