Why interstellar comet 3I/ATLAS' close Earth approach is an early Christmas gift

[Buster's Uncle]

Space
Why interstellar comet 3I/ATLAS' close Earth approach is an early Christmas gift for astronomers
Darryl Seligman
Mon, December 15, 2025 at 6:00 AM EST
5 min read



NASA’s Hubble Space Telescope re-observed the interstellar comet 3I/ATLAS on Nov. 30, 2025 with its Wide Field Camera 3 instrument. | Credit: NASA, ESA, STScI, D. Jewitt (UCLA). Image Processing: J. DePasquale (STScI)


Darryl Z. Seligman is an Assistant Professor of Physics and Astronomy at Michigan State University.

Comet 3I/ATLAS is the third large interstellar visitor that we have ever discovered —- an asteroid or comet that originated outside of our solar system and was discovered passing through. Astronomers can glean information about celestial bodies by observing the light reflected off them with telescopes. When 3I/ATLAS is closest to the Earth, all the features that we are looking for will be easier to detect with our telescopes.

This interstellar vagabond presumably formed in a protoplanetary disk of gas and dust swirling around another star, the sites of active planet formation. Most likely, 3I/ATLAS was ejected by a close gravitational slingshot off a giant exoplanet. It has since been careening through the interstellar medium of the Milky Way galaxy for billions of years. And we get front-row seats to watch as it gets close to our sun, for what is almost surely the first time it has ever gotten close to a star.

On Dec. 19, just six days before Christmas, this erratic wanderer will be the closest to Earth that it will ever be over the lifetime of the entire universe. You'll be able to get a glimpse of it up close with a small telescope or very powerful binoculars. This close approach also offers astronomers our best opportunity to look up close and learn about how planet formation in exoplanetary systems is similar or different to how it unfolded in our solar system.

In the past seven years, we have discovered three members of an entirely new population of celestial bodies: interstellar objects. These objects have hyperbolic orbits, as opposed to the bound circular or elliptical orbits of everything native to the solar system. That's how we know that they come from elsewhere: they come and leave and never return. All we get is a fleeting look into the lifetime of these objects, and the measurements we take during their brief passage through our solar system could provide critical clues into our understanding of planet formation throughout the galaxy.

We know that the solar system ejected a huge amount of material into the Milky Way galaxy in the form of interstellar comets. Our best computer simulations have shown that, in order to reproduce the structure of the solar system that we see today, there was most likely a violent period during which the giant planets Jupiter, Uranus, Saturn, and Neptune migrated, flinging material into the Kuiper Belt and the Oort Cloud. During that process, we probably liberated about 30 Earth masses of 3I/ATLAS-sized comets into the interstellar medium.

In the last 30 years, we have discovered that planets are surprisingly common around other stars. It is therefore not surprising that other planetary systems should also have ejected comets into the Milky Way galaxy. The first known interstellar object, 1I/'Oumuamua, was discovered in 2017. Two years later, we discovered 2I/Borisov, which displayed a prominent cometary tail, and its composition was much different than those of solar system comets. Our telescopic observations revealed that it contains more carbon monoxide than water. Most solar system comets are comprised of a lot more water than any other type of ice.

The ices we see in a comet can tell us something about the conditions in which they formed. For example, water freezes at cold temperatures. The farther away from the sun a comet forms, the colder it is. Therefore, the fact that comets in our solar system have water as their main ice tells us that they mostly formed around where Jupiter is now, about five times more distant than the Earth. Carbon monoxide and carbon dioxide freeze at much colder temperatures than water. Therefore, 2I/Borisov's carbon monoxide tells us that it probably formed at a much farther distance from its star than the typical comets left in our solar system.

Astronomers had been looking for interstellar objects unsuccessfully for six years until we spotted 3I/ATLAS this July. And 3I/ATLAS has been well worth the wait. We have been monitoring it since we discovered it, and our early observations with facilities like the James Webb Space Telescope have revealed that it is enriched in carbon dioxide. This is probably telling us that, like 2I/Borisov, 3I/ATLAS formed much farther out in its progenitor star system than our solar system comets did.

Mere days before Christmas 3I/ATLAS will be at its closest approach to the Earth. This is exciting for everyone, because anyone can go see 3I/ATLAS with a powerful amateur telescope. And for us astronomers, all these critical ice features are easier to detect the closer 3I/ATLAS is to Earth.

These observations might be telling us that comet formation occurs in much farther regions than we previously thought possible based on our inferences from the solar system. In that way, the solar system would be somewhat unique. Alternatively, it's possible that we also produced such distant comets, but all of them were subsequently ejected. And then maybe the solar system is not so unique after all. Either way, 3I/ATLAS is giving us a new window to put our solar system into its cosmic context this Christmas.

https://www.yahoo.com/news/articles/why-interstellar-comet-3i-atlas-110000339.html