It has long been agreed that the Chicxulub impactor, a space rock that crashed into the Earth 66 million years ago, was responsible for triggering the catastrophic global events that wiped out the dinosaurs.


The impact left behind a crater off the coast of Mexico’s Yucatan Peninsula that measures 150km across and 20km deep, and is thought to have triggered tsunamis, earthquakes and volcanic eruptions that not only led to the demise the dinosaurs but to almost three quarters of all plant and animal life on Earth.

However, it is still unclear exactly what the Chicxulub impactor was – was it an asteroid or a comet? – where it came from, or what caused it to strike the Earth with such force.

Now, in a new study, researchers at Harvard University say the Chicxulub impactor may have been a comet originating from the Oort cloud, an extended shell of icy debris located at the edge of the Solar System, that was bumped off course by Jupiter’s gravitational field during orbit.

Read more about the fate of the dinosaurs:

“The Solar System acts as a kind of pinball machine,” said the study’s co-author Harvard University undergraduate student Amir Siraj.

“Jupiter, the most massive planet, kicks incoming long-period comets into orbits that bring them very close to the Sun.”

As they pass close to the Sun, the comets – nicknamed sungrazers – can experience powerful tidal forces that break apart pieces of the rock and ultimately produce shrapnel-like chunks of comet.

“In a sungrazing event, the portion of the comet closer to the Sun feels a stronger gravitational pull than the part that is further, resulting in a tidal force across the object,” said Siraj.

“You can get what’s called a tidal disruption event, in which a large comet breaks up into many smaller pieces.

“And crucially, on the journey back to the Oort cloud, there’s an enhanced probability that one of these fragments hit the Earth.”

Compared to previous calculations, the Harvard study puts the chances of long-period comets impacting upon Earth up by a factor of about 10, and shows that about 20 per cent of long-period comets become sungrazers.

“Our paper provides a basis for explaining the occurrence of this event,” lead author of the study Prof Avi Loeb said.

“We are suggesting that, in fact, if you break up an object as it comes close to the Sun, it could give rise to the appropriate event rate and also the kind of impact that killed the dinosaurs.”

Further backing up their claim is the fact that analysis of the Chicxulub crater suggests the rock was composed of carbonaceous chondrite – a type of meteorite rare amongst main-belt asteroids, but possibly widespread amongst long-period comets.


The researchers say their theory can be tested by further studying other similar craters, and even ones on the surface of the Moon to determine the composition of the impactors.

Reader Q&A: Could dinosaurs survive in today's climate conditions?

Asked by: Liam Farmer, Birmingham

It’s doubtful. Tyrannosaurus Rex, and Triceratops for example, lived in the Cretaceous Period 145-66 million years ago (whatever Jurassic Park would have you believe). The average global temperature at the time was about 4ºC higher than today, with much less difference between the temperature of the equator and the poles. The sea temperature averaged 37ºC, so even tropical seas today would be too cold for marine life of the time.

But land dinosaurs would be quite comfortable with the climate of tropical and semi-tropical parts of the world. That is, until they all died of altitude sickness. Studies of air bubbles trapped in amber show that the atmosphere of the Cretaceous may have had up to 35 per cent oxygen, compared to today’s 21 per cent. For T. Rex this would feel like he was at the base camp of Everest. In such thin air dinosaurs would be too breathless to chase hapless tourists.

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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.