When a Star Flew Too Close: Could HD 7977 Have Sparked an Ancient Comet Shower?

The paper by Cao et al. explores a remarkable possibility: about 2.5 million years ago, a Sun-like star called HD 7977 may have passed unusually close to our Solar System. Using computer simulations and analytical calculations, the authors investigate how this flyby could have disturbed the distant Oort Cloud, the vast, icy reservoir of comets that surrounds the Sun. Their goal is to understand whether this event might have unleashed a pulse of comets into the inner Solar System, possibly hitting Earth, right around the Pliocene-Pleistocene transition, a time of major environmental change.

Structure of the Oort Cloud and the Proposed Encounter

The authors begin by introducing the Oort Cloud and explaining how its comets can be nudged inward by gravitational tugs from passing stars. They describe the Inner Oort Cloud (IOC) and Outer Oort Cloud (OOC), emphasizing that close stellar encounters mainly shake up the IOC. Since HD 7977 may have come within only 2,300 AU of the Sun, a tiny distance by galactic standards, Cao and collaborators propose that this specific encounter deserves detailed study. Their introduction connects this idea to past work showing that comet showers have been considered possible triggers for climatic shifts and even extinction events.

Methods: Simulations and Analytical Modeling

To study this scenario, the team models how comets respond when a star sweeps by. They combine N-body simulations (which track particle motions in 3D) with a faster impulse-approximation method. Before the flyby, the authors assume the IOC has an “empty loss cone,” meaning very few comets have orbits that bring them near the Sun. After the flyby, however, the story changes: many comets receive sudden kicks that shrink their perihelion distance (the closest point to the Sun). Figures in the paper show how the angular momentum of these comets drops, allowing some to plunge into the inner Solar System. This effect is most dramatic when HD 7977 passes at the closest allowed distance of about 2,300 AU.

Estimating Comet Delivery and Earth Impact Rates

Using their analytic model, the authors estimate how many of these newly shifted comets would reach Earth’s orbital region. They compute how long each comet spends inside a sphere of radius 1 AU and use this to calculate impact rates. They find a sharp spike in the number of kilometer-scale comets hitting Earth shortly after the flyby, up to ten times higher than the normal background rate. Comets 2.25 km across would have a near-certain (P=1) chance of striking Earth during the inferred million-year-long comet shower. For typical 1-km comets, they estimate roughly five impacts in that same interval, far above what is expected in normal times.

Sensitivity to Flyby Distance and Additional Contributions

The paper then explores how sensitive this result is to the distance of the flyby. Because Gaia observations allow flyby distances between 2,300 and ~13,000 AU, the authors repeat their analysis over this range. The shower becomes dramatically weaker at larger distances; for a 12,000-AU passage, the Earth-impact probability for 1-km comets drops to about 0.3. They also note that uncertainties in the IOC’s true population could shift these numbers up or down. Interestingly, they consider whether HD 7977 itself might carry its own comet cloud, which the Sun could have passed through. Though this contribution is uncertain, their estimates suggest it could slightly enhance the overall comet flux.

Links to Geological and Lunar Records

Finally, the authors discuss how such a comet shower might appear in geological and lunar records. They review known impact craters dated between 3 and 1 million years ago, including El’gygytgyn, Bosumtwi, and Zhamanshin, and point out that many of these were likely made by impactors in the 1–2 km diameter range, consistent with their modeled shower. While none of these craters are definitively identified as cometary, the timing overlaps intriguingly with HD 7977’s passage. They emphasize that the Pliocene-Pleistocene transition was gradual, and a long-lasting increase in impacts, rather than a single catastrophic event, matches both their results and geological expectations.

Conclusions and Implications

In the end, Cao and colleagues do not claim a discovery but present a compelling hypothesis: HD 7977’s close flyby could have stirred the Oort Cloud, sending comets toward Earth during a climatically important period. Their analytical method provides a new way to estimate comet-impact probabilities from stellar encounters, and their results highlight how the Solar System’s environment continues to be shaped by the stars that wander near it.

Source: Cao