A Star Passed Close to the Sun 2.5 Million Years Ago. We're Still Feeling It.

About 2.5 million years ago — a blink in geological time — a Sun-like star named HD 7977 appears to have brushed past the outer edge of our Solar System. New research published in The Planetary Science Journal (accepted June 2026) argues that we are still living in the aftermath of that encounter. The evidence is hiding in plain sight, written in the orbits of the long-period comets that continue to fall toward the Sun today .

This article explains the key evidence, why the flyby's distance remains a major uncertainty, and what the findings mean for our understanding of the Oort Cloud, cometary impacts on Earth, and the orbital stability of the planets.

The New Evidence: An Orbital Fingerprint in Today's Comets

A team led by Planetary Science Institute Senior Scientist Nathan Kaib and Sean Raymond of Université de Bordeaux has produced the first dynamical evidence that the HD 7977 flyby left a quantifiable signature still visible in the orbits of long-period comets (LPCs) entering the inner solar system today .

The key finding is a pattern in the argument of perihelion (ω) — a parameter that describes where in its orbit a comet makes its closest approach to the Sun. The observed ω distributions for both "new" (first-visit) and "returning" LPCs can be accurately replicated in simulations only if HD 7977 passed within approximately 6,000–10,000 AU of the Sun about 2.5 million years ago . Without this ancient encounter, the model cannot match the observed pattern.

Importantly, the model is not a perfect fit. While it successfully matches the ω distribution, it does not fully reproduce the observed distribution of orbital energies (semimajor axes) of LPCs, suggesting that other stellar encounters or dynamical effects also play a role .

Still in the Tail End of a Comet Shower

If the Kaib & Raymond model is correct, our Solar System is currently in the latter stages of a comet shower — a sustained period of elevated LPC flux that has not yet subsided to the long-term background rate .

The effect is substantial. The simulations imply that today's observed LPC flux is approximately twice as high as the longer-term, tide-dominated rate . This means estimates of the Oort Cloud's total population likely need to be revised downward by a factor of about 2, because we have been mistaking the shower tail for the steady state .

This is consistent with broader patterns. Gaia-based analyses indicate that close stellar passages are a regular feature of the Sun's Galactic environment, with approximately 20 stellar passages within 1 parsec of the Sun per million years . A separate 2024 analysis found that roughly 87 stars per million years pass within 6.5 light-years — about double previous estimates .

The Critical Uncertainty: How Close Did HD 7977 Actually Get?

The flyby distance is the single most important unknown. Gaia data give a median closest approach of about 13,000 AU, but with a wide range of possible distances .

• There is a 5% probability that HD 7977 came within ~4,000 AU of the Sun.
• There is a 5% probability it never came closer than ~24,000 AU .

The Kaib & Raymond model best fits the comet data at the closer end of the uncertainty range: 6,000–10,000 AU . But other research has proposed even closer approaches. A study by Bobylev (2023) estimated a minimum distance of 0.071 ± 0.027 parsecs (roughly 14,600 AU) . An earlier analysis by Dybczyński (2022) noted the possibility of a passage as close as 1,000 AU .

What This Means for Earth: Impact Risk and Climate

The implications for Earth depend heavily on this unknown distance. Two scenarios dominate the discussion:

The Close Approach Scenario (~2,300 AU)

A paper led by Loeb and co-authors (accepted in Nature, 2025) investigates what happens if HD 7977 came within approximately 2,300 AU of the Sun . Under this extreme but statistically possible scenario:

• The terrestrial impact probability of 1 km-class comets increases by an order of magnitude compared to the steady state, slightly exceeding the asteroid impact probability at this size scale .
• The impact flux is dominated by Oort Cloud comets for objects smaller than 1 km .
• The threshold comet diameter for which the expected number of impacts reaches 1 is about 2.25 km .

This scenario would place a significant comet storm at roughly the same time as the Pliocene-Pleistocene transition (~2.6 million years ago) — a period of dramatic climate change when the Earth shifted from a warm, relatively stable climate to the glacial-interglacial cycles of the Pleistocene . The timing has led to speculation, though not proof, that the enhanced comet influx could have contributed to the environmental pressures shaping early human evolution .

The More Probable Scenario (6,000–10,000 AU)

At the flyby distances that best fit the Kaib & Raymond model, the comet shower is real but more modest: a factor-of-2 enhancement in LPC flux rather than a catastrophic storm .

A Contradictory View on Planetary and Climate Influence

Not all research agrees that the HD 7977 flyby had any measurable effect on Earth. A 2025 study by Zeebe & Hernandez tested whether an extreme flyby (within ~3,900 AU about 2.8 million years ago) could have altered Earth's orbit. They found no discernible change in Earth's orbital evolution over the past 70 million years compared to the standard model . A separate study (2025) concluded that stellar flybys have had no influence on paleoclimate reconstructions over the past 56 million years .

This means that while cometary orbits were strongly affected, the planetary orbits themselves appear robust to this encounter . The connection between the HD 7977 flyby and Earth's climate or human evolution remains an open, actively debated question.

The Bigger Picture: Stellar Flybys Are Not Rare

The HD 7977 encounter is not an isolated event — it is part of a broader pattern. As mentioned, the Sun experiences approximately 20 close stellar passages per million years . Some notable examples include:

• Scholz's Star: Passed through the Oort Cloud about 70,000 years ago .
• Gliese 710: Will pass through the Oort Cloud at about 69,000 AU from the Sun in about 1.3 million years, and is the most significant known future perturber .
• Gamma Microscopii: A G6 giant star that may have passed within 0.35–1.34 parsecs around 3.8 million years ago .

The new evidence from HD 7977 provides the first direct orbital fingerprint of one such encounter, but it is unlikely to be the last. Each discovery refines our understanding of the Oort Cloud's dynamics, the true population of comets in the Solar System's outer reaches, and the long-term history of impacts on Earth.

Bottom line: The HD 7977 flyby left a measurable signature in comet orbits that we can still detect today. We are likely living in the tail of a comet shower that doubled the long-period comet influx. But the magnitude of that shower — and its past impact on Earth — depends on a flyby distance that is still uncertain by a factor of ~5.