Leading astronomers propose a compelling alternative to the dinosaur extinction: a comet from the Oort Cloud, deflected by Jupiter's gravity, grazed the Sun and fragmented, with one piece striking Earth.
Approximately 66 million years ago, a 10-kilometer-wide object from the asteroid belt—or so the prevailing theory goes—collided with Earth at Chicxulub, Mexico. The cataclysmic impact and its aftermath wiped out about 75% of Earth's species, including non-avian dinosaurs, marine life, and plants.
This hypothesis, dominant since the 1980s, stems from global Cretaceous-Paleogene boundary layers rich in iridium—a rare Earth element abundant in asteroids. The Chicxulub crater, identified in the late 1970s and confirmed by 2016 drilling samples showing extreme pressure deformation, supports this view.
Now, Harvard researchers offer a fresh perspective, detailed in Scientific Reports.
Harvard undergraduate Amir Siraj and astrophysicist Avi Loeb analyzed impact rates on Earth-like exoplanets and long-period comets from our Solar System's distant Oort Cloud, roughly 100,000 astronomical units from the Sun.
Their findings: Jupiter's gravity frequently deflects these comets toward the Sun. Many Earth-crossing comets show prior solar close encounters. Simulations indicate comets 10-60 kilometers wide fragment under the Sun's tidal forces.
Such disruptions produce fragments at rates making Chicxulub-scale impacts more likely from comets than asteroids. The impactor's carbonaceous chondrite composition aligns with Oort Cloud materials, rarer in the asteroid belt.
Similar craters—like South Africa's Vredefort (2 billion years ago) and Kazakhstan's Zhamanshin (less than 1 million years ago)—match this profile. Models predict hits every 250,000 to 730,000 years.
While simulation-based, the theory awaits validation. Loeb anticipates data from Chile's Vera C. Rubin Observatory, launching next year: "Hopefully, we'll gain better statistics on long-period comets and spot fragment evidence."
