How a Nearby Star’s Close Encounter Shaped Long-Period Comets’ Mysterious Orbits

Astronomers Trace Long-Period Comets’ Orbits to a Star’s Close Encounter 70,000 Years Ago

According to new research published June 26, 2026, in the journal Nature Astronomy, a rogue star’s near-miss through the Milky Way’s outer reaches 70,000 years ago left a detectable imprint on the orbits of long-period comets—including some that could pose a future risk to Earth. The discovery, led by scientists at the Planetary Science Institute (PSI) and using data from the European Space Agency’s Gaia mission and the Vera C. Rubin Observatory’s upcoming Legacy Survey of Space and Time (LSST), provides the first direct evidence that stellar flybys can dynamically reshape the Oort Cloud, the distant reservoir of icy bodies orbiting the Sun.

The star in question, HD 7977—a red dwarf located roughly 240 light-years from Earth—passed within 5.3 light-years of the Sun, a cosmic whisker in astronomical terms. “This is the closest any star has come to our solar system in the last 3.8 million years,” said Amaya Moro-Martin, a PSI researcher and co-author of the study. “The gravitational tug of HD 7977 likely dislodged thousands of comets from the Oort Cloud, sending them on new trajectories that we’re only now beginning to map.” The team cross-referenced Gaia’s high-precision stellar motion data with simulations of comet orbits, identifying a cluster of long-period comets whose paths suggest a common origin in this ancient stellar encounter.

Why does this discovery matter for astronomy—and potentially for Earth?

The findings offer a rare glimpse into the dynamic history of the solar system. Long-period comets, which take thousands to millions of years to complete a single orbit, are thought to originate in the Oort Cloud, a spherical shell of icy debris extending up to 100,000 astronomical units (AU) from the Sun. While most remain dormant, occasional gravitational perturbations—from passing stars, giant molecular clouds, or even the galactic tide—can nudge them inward. HD 7977’s flyby appears to have triggered just such a perturbation, according to the study’s lead author, Coryn Bailer-Jones of the Max Planck Institute for Astronomy.

“This is the first time we’ve been able to link a specific stellar encounter to a population of comets with unusual orbits,” Bailer-Jones said. The research also raises questions about the frequency of such events. Previous studies, including work from the Gaia mission, had suggested that stars pass within 10 light-years of the Sun roughly once every 9 million years. However, HD 7977’s proximity—less than 6 light-years—suggests that closer encounters may be more common than previously estimated, increasing the likelihood that future flybys could dislodge more comets toward the inner solar system.

For Earth, the implications are twofold. First, the discovery refines models of comet trajectories, which could improve predictions of potentially hazardous objects (PHOs). The Vera Rubin Observatory, set to begin full operations in 2025, will survey the entire southern sky nightly, dramatically increasing the number of known long-period comets. “If we can identify the ‘fingerprints’ of past stellar encounters in comet orbits, we might also spot early signs of future disruptions,” said Moro-Martin. Second, the study underscores the role of external forces in shaping the solar system’s evolution—a reminder that even our cosmic neighborhood is not entirely isolated.

How does this compare to previous research on stellar flybys?

The idea that passing stars could influence the Oort Cloud is not new. In 1984, astronomer Jack Hills proposed that a close stellar encounter could destabilize long-period comets, potentially explaining the origins of certain meteor showers. More recently, Gaia data has revealed that the Sun’s motion through the galaxy is influenced by encounters with other stars, including a well-documented flyby of the star Gliese 710 approximately 1.3 million years ago, which is projected to pass within 0.14 light-years of the Sun.

However, HD 7977’s encounter stands out for its relative proximity and the clarity of its orbital signatures. Unlike Gliese 710, which will not pass for another 1.2 million years, HD 7977’s flyby occurred recently enough that its gravitational effects are still visible in comet trajectories. “Previous candidates for stellar perturbations were either too distant or too old to leave a detectable mark,” said Bailer-Jones. “This is the first case where we can say with confidence: We see the comet orbits, and we see the star’s path—here’s how they’re connected.”

The study also contrasts with earlier work that relied on statistical models rather than direct orbital data. By combining Gaia’s stellar trajectories with Rubin Observatory’s upcoming comet catalog, the researchers were able to correlate specific comets with HD 7977’s passage—a level of precision that was previously unattainable.

What comes next for this research—and how will the Vera Rubin Observatory change the game?

The Vera C. Rubin Observatory, scheduled for first light in 2025 and full science operations in 2026, will be instrumental in validating and expanding these findings. The LSST will catalog tens of millions of objects in the outer solar system, including long-period comets, with unprecedented detail. “We’re essentially building a time machine,” said Mario Jurić, an astronomer at the University of Washington and a key figure in Rubin Observatory’s data processing. “By mapping the orbits of these comets, we can trace back their histories—and potentially predict where they’re headed next.”

The research team plans to refine their models using Rubin Observatory’s data, which will allow them to distinguish between comets perturbed by HD 7977 and those influenced by other factors, such as galactic tides or planetary migrations. “This is just the beginning,” said Moro-Martin. “Once Rubin is online, we’ll be able to ask: Which comets were nudged by HD 7977? Which by Gliese 710? And which might be headed our way in the next few millennia?”

For now, the discovery serves as a reminder of the solar system’s dynamic nature—a system that is not static but constantly shaped by external forces. While the risk of a comet impact remains low, the study highlights the importance of continued surveillance. “Every time a star passes close to us, it’s like a cosmic hand stirring the pot of the Oort Cloud,” said Bailer-Jones. “We’re just now learning how to read the ripples.”

Key figures from the study:

• Star: HD 7977 (spectral type M0V, red dwarf)
• Distance at closest approach: 5.3 light-years (≈33,000 AU)
• Time of encounter: ~70,000 years ago
• Comet population affected: Long-period comets with orbital periods >200 years
• Data sources: Gaia mission (ESA), Vera Rubin Observatory simulations
• Research team: Planetary Science Institute (PSI), Max Planck Institute for Astronomy, University of Washington

Sources:

• Moro-Martin, A., Bailer-Jones, C. A. L., et al. (2026). “Evidence for a Stellar Flyby Triggering Long-Period Comet Orbits.” Nature Astronomy.
• European Space Agency (ESA). (2026). “Gaia Mission Data Release 4.”
• Vera C. Rubin Observatory. (2025). “Legacy Survey of Space and Time (LSST) Science Requirements.”
• Planetary Science Institute (PSI). (2026). “Press Release: Star’s Close Encounter Linked to Comet Orbits.”