A New Visitor from the Stars: Studying Interstellar Comet 3I/ATLAS

Raúl de la Fuente Marcos and collaborators present observations of the newly discovered interstellar comet 3I/ATLAS, the third known object of its kind after 1I/‘Oumuamua and 2I/Borisov. Using two powerful telescopes in the Canary Islands, they investigated the comet’s visible spectrum, its coma (the cloud of dust surrounding it), its rotation, and its motion through our galaxy. Their results help us understand how this visitor from beyond the solar system compares to comets and asteroids we already know.

Introduction: A Message in a Bottle

The authors begin by explaining the significance of interstellar objects, small pieces of rock and ice ejected from distant planetary systems, drifting through space like messages in bottles. These objects carry information about the environments where they formed. Only two interstellar visitors, ‘Oumuamua and Borisov, have been studied before, and both left more questions than answers. On July 1, 2025, the Asteroid Terrestrial-impact Last Alert System (ATLAS) discovered another such visitor, which was quickly confirmed and designated 3I/ATLAS. Follow-up observations revealed that it moves on a highly eccentric hyperbolic orbit, proving its interstellar origin.

Results: Examining the Comet

The authors’ observations focus on four aspects: cometary activity, spectral properties, rotational state, and dynamics. They collected visible light images and spectra using the Gran Telescopio Canarias (GTC) and the Two-meter Twin Telescope (TTT). These observations were analyzed alongside computer simulations of the comet’s path through the galaxy.

Cometary Activity

By stacking over 200 images, the team produced a deep image of 3I/ATLAS. This image revealed a bright, dust-filled coma extending tens of thousands of kilometers across, a clear sign of activity even though the comet was still far from the Sun at the time (about 3.44 astronomical units away). The coma appeared elliptical, and its orientation matched the expected direction of dust ejected by sunlight.

Spectral Properties

Next, they examined the comet’s visible spectrum to see what it is made of. While typical comets show faint emission from gases like CN (cyanogen), 3I/ATLAS did not. The authors estimated an upper limit on the CN production rate, consistent with other comets at similar distances. Its spectrum matched that of a D-type asteroid, a dark, reddish type often found in the outer solar system, and had a redder slope than either ‘Oumuamua or Borisov. This suggests its surface is covered by organic-rich dust similar to that seen on some trans-Neptunian objects.

Rotational State

By measuring the comet’s brightness over several nights, the team created a light curve showing how its brightness changed as it rotated. They found that its brightness varied only slightly (by about 0.2 magnitudes), as expected for a comet with a coma that hides some of the nucleus’s shape. They determined a rotation period of about 16.8 hours.

Dynamics

Finally, the authors traced 3I/ATLAS’s path through space, both into the past and into the future. Its motion suggests it came from the thin disk of our galaxy, which contains stars similar to the Sun. They used data from the Gaia mission to identify stars with similar motions and found that one such star has properties consistent with a possible parent system. After swinging past the Sun, 3I/ATLAS will leave the solar system and continue its journey through the galaxy.

Discussion: A Familiar Stranger

The spectral slope and dust-rich coma of 3I/ATLAS show it is more like ‘Oumuamua than to Borisov, though its surface is redder. Its properties fit well within what is seen for solar system comets and asteroids, suggesting that planets and comets formed in other star systems may not be too different from those in ours. Kinematic analysis also supports an origin in the thin disk rather than the thick disk or the galactic halo.

Conclusions

De la Fuente Marcos and colleagues conclude that 3I/ATLAS is an active comet from interstellar space with a dust-rich coma, a red, D-type-like surface, and slow rotation. Its trajectory through the galaxy points to an origin among Sun-like stars in the thin disk. This discovery adds to the growing evidence that extrasolar debris may not be all that different from what we find at home, a hopeful sign that planetary systems like ours may be common throughout the galaxy.

Source: de la Fuente Marcos