Interstellar comet 3I/ATLAS has a surprising chemical link to icy worlds beyond Neptune? What James Webb discovered and what it reveals about planets around other stars

Astronomers led by Dr Silvia Protopapa at the Southwest Research Institute (SwRI) in the US used the James Webb Space Telescope to study the ice in 3I/ATLAS. The ice grains are tiny, and they’re mixed with dark, rocky dust.

3I/ATLAS is only the third interstellar object ever found. It came from outside our Solar System, and its chemistry turns out to have more in common with the icy bodies beyond Neptune than anyone expected.

3I/ATLAS: What James Webb found about icy worlds beyond Neptune

Interstellar comets are small, icy bodies. They form in the disks of gas and dust around other stars, get thrown out, and drift through space for a very long time. A few pass through our Solar System.


Comets that belong to our Solar System go round the Sun again and again. An interstellar comet doesn’t. Once it leaves, it’s gone from view for good, so every observation counts.

The ATLAS telescope in Chile discovered 3I/ATLAS on 1 July 2025. The two earlier interstellar objects were 1I/ʻOumuamua and 2I/Borisov. Some studies suggest 3I/ATLAS is nearly as old as the Universe itself. That would make it a sample of material from a star system that formed very early.

Webb observed it with its NIRSpec instrument on three dates: 6 August 2025, 22 December 2025 and 1 April 2026. The comet was 3.3, 2.4 and 5.7 astronomical units from the Sun on those dates. One astronomical unit is the average distance between the Earth and the Sun, about 150 million km. The three looks together cover eight months around the comet’s closest approach to the Sun.

A comet’s ice doesn’t just sit still. As the comet warms up, gas escaping from its interior carries ice and dust grains into the cloud around it, called the coma. Webb studies the light from this cloud, and that’s how the team could read what the grains are made of.

They found that the water ice is mixed closely with dark, rocky material in grains measured in micrometres. A micrometre is a thousandth of a millimetre. According to Protopapa, the grains are about the same size as those in comets from our own Solar System. What sets 3I/ATLAS apart is how much dust is inside them.

The spectrum also looks different from other water-ice-rich comets in our Solar System. It shows strong absorption at 3 micrometres, but the water-ice bands at 1.5 and 2.0 micrometres are weak or missing. In plain terms, the ice leaves a different fingerprint in the light here.

This fits what Webb saw earlier. Webb’s first spectrum showed a coma dominated by carbon dioxide, with about eight times as much of it as water.

Why the region beyond Neptune matters here

The researchers also compared 3I/ATLAS with other comets and with several classes of trans-Neptunian objects. These are icy bodies that orbit the Sun beyond Neptune. Their spectra also show carbon dioxide ice and organic material.

The spectrum of 3I/ATLAS sits between that of our comets and several classes of mid-sized trans-Neptunian objects. It doesn’t match either group exactly. It falls in between.

That led the team to a broader idea. Icy building blocks formed around different stars may cover a range of ice types, from nearly pure ice grains to ice mixed with varying amounts of dust. 3I/ATLAS lies toward the dusty end of that range.

“At first glance, 3I/ATLAS looks different from the icy comets we know,” Protopapa said. The new work suggests the difference is smaller than it looked. Our own Solar System’s outer edge appears to hold material of a similar kind.

Webb’s last look came on 1 April 2026, when 3I/ATLAS was 5.7 astronomical units from the Sun, farther out than at the first observation. The comet is now leaving, and no telescope will see it again.