Rocky fireball challenges theories about origins of our solar system

We really need to know what's in the Oort Cloud and beyond.
Ameya Paleja
The fireball as captured by GFO cameras
The fireball as captured by GFO cameras

University of Alberta  

Last year, astronomers and stargazers captured images of a grapefruit-sized meteoroid that 'fireball'ed over central Alberta. Tracing back its journey, scientists were sure that it originated from the Oort cloud and was made up of ice.

Except that it was not and as it broke apart in the skies, the fireball dropped stony meteorites. This rare incident has now raised questions about the theories we have about the origins of our solar system, Phys.org reported.

The Oort cloud is a collection of icy objects sailing through space between our solar system and halfway to the nearest stars. Passing stars sometimes nudge these icy objects toward our Sun and astronomers have observed them as comets with long tails.

No human has actually looked at the objects that have come from the Oort Cloud directly but everything that has come that way has been made of ice. This has led us to theorize that Oort Cloud is made only of icy objects and nothing else.

Twist in the tale

The scientific theory was put into doubt last year when researchers at the University of Western Ontario captured images and videos of a fireball that was clearly rocky. The images were captured using state-of-the-art Global Fireball Observatory (GFO) cameras.

The meteor was rather a small weighing no more than 4.4 pounds (two kgs) and using the Global Meteor Network tools, the researchers found that the orbit it had followed was one seen with icy comets with long tails.

Tracing back its origins showed that the meteor undoubtedly came from the center of the Oort Cloud, even though all previous observations of rocky meteors had their origins much closer to our planet.

"In 70 years of regular fireball observations, this is one of the most peculiar ever recorded," said Hadrien Devillepoix, research associate at Curtin University, Australia, and the principal investigator of the GFO in the press release.

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Understanding the origins of the solar system

Comets are usually like snowballs mixed with dust that vaporize as they approach the Sun. The Alberta meteor, on the other hand, entered the Earth's atmosphere and flew much deeper than what a comet would do. Finally, it broke into smaller stony meteorites giving us ample evidence that it was essentially made of rock.

This observation was made possible by a strategy GFO began using five years ago where they broadened their horizons to monitor nearly two million square miles (five million sq. km) of the skies while bringing together experts from around the globe.

Having captured a rare event, the researchers now intend to understand how the rocky meteoroid originated in the Oort Cloud and how it ended up so far away. To do so, the researchers intend to paint the most accurate picture of the origins of our solar system and how we ended up here today.

The research observations were published in the journal Nature Astronomy.

Abstract

The Oort cloud is thought to be a reservoir of icy planetesimals and the source of long-period comets (LPCs) implanted from the outer Solar System during the time of giant-planet formation. The abundance of rocky ice-free bodies is a key diagnostic of Solar System formation models as it can distinguish between ‘massive’ and ‘depleted’ proto-asteroid-belt scenarios and thus disentangle competing planet formation models. Here we report a direct observation of a decimetre-sized (~2 kg) rocky meteoroid on a retrograde LPC orbit (eccentricity ~1.0, inclination 121°). During its flight, it fragmented at dynamic pressures similar to fireballs dropping ordinary chondrite meteorites. A numerical ablation model fit produces bulk density and ablation properties also consistent with asteroidal meteoroids. We estimate the flux of rocky objects impacting Earth from the Oort cloud to be 1.08+2.81−0.951.08−0.95+2.81 meteoroids per 106 km2 yr−1 to a mass limit of 10 g. This corresponds to an abundance of rocky meteoroids of ∼6+13−5∼6−5+13% of all objects originating in the Oort cloud and impacting Earth to these masses. Our result gives support to migration-based dynamical models of the formation of the Solar System, which predict that significant rocky material is implanted in the Oort cloud, a result not explained by traditional Solar System formation models.