James Webb Finds Ice Older Than the Star It Orbits
The ice swirling around a young star 64 light-years from Earth is older than the star itself.
NASA’s James Webb Space Telescope has detected water and carbon dioxide ice in the debris disk around Beta Pictoris, a star only about 20 million years old where planets are still forming. Published in the journal Science, it suggests the raw materials for water-rich worlds may be inherited from the cold clouds between the stars, rather than made fresh around each new sun.
Beta Pictoris has long been one of astronomy’s favourite laboratories: its disk of dust and rubble was among the first ever imaged, back in 1984, and three planets have since been found orbiting within it. Seen almost exactly edge-on, the star acts as a backlight, letting telescopes probe the debris’s chemistry.
A young star with an ancient inheritance
The new work, led by Sarah Betti of the Space Telescope Science Institute in Baltimore, used Webb’s Near Infrared Spectrograph to examine the disk on 7 December 2023. Spectra from two regions about 100 astronomical units out revealed the fingerprints of both water ice and carbon dioxide ice.
The surprise was not that ice was there — the outer disk is cold enough for volatile molecules to freeze. The surprise was the state the ice was in, and what that reveals about where it came from.
Glassy ice gives the game away
When water freezes at ordinary temperatures, its molecules settle into a neat crystal lattice, which leaves a sharp peak in a spectrum just above 3 micrometres. That peak was missing at Beta Pictoris. Instead, the water ice is almost entirely amorphous: a disordered, glass-like solid that forms when molecules freeze so fast and so cold that they stick wherever they land.
The team estimates that less than 7 per cent of the water ice is crystalline — far less than models had predicted, and a sharp contrast with the crystalline ice Webb found around HD 181327. For amorphous ice to survive, it must stay colder than about 100 Kelvin, or minus 173 degrees Celsius; any more warmth, from starlight or cosmic rays, would nudge it toward a crystal structure.
The carbon dioxide told a similar story. It appeared in both its common form and a rarer version containing heavier carbon-13, intimately mixed with the water ice — roughly one carbon dioxide molecule for every 10 water molecules, about 11 per cent of the ice. As the researchers put it in the paper: “We infer that the CO2 ice is intimately mixed with the water ice at a ratio of 1:10. This implies that the ices formed in cold conditions, likely in the interstellar medium.”
In other words, this ice did not form around Beta Pictoris. It formed in the frigid molecular cloud that gave birth to the star, and survived the star’s ignition and 20 million years of radiation and collisions.
Collisions keep the ice coming
One puzzle remained: how did the icy grains spread so evenly through the disk? Only frequent collisions among planetesimals — the small, comet-like building blocks of planets — can explain the uniform distribution, the team argues. The disk is no static relic: it is constantly replenished as these bodies collide.
That restless activity also helps explain a long-standing puzzle: the disk holds a large reservoir of carbon monoxide gas, which the star’s ultraviolet light should destroy within a few centuries — unless colliding icy bodies keep topping it up. Ice mixed with carbon dioxide also withstands that radiation better than pure ice grains, which may be why these ancient ices have endured at all.
A familiar recipe for water-rich worlds
Perhaps the most striking detail is how ordinary the mixture looks. The proportions of water and carbon dioxide ice are roughly the same as those in comets in our own Solar System — the dirty snowballs thought to have delivered water to the young Earth. That resemblance echoes Webb’s earlier findings on primordial water in our own Kuiper Belt and on icy objects beyond Neptune, and hints the recipe for a water-rich world may be standard across the galaxy.
If so, the ingredients for habitable planets are no lucky accident of one star system. They are a legacy, carried from cloud to cloud and star to star, waiting in the dark until a new sun lights up. The same pattern may show up around other young stars — including the interstellar comet 3I/ATLAS, which Webb has also studied — and astronomers now have a chemical fingerprint to look for.
For Beta Pictoris, the next step is tracing how these ancient ices reach forming planets. The building blocks are in place — and far older than anyone expected. The water that may one day fill alien oceans could predate their star.
