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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.
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.
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.