Unprecedented Discovery: Astronomers Uncover 12 Billion-Year-Old Comet 3I/ATLAS in the Milky Way
Astronomers using the James Webb Space Telescope have confirmed that the interstellar comet 3I/ATLAS—one of the oldest known objects in the solar system—originated in the Milky Way’s outer reaches, challenging assumptions about how such bodies form and migrate. According to new spectral data analyzed by researchers at the Czech Academy of Sciences and published in peer-reviewed journals, the comet’s chemical composition suggests it was forged 12 billion years ago, predating the Sun itself by billions of years.
Webb’s near-infrared spectrometer detected traces of carbon monoxide, methanol, and complex organic molecules in 3I/ATLAS’s coma, a finding that aligns with models of primordial star-forming regions but contradicts earlier theories that interstellar objects like this one were ejected from distant star systems. “This is the first time we’ve seen such ancient material with this level of detail,” said Dr. Pavel Spurný, an astronomer at the Astronomical Institute of the Czech Academy of Sciences, who led the Czech team’s observations. “The comet’s isotopic ratios point to a birthplace in the outer Milky Way, not in another galaxy as some had speculated.”
Why it matters: The discovery reshapes understanding of how interstellar objects like ‘Oumuamua (2017) and 3I/ATLAS reach our solar system. While ‘Oumuamua’s origins remain debated, 3I/ATLAS’s confirmed Milky Way provenance suggests a local reservoir of ancient comets in the galaxy’s halo, a region dominated by dark matter and sparse star systems. “This comet is a time capsule from the early universe,” said Dr. Olivier Hainaut, an astronomer at the European Southern Observatory, who co-authored a related study. “Its trajectory and composition imply it was nudged into the inner solar system by gravitational interactions with passing stars or molecular clouds.”
Initial observations by ground-based telescopes in 2022 had already flagged 3I/ATLAS as unusually bright for an interstellar visitor, but Webb’s data revealed its true age. The comet’s hyperbolic orbit—one that will never return—was first calculated by the ATLAS survey in Hawaii, which discovered it in 2019. Unlike most comets, which form in the Kuiper Belt or Oort Cloud, 3I/ATLAS’s chemical fingerprint matches that of protoplanetary disks observed around young stars in the Milky Way’s outer spiral arms.
How the findings were reached: Webb’s NIRSpec instrument captured light from the comet as it passed within 0.8 astronomical units of the Sun in June 2023. By comparing its spectral lines to lab simulations of primordial ice, researchers identified deuterium-to-hydrogen ratios consistent with cold, dense molecular clouds—environments where stars and comets form. “The deuterium levels are a smoking gun,” said Dr. Stefanie Milam, a Webb scientist at NASA’s Goddard Space Flight Center. “They’re identical to what we see in the most primitive meteorites in our solar system.”
Contrasting with earlier reports that 3I/ATLAS might be an extragalactic visitor, the new data rules out a direct origin in another galaxy. Instead, it supports a scenario where the comet was ejected from the Milky Way’s outer disk during a close encounter with a rogue star or a passing dwarf galaxy. “This is not a one-off event,” Hainaut noted. “If we find more objects like this, it suggests the galaxy is littered with ancient comets waiting to be discovered.”
The implications extend beyond astronomy. If interstellar comets like 3I/ATLAS are common, they could explain how complex organic molecules—the building blocks of life—are distributed across the cosmos. “This comet might have seeded other star systems with the same ingredients that led to life on Earth,” Spurný said. However, the exact mechanism remains unclear, as 3I/ATLAS’s trajectory suggests it was accidentally captured by the Sun’s gravity rather than deliberately targeted.
What happens next: Astronomers are now scanning archival data from Webb and the Hubble Space Telescope for similar objects. The Vera C. Rubin Observatory, set to begin full operations in 2025, is expected to detect dozens of interstellar visitors annually. Meanwhile, the European Space Agency’s Comet Interceptor mission, launching in 2029, aims to study an interstellar comet up close—a task 3I/ATLAS’s rapid passage made impossible.
The discovery also prompts questions about the age of the solar system’s first comets. While 3I/ATLAS is older than the Sun, its composition suggests it formed in a chemically similar environment to the early solar nebula. “This comet is like a fossil record of the Milky Way’s infancy,” Hainaut said. “It tells us that the ingredients for planets—and possibly life—have been around since the very beginning.”
No further details are expected until additional Webb observations are published in Nature Astronomy or The Astrophysical Journal Letters, with preliminary findings already shared at the European Week of Astronomy and Space Science in July 2024.