Planetary Flybys May Have Triggered Earth's Mass Extinctions, New Theory Proposes

Earth's history is not a smooth climb toward complexity. It is a series of collapses and recoveries, punctuated by moments when vast swaths of life simply vanished. The asteroid that killed the dinosaurs 64 million years ago is the most famous of these catastrophes, partly because it left unmistakable evidence—a layer of iridium in the rock, a crater in the Yucatan. But not every extinction event comes with such clear fingerprints. Some of the largest die-offs in Earth's past remain mysterious, their causes still debated.

Daniele Fargion, a research professor at Rome University and the Capodimonte Observatory in Naples, has proposed a different kind of cosmic culprit: the gravitational tug of massive objects wandering through the inner Solar System. The idea appears in a conference paper titled "Mass Extinctions by Gravitational Tides," presented in Palermo in June 2025. Fargion argues that planetary-mass bodies from the outer Solar System—objects similar in size to dwarf planets or larger—may have occasionally drifted inward and passed near Earth. A direct collision would be rare. But a close approach would be far more likely, and the gravitational pull alone could have been catastrophic.

The mechanism is straightforward in theory. As a massive object passed near Earth, its gravity would tug on the planet's oceans, crust, and interior. The result would be giant waves, sudden shifts in sea level, violent volcanic eruptions triggered by tidal heating deep underground, and rapid changes in climate. The same gravitational disturbance could also knock asteroids and comets out of their orbits, sending them hurtling toward Earth. All of these effects—tsunami, volcanism, climate shock, meteor shower—could combine to destabilize ecosystems already stressed by other changes. Fargion points out that Earth's major extinctions do correlate in time with volcanic activity, climate shifts, and meteorite impacts. But none of these alone fully explains what happened. Perhaps, he suggests, they were all triggered by the same underlying cause: a planetary flyby.

The largest extinction in Earth's history occurred 251 million years ago, at the boundary between the Permian and Triassic periods. Between 80 and 95 percent of all species vanished. Yet geologists have found no iridium layer and no impact crater from that time. The cause remains a puzzle. Fargion's theory offers a possible answer: a massive object passed near Earth, generating tidal forces powerful enough to reshape the planet's surface and atmosphere, and no asteroid impact was necessary.

The Solar System itself offers hints that such events are plausible. Uranus rotates on its side, likely knocked into that position by a collision billions of years ago. Neptune's largest moon, Triton, appears to be a captured object from the Kuiper Belt. The Late Heavy Bombardment, a period of intense impacts early in Solar System history, may have been triggered by gravitational disturbances from a wandering planet. If Earth's neighbors experienced such cosmic encounters, Fargion argues, Earth almost certainly did too.

Evidence may be preserved in the Earth-Moon system. Fossil corals from the end of the Devonian period show that the number of days in a year suddenly decreased—the planet's rotation slowed abruptly. This slowdown is normally caused by the Moon's tidal forces, a process that happens gradually over millions of years. But the fossil record shows it happened suddenly. A collision would create an instantaneous change. A planetary flyby, however, could jolt the Moon outward through tidal forces, suddenly altering Earth's rotation rate. The same flyby would generate years-long tsunamis and crustal deformation, triggering massive volcanic eruptions.

Fargion acknowledges the limits of his proposal. Estimating how many flybys occurred, how massive the objects were, and precisely when they happened is extremely difficult with current evidence. He notes that Jupiter, based on its tilted axis and unexplained heat source, may have been struck by roughly 16 objects of Earth-mass size over its history. Similar calculations for other planets yield less reliable numbers. Yet the basic premise—that the outer Solar System once held a larger population of dwarf planets and larger bodies, and that some were occasionally scattered inward—is consistent with what astronomers know about planetary formation and dynamics.

If such flybys happened in the past, Fargion argues, they could happen again. He proposes two strategies: continuous monitoring of the deep sky to detect incoming objects, and the construction of high-altitude refuges on mountain chains at 2 to 3 kilometers elevation where humans and other species could shelter from the tsunamis and climate chaos that would follow. For smaller objects, deflection might be possible. For planetary-mass bodies, he suggests, survival would depend on preparation and refuge. The proposal is speculative, grounded in plausible physics but lacking the hard evidence that would move it from hypothesis to accepted theory. Yet it illustrates how much remains unknown about the forces that have shaped life on Earth.