Solar System

The Oort Cloud Could Have More Rock Than Previously Believed

The Oort Cloud is a collection of icy objects in the furthest reaches of the Solar System. It contains the most distant objects in the Solar System, and instead of orbiting on a plane like the planets or forming a ring like the Kuiper Belt, it’s a vast spherical cloud centred on the Sun. It’s where comets originate, and beyond it is interstellar space.

At least that’s what scientists think; nobody’s ever seen it.

A new study shows that the Oort Cloud is not exclusively the domain of frozen objects. There’s more rock there than we thought. And if there’s more rock there than we thought, it changes our understanding of how the Solar System formed. The study is based on a meteoroid that burned up in the sky over Alberta in 2021.

“This discovery supports an entirely different model of the formation of the Solar System.”

Denis Vida, study lead author, Western University.

The study is “Direct measurement of decimetre-sized rocky material in the Oort cloud,” published in Nature Astronomy. The lead author is Denis Vida, a meteor physics postdoctoral researcher at Western University in London, Ontario, Canada.

“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,” said Vida. “This result is not explained by the currently favoured Solar System formation models. It’s a complete game changer.”

Door camera videos of the fireball created a stir when the meteoroid burned up over Alberta in February 2021.

Scientific cameras with the Global Fireball Observatory (GFO) program also captured the fireball. The GFO is a global collaboration including institutions like NASA Ames Research Center, the Lunar and Planetary Institute, Western University, and many others. The GFO takes pictures of fireballs so scientists can recover the ones that reach Earth.

This image of the fireball is from the Global Fireball Observatory camera at Miquelon Lake Provincial Park, Alberta. Image Credit: University of Alberta.

The researchers in this study used tools from another collaboration, the Global Meteor Network (GMN), to calculate its origins. The GMN is a citizen-science project, a network of cameras on homes worldwide aimed at the sky. There are over 500 of them in 31 countries, and they log data each night and send it to a central repository. Scientists use it to determine the orbits of meteors.

A specialized satellite provided data for the study, too. It’s called the Geostationary Lightning Mapper (GLM.)

Bright flashes from fireballs can over-saturate some cameras, but the GLM excels at spotting them because it’s designed to map lightning. The GLM takes 500 images per second, enough data to reveal the detail in a meteoroid’s path through the atmosphere. When the researchers combined GLM observations with the coverage from ground-based cameras, the data set increased in breadth and depth.

With all these ground-based and satellite-based data, Vida and his colleagues understood they were dealing with something unusual.

“In 70 years of regular fireball observations, this is one of the most peculiar ever recorded.”

Hadrien Devillepoix, Curtin University, Australia, principal investigator of the GFO

Objects on similar orbits as this one typically burn up in the atmosphere’s upper reaches more quickly because they’re icy and less dense. But this one took much longer to burn up and travelled more deeply into the atmosphere. That told researchers that the Alberta fireball had to be a rocky object. The research team says it was a 2kg chunk of rock the size of a grapefruit. And this rock came from much further away than previous rocky fireballs.

“In 70 years of regular fireball observations, this is one of the most peculiar ever recorded. It validates the strategy of the GFO established five years ago, which widened the ‘fishing net’ to ~5 million square kilometres of skies and brought together scientific experts from around the globe,” said Hadrien Devillepoix, research associate at Curtin University, Australia, and the principal investigator of the GFO. “It not only allows us to find and study precious meteorites, but it is the only way to have a chance of catching these rarer events that are essential to understanding our Solar System.”

These images show the fireball as seen from the two GFO stations. It was observed for a total of 2.4 seconds with a path length of 148.5 km. Top: Miquelon Lake. Bottom: Vermilion (the Big Dipper can be seen on the left side). The fireball is moving left to right, and the periodic breaks in the fireball are used to encode the absolute time to an accuracy of 1 ms. Image Credit: GFO/Vida et al. 2022

The rocky object travelled at a velocity of 62.1 km s1 and penetrated down to a height of 46.5 km, just inside the stratosphere. An icy object should never have made it that close to Earth.

The chunk of rock is undoubtedly rare, or at least its pathway to Earth is. It follows a Long Period Comet (LPC) orbit, and LPCs come from the Oort Cloud. LPCs have orbits longer than 200 years and are highly inclined compared to the ecliptic, the orbital plane of the planets around the Sun. Its orbit is also retrograde, meaning it’s reversed compared to the planets.

This image shows the orbit of a long-period comet. Since these comets come from the distant Oort Cloud, a spherical region on the Solar System’s edges, their orbits are highly inclined. Image Credit: NAOJ.

The question this study poses is, “How did this chunk of rock originate in the Oort Cloud?” Finding an answer to that will shed more light on our Solar System’s formation because the existence of the icy bodies that make up the Oort Cloud is a fundamental part of our understanding.

“We want to explain how this rocky meteoroid ended up so far away because we want to understand our own origins. 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.”

The meteoroid behaved like most meteoroids, most of which come from asteroids. “During its flight, it fragmented at dynamic pressures similar to fireballs dropping ordinary chondrite meteorites,” the paper states. “A numerical ablation model fit produces bulk density and ablation properties also consistent with asteroidal meteoroids.”

Monitoring meteoroids and how far they penetrate the atmosphere is its own scientific endeavour. Scientists use the PE (Penetrate Earth) factor in describing a meteor’s structural strength, resistance to ablation, and how far it penetrates Earth’s atmosphere. It’s an imperfect measurement, but it’s still helpful in comparing meteoroids.

This figure from the study shows how the Alberta meteoroid’s PE compares to others in the Meteor Observation and Recovery Project dataset. The Alberta meteoroid is in a different region of the graph than softer cometary objects. It also shows how another noteworthy fireball called the Karlštejn event compares. Image Credit: Vida et al. 2022.

In their study, the researchers expand on the significance of the rocky object originating in the Oort Cloud. It won’t be the only one, and the study shows that the Cloud contains significant amounts of rocky material. But the rocky material didn’t form there. The ancient migration of planets in the Solar System drove the material into the Oort Cloud’s icy reaches.

“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,” the authors write in their paper.

The authors say the data rules out an icy object. “The fireball fragmented under dynamic pressures similar to those observed for rocky meteoroids,” they explain. This points out the need for a new model to explain how these rocky objects got into the Oort Cloud.

While a new model for the Solar System is beyond the scope of this paper, the researchers mention a couple of things.

They calculate the ratio of icy objects to lithic (rocky) objects in the Oort cloud. They constrain the ratio “… of icy/rocky objects to between 130:1 and 5:1 for masses >10 g.” The rocky objects can’t have formed there, so a Solar System model needs to have an ejection mechanism, even if the objects never even originated in our Solar System. “Even in a scenario where most of the Oort cloud objects are captured from other star systems, an ejection mechanism still needs to be present to explain the radial mixing of material,” they write.

The authors say that the rocky objects were implanted in the Oort Cloud during the formation of the Solar System and that the icy/rocky ratio is an intrinsic parameter of the Cloud. When it comes to the Alberta fireball, they think that object was likely not an intact primordial object. “The interstellar-medium erosion model predicts that all primordial objects smaller than a few metres should have been eroded away, indicating that the Alberta fireball possibly originated from a larger parent asteroid,” they write.

The researchers conclude that the rocky objects embedded in the Oort Cloud came from a proto-asteroid belt. They point to previous research showing that only the Grand Tack Hypothesis can explain how material from a proto-asteroid belt became embedded in the Oort Cloud. They also say that the pebble accretion theory, which describes how particles in a protoplanetary disk combine over time to form planetesimals, can’t explain their results.

It all adds up to a big challenge for our scientific models of how the Solar System formed.

“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 reflectance spectra data,” they write in their conclusion.

This study shows how much we still have to learn about our Solar System. For decades, the widespread understanding was that the Oort Cloud was an icy enclave of primordial objects and that an occasional perturbation would send one of them into the inner Solar System as a comet. The Cloud is named after Danish astronomer Jan Oort, who proposed its existence back in 1950.

This image shows part of the abstract from Jan Oort’s 1950 paper “The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin.” Image Credit: Oort, 1950/Bulletin of the Astronomical Institutes of the Netherlands.

But the Alberta fireball shows there’s more to the Cloud than we thought.

The study also shows the power of scientific collaboration between scientists and institutions and the rest of us who want to aid the effort.

If you’re interested in participating in this effort, check out the Global Meteor Network.


Evan Gough

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