Grapefruit-size fireball from mysterious Oort Cloud could rewrite the history of the solar system

A fireball streaks across the sky over Alberta, Canada
A fireball streaks across the sky over Alberta, Canada (Image credit: University of Alberta)

A dazzling fireball that ended its cosmic journey over central Alberta, Canada could change astronomers' understanding of how the solar system formed 4.5 billion years ago.

Caught on camera on Feb. 22, 2021, the grapefruit-size rocky meteoroid is thought to have come from the Oort Cloud, a reservoir of celestial objects that encircles the entire solar system and separates it from interstellar space. Scientists have never directly observed rocky objects in the Oort Cloud and have long believed that it holds only icy objects. But the rocky object that burnt up over Canada challenges popular theories of the Oort Cloud's formation, and the early solar system's formation in general, according to a study published Dec. 12 in the journal Nature Astronomy (opens in new tab).

"This discovery supports an entirely different model of the formation of the solar system, one which backs the idea that significant amounts of rocky material co-exist with icy objects within the Oort cloud," lead study author Denis Vida, a meteor physics postdoctoral researcher at Western University in London, Ontario, Canada, said in a statement. "This result is not explained by the currently favored solar system formation models. It's a complete game changer."

According to NASA, the Oort Cloud is thought to have formed when gravity from the newly formed planets pushed icy objects away from the sun. Gravity from the Milky Way galaxy caused the objects to settle on the edge of the solar system instead. 

A popular current theory about how the solar system formed is the pebble accretion model, which describes millimeter-size pebbles being sucked together over time to form celestial bodies. 

"These findings challenge solar system formation models based on pebble accretion alone, which currently cannot explain the high observed abundance of rocky material in the Oort cloud as derived from fireball measurements and telescopic data," the authors wrote in the new study. 

Rather, these results support what's known as the "Grand Tack" theory of solar system formation. This model proposes that Jupiter formed closer to the sun and migrated towards it before gravitational effects between Jupiter and Saturn forced both planets farther out. Only this model can account for sufficient amounts of rocky material from the inner solar system being ejected to the Oort cloud to explain the fireball, according to the researchers.

The fireball was picked up by Global Fireball Observatory (GFO) cameras run by the University of Alberta. The GFO is a global collaboration between organizations including the Lunar and Planetary Institute, NASA Goddard Space Flight Center and several universities. Its aim is to image fireballs so that meteorites can be recovered. 

Calculations of the fireball's trajectory show that it traveled from the outer reaches of the solar system, similar to the trajectories of icy comets — the objects thought to inhabit the Oort Cloud. The fireball's rocky nature was confirmed by its descent deeper into Earth's atmosphere than icy objects traveling on a similar orbit could survive. It also then broke apart, just as a regular rocky fireball does.

However, the Alberta fireball is not a one-off. The researchers found a similar fireball in a historical database that never got noticed at the time. These multiple rocky bodies suggest that between 1% and 20% of meteoroids coming from the Oort Cloud are rocky, the authors said. 

"The better we understand the conditions in which the solar system was formed, the better we understand what was necessary to spark life," said Vida. "We want to paint a picture, as accurately as possible, of these early moments of the solar system that were so critical for everything that happened after."

Live Science contributor

Jamie Carter is a freelance journalist and regular Live Science contributor based in Cardiff, U.K. He is the author of A Stargazing Program For Beginners and lectures on astronomy and the natural world. Jamie regularly writes for Space.com, TechRadar.com, Forbes Science, BBC Wildlife magazine and Scientific American, and many others. He edits WhenIsTheNextEclipse.com.