A startling Christmas surprise for astronomers: interstellar visitor 3I/ATLAS shifts our view of the cosmos
Darryl Z. Seligman, an Assistant Professor of Physics and Astronomy at Michigan State University, has written about the interstellar traveler 3I/ATLAS. This object is only the third known body from outside our solar system to voyage through our neighborhood, whether it behaves like a comet or an asteroid. By studying the light that reflects off 3I/ATLAS with our telescopes, scientists can extract vital clues about its composition and origin. When the object reaches its closest approach to Earth, the features we seek—such as its ice makeup and surface properties—will be easier to observe and analyze.
On December 19, six days before Christmas, 3I/ATLAS will come nearest to Earth in its entire lifetime. With a modest telescope or high-powered binoculars, keen observers can glimpse this enigmatic visitor. For astronomers, this close pass represents our best opportunity to examine how planet formation in distant star systems compares to the process that shaped our own solar system.
In the past decade and a half, we have identified three members of a new class: interstellar objects. These bodies travel on hyperbolic trajectories, unlike the bound orbits typical of objects native to our solar system. Their fleeting passage provides us with rare, near-term data about materials and processes from other stellar neighborhoods, potentially shaping our understanding of planet formation across the galaxy.
The solar system itself likely ejected a substantial amount of material into the Milky Way during its early history. Computer simulations suggest a turbulent era when the giant planets—Jupiter, Saturn, Uranus, and Neptune—migrated and flung countless objects into the Kuiper Belt and the distant Oort Cloud. Roughly, such dynamical upheaval could have liberated around 30 Earth masses worth of 3I/ATLAS-sized comets into interstellar space.
As exoplanet studies have accelerated over the last 30 years, we’ve learned that planetary systems are common beyond our own. This makes it unsurprising that some systems would also eject comets. The first interstellar object detected, 1I/'Oumuamua, appeared in 2017, followed by 2I/Borisov in 2019, which showcased a prominent cometary tail and a composition notably different from typical solar-system comets—containing more carbon monoxide relative to water ice.
Ices carried by comets reveal the conditions in which they formed. Water ice freezes at relatively warm, moderate distances from a star, so comets rich in water generally formed closer to their suns—around the region where Jupiter resides today. In contrast, carbon monoxide and carbon dioxide condense at far colder temperatures, implying that 2I/Borisov originated much farther out. These contrasts help astronomers map where and how different comets formed across the galaxy.
Interstellar objects remained elusive for years, but 3I/ATLAS finally offered a turning point when it was identified this July. Ongoing observations, including data from the James Webb Space Telescope, indicate that 3I/ATLAS is enriched in carbon dioxide. This signature suggests that, like 2I/Borisov, it formed far from its parent star, beyond the regions where many solar-system comets take shape.
The imminent Christmas-week close approach of 3I/ATLAS excites both amateur skywatchers and professional scientists. A powerful amateur telescope can reveal details for curious observers, while the proximity makes the delicate ice signatures more accessible to our instruments.
What 3I/ATLAS teaches us could reshape our ideas about where comets form. If such distant formation is common, the solar system might be less unique than we once believed; alternatively, it could imply that many distant comets were ejected from their birth systems, leaving our neighborhood as a selective sampling. Either way, 3I/ATLAS provides a rare, direct window into the broader cosmic context of our solar system this holiday season.
Darryl Z. Seligman is a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellow and an Assistant Professor at Michigan State University. He earned degrees in Mathematics and Physics from the University of Pennsylvania in 2015 and completed his Ph.D. in Astronomy at Yale University in 2020, where he received the Dirk Brouwer Memorial Prize for Outstanding Ph.D. Thesis. He is recognized for his work on comets, asteroids, planetary science, and exoplanets.
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