Ever since interstellar object 3I/ATLAS streaked into our solar system, astronomers have been scrambling to understand how this likely comet got here—and more importantly, perhaps, where it came from.
As only the third interstellar object ever discovered, 3I/ATLAS offers a rare chance to study remnants of solar systems beyond our own, briefly bringing the distant galaxy within reach.
But before we get into the new study, imagine the Milky Way as a kind of celestial sandwich: Inside is a thin disk, full of stars and dust, encased by a puffy thick disk, which has different chemistry and composition to the thin disk and which contains older stars. Early observations of 3I/ATLAS pointed to an origin point in that more ancient, puffy layer, but a new study posits a totally different theory.
A team of astronomers led by Yiyang Guo of Shanghai Jiao Tong University analyzed the current orbit of 3I/ATLAS and the orbits of nearby stars to try and trace its origin. Previous findings suggested it came from the Milky Way’s thick disk—a region that contains about 10% of the galaxy’s stars, many of them among the most ancient. But Guo’s study, which was posted earlier this week to the preprint server arXiv, contradicts that idea, pointing to the galaxy’s thin disk instead.
To reach their conclusion, Guo and his colleagues first traced the past orbits of 3I/ATLAS and 30 million nearby stars to see if any came close enough to fling the object into interstellar space. This is a common mechanism for interstellar object formation. When a celestial body—such as an asteroid or comet—encounters a passing planet or star, the gravitational force can slingshot it out of its home star system. If the researchers could pinpoint the star that ejected 3I/ATLAS they would know exactly where it came from.
They identified 25 stars that passed within 1 parsec (3.26 light years) of 3I/ATLAS. While this is theoretically close enough to throw the object out of its home system, the relative speeds were too high for any of the stars to have done so. Overall, the researchers concluded that these stars’ gravitational nudges likely had little impact on the object’s speed or direction.
So, the researchers took a different tack, analyzing the object’s current motion instead, revealing that its velocity distribution closely resembles that of stars in the thin disk. Coupled with the fact that most of the nearby stars are in the thin disk, this suggested to the researchers that 3I/ATLAS most likely originated from this region.
The findings are just the latest in a string of puzzling details to emerge about 3I/ATLAS. For example, its coma—the cloud of gas and dust that surrounds the object’s nucleus—contains one of the highest ratios of carbon dioxide to water ever observed in any comet. This suggests the object might have formed in an environment with higher levels of radiation than our solar system, or near the CO2 ice line of a star’s protoplanetary disk.
Newly released images of 3I/ATLAS, captured by the Gemini South telescope in Chile in late August, reveal that the comet is becoming increasingly active as it hurtles toward the Sun. This will help astronomers gather more data about its properties and origin in the coming weeks.