Interstellar visitors like comet 3I/ATLAS are the most common objects in the Milky Way: 'There's almost always one within the solar system'

Objects such as 'Oumuamua, Borisov and recently 3I/ATLAS have opened our eyes to the reality that outsiders regularly visit our solar system — and we're about to start spotting a whole lot more of them.

3I/ATLAS, an interstellar object currently zipping through the solar system, is capturing the attention of astronomers, the public and even most of our deep space assets. Its hyperbolic orbit tells us that it is not gravitationally bound to the sun and is just passing through.

This rare and novel event is, however, representative of a much more regular occurrence, according to astronomers at the Europlanet Science Congress (EPSC) and Division for Planetary Sciences (DPS) joint meeting, which was held in Helsinki last week.

"These sorts of interstellar objects are the most common macroscopic objects in the galaxy," Chris Lintott, a professor of astrophysics at the University of Oxford, said during the meeting. "The fact that we've only found three of them is quite fun."

Interstellar objects, or ISOs, are planetesimals — the rocky building blocks of planets — that have been flung out of their own, original star systems and look like either asteroids or comets. Astronomers think there are on the order of a hundred million trillion to a trillion trillion of these ISOs in our Milky Way galaxy, Lintott said.

"Therefore, there's almost always one within the solar system. But because they're small, dark and fast-moving, they are difficult to spot," Lintott said. That's expected to change, however, with new, powerful capabilities coming online.

Rosemary Dorsey, a postdoctoral researcher at the University of Helsinki, presented her research on simulating surveys and predicting how many ISOs a telescopic campaign may detect,

Dorsey found that the Legacy Survey of Space and Time (LSST), a 10-year, deep astronomical survey conducted by the new Vera C. Rubin Observatory, is expected to spot between six and 51 ISOs, though simulating ISOs faces challenges in terms of unknowns such as the size distribution and luminosity of the objects.

Despite the challenges of detection, ISOs offer a tremendous opportunity to learn about the galaxy beyond the boundaries of our own solar system.

"These are our chance to get a sample-return mission from a distant planetary system," said Lintott. He noted that astronomers are working on models to show the expected distribution of interstellar objects based on their composition, and also seeking to use ISOs to better understand the chemical evolution of the Milky Way.

For now, 3I/ATLAS is the focus of attention and providing new insights as ground-based telescopes, space observatories such as the James Webb Space Telescope and Hubble, and other spacecraft train their sights on the object.

There is a good chance that 3I/ATLAS is around eight billion years old, said Lintott. If that's the case, he added, it's "the oldest thing we've ever seen" in our neck of the cosmic woods.