Images of Uranus and Neptune taken by the Voyager 2 probe

Uranus (left) and Neptune (right) are the most distant planets from the Sun within our Solar System. Image adapted from © NASA/JPL-Caltech, licensed under Public Domain via NASA and NASA.

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Outer solar system near Neptune and Uranus was source of rare asteroids

The origin of one of the rarest group of meteorites may have been identified.

Researchers studying samples returned from the asteroid Ryugu found that their iron makeup is indistinguishable from stony meteorites known as CI chondrites, offering an insight into the Solar System's formation.

A large part of the asteroid belt between Mars and Jupiter may have been formed on the fringes of the Solar System.

A new study, published in the journal Science Advances, suggests that a group of asteroids known as the Cb-type formed in a region of space towards the edge of the Sun's influence, billions of kilometres away from Earth.

Using samples taken from the near-Earth asteroid Ryugu by the Hayabusa2 probe and modelling, researchers suggest that these asteroids may have formed in the Kuiper Belt, or perhaps even further out.

Some of these asteroids are thought to have been thrown into the main asteroid belt during the formation of Uranus and Neptune. They make up around 10-20% of the C-type asteroids present there today, with the rest thought to have come from the region surrounding the gas giants Jupiter and Saturn.

Professor Sara Russell, a Senior Research Lead at the Museum who co-authored the paper, says, 'It's only within the last decade we've begun to appreciate just how far objects in the Solar System can move towards and away from the Sun.'

'While there was general acceptance that material from the outer Solar System could have been moved inwards by the giant planets, this is one of the first which suggests they come from not just the Jupiter region, but as far out as Neptune.

'This adds an extra layer of detail to our knowledge of how the Solar System formed.'

An artist's impression of the Solar System forming

The formation of the Solar System took place around 4.6 billion years ago. Image © Mopic/Shutterstock

How did the Solar System's giant planets form?

The Solar System formed from a collapsing gas cloud, with the vast majority of it becoming part of the Sun. The remaining 0.1% of matter formed a protoplanetary disk from which the planets and asteroids were born.

Closer to the Sun, only rocky and metallic elements such as aluminium and calcium could remain solid giving rise to planets such as Earth and Mars. Further out, gas and dust condensed to form the gas giants Jupiter and Saturn.

At this time, the orbits of the planets were not as stable as they are now, so interactions with other bodies affected the path that the developing gas giants took.

'Recent models of the Solar System suggests Jupiter and Saturn migrated inwards when they were young and absorbed some of the material before migrating out again,' Sara explains. 'This could explain why Mars is a small planet relative to its neighbours, as much of the material it could have developed from was taken.'

'The movement of Saturn and Jupiter is also responsible for moving icy bodies from the outer Solar System inwards. This enabled water to be delivered to the inner planets, including Earth.'

Further beyond the gas giants are the ice giants, Uranus and Neptune. Located more than three billion kilometres from the Sun, their formation is something of a mystery.

Planets at this distance take longer to form than those closer to the Sun. In fact, they should take so long to form that the protoplanetary disk would have disappeared before they could gain the size and structure they have now.

As a result, it is hard to reconcile current theories of Uranus and Neptune's formation with what we know of the ice giants today. It is likely that they formed closer to the Sun than they are now, and migrated outwards in their early history.

What is not in dispute, however, is their influence on their surroundings. With Neptune and Uranus the third and fourth biggest planets of the Solar System, the gravitational force they exert on surrounding space is immense.

As the planets migrated, they are believed to have scattered much of the surrounding debris.

'The early Solar System was an incredibly chaotic place,' Sara says. 'While the planets cleared their orbits, smaller bodies would have been thrown around and caused many large impacts, such as the collision which created the Moon.'

'The migration of the giant planets would have thrown a significant amount of material from the outer Solar System inwards, where it would have been captured by the asteroid belt.'

The new paper suggests that these asteroids from the outer Solar System include a group of carbon-rich bodies known as the Cb-type asteroids.

An image of Ryugu taken from 64 metres above its surface

Ryugu's surface was sampled by the Hayabusa2 probe, with a canister containing the samples later returned to Earth. Image © JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST.

What does Ryugu reveal about the formation of the asteroid belt?

For this study, the researchers used samples taken from a Cb-type asteroid known as Ryugu. In 2020, the Hayabusa2 mission returned five grams of the asteroid to Earth, and these samples are now being studied by scientists from all over the world.

The researchers were particularly interested in assessing whether Cb-type asteroids could be the parent body of a rare group of meteorites known as carbonaceous chondrites, and in particular the rarest form, carbonaceous Ivuna type (CI).

'We've been collecting meteorites for over 200 years at the Museum, but for most of that time we've had very little clue where they came from,' Sara says. 'In the past few years, we've used observations to help calculate the orbits of meteorites such as Winchcombe, but we still don't know where most are from.'

'By comparing the forms of iron in both the asteroids and meteorites, we've been able to find out that Ryugu is a very close match to CI chondrites. These are the rarest type of carbonaceous meteorite, and I'm really excited about that as the type specimen, Ivuna, is within the Museum's collections.'

The study suggests that both Ryugu and the CI chondrites originate from the same region of space, and possibly even share the same parent body.

To investigate the formation of the outer Solar System in further detail, the paper suggests that missions could be sent to asteroids believed to have come from a region of space beyond Neptune similar to where the CI and Cb asteroids could have originated in the main belt, such as 203 Pompeja and 269 Justitia.

Further insights may also be gained from visiting Uranus and Neptune, which have only been briefly visited by the Voyager 2 probe. A number of missions have been proposed by the world's space agencies, which could see the ice giants visited in the coming decades.

'It would be ideal to get a sample returned from Uranus or Neptune or one of their moons,' Sara adds. 'If we could sample them, it would allow us to assess whether these planets have any similarities to the CI chondrites, which would really confirm whether this hypothesis is correct or not.'

As well as inspiring missions to outer space, the paper also suggests that researchers can explore the universe while staying closer to home – by exploring the Museum's collections.

'This discovery is very exciting for me as it means that the Museum's meteorite collection is sampling the whole of our Solar System,' Sara says. 'Along with other types of meteorites, such as the enstatite chondrites from the inner Solar System and ordinary chondrites from the asteroid belt, we can study huge swathes of space from here in London.'