Astronomers Trace Origin of New Interstellar Comet

The third object known to have entered the Solar System from interstellar space has an origin distinct from the previous two.

Comet 3I/ATLAS, according to a team led by astrophysicist Matthew Hopkins of the University of Oxford, originated from the thick disk of the Milky Way. This region differs from the current environment of the Sun.

The team's findings, based on observations made shortly after the object's discovery, have been submitted to The Astrophysical Journal Letters and are available on arXiv.

Background

The Solar System contains many rocks, most of which formed from the material left over after the Sun's formation. Due to gravitational interactions and ongoing changes within the system, these objects follow a variety of orbital paths, ranging from long-period orbits lasting millions of years to orbits inside that of Venus.

• On three occasions, astronomers have observed objects moving through the Solar System on trajectories that suggest an origin outside the Solar System.
• The first two such objects were 'Oumuamua and comet 2I/Borisov, observed in 2017 and 2019, respectively.

Comet 3I/ATLAS, discovered on 1 July 2025, is the third such object. Astronomers are studying its trajectory, appearance, and possible origin within the galaxy.

Analysis

Hopkins and colleagues used the Ōtautahi-Oxford interstellar object population model, developed by astronomers from New Zealand and the UK, to trace 3I/ATLAS's origin within the Milky Way. This model incorporates Gaia data, chemical models of the Milky Way's disk, and object movement to map interstellar object populations.

3I/ATLAS was observed traveling at approximately 57 kilometers (about 35 miles) per second. Its path will bring it just inside Mars's orbit for its closest approach to the Sun in October 2025 before it exits the Solar System. The object is estimated to be 10 to 20 kilometers in diameter, with a bluer surface and a redder coma compared to most Solar System comets.

The research team determined that the comet's velocity is consistent with an origin in the thick disk of the Milky Way, which surrounds the main thin disk. The thin disk contains most of the Milky Way's stars, including the Sun, while the thick disk contains about 10 percent of the galaxy's stars, most of which are over 10 billion years old.

If confirmed, this would indicate that 3I/ATLAS is significantly older than the Solar System. The researchers estimate its age to be between 7.6 and 14 billion years, compared to the Sun's age of 4.6 billion years.

This finding suggests that the three known interstellar visitors likely originated from different locations. 3I/ATLAS appears to be the first observed interstellar object from the thick disk of the Milky Way.

It is not feasible to trace these objects back to a specific star due to uncertainties in their ages.

"We rule out the possibility that 3I/ATLAS comes from the same star, or same cluster, as either 1I or 2I, but the velocity does tell us about its origin: it is a member of the Milky Way's thick disk, and the first interstellar object observed from this population," the researchers write in their paper.

"Interstellar objects provide the opportunity to gain evidence of the process of planetesimal formation and evolution from a host of galactic environments, and further observations of 3I/ATLAS will allow us to constrain and test the assumptions made in the Ōtautahi-Oxford model."

Their analysis is available on arXiv.