Origin stories have a special way of capturing our imaginations. Whether it’s a particular species of plant or animal, or a favourite fictional character, knowing how they came to be makes for captivating stories. It’s no different in space. Especially the small corner of it we live in: the Solar System.
How did it form and why does it look the way it does today? How does it compare to other solar systems and is the one we’re part of special or just like all the others? In recent years, astronomers have got closer to answering these important questions and uncovering the story of our Solar System.
How they’re doing it is a story in itself. Humans may not have been around to see and record how the Solar System began, but there were witnesses, of a sort, who can relate that part of the tale. The rocky asteroids and icy comets that silently patrol the Solar System date back to its earliest days. Many are older than Earth.
“Their chemical composition and the distribution of their orbits contain clues as to what happened in the Solar System’s past,” says Prof Alan Fitzsimmons, an expert on asteroids and comets at Queen’s University Belfast.
Once upon a time...
The story starts around 4.6 billion years ago, when a cloud of interstellar gas and dust buckled under its own weight and collapsed until newborn stars lit up inside. The Sun was one of those stars.
Leftover material was hurled around these fledglings, forming flat bands known as protoplanetary discs. Gravity then took over, pulling and sculpting debris until it snowballed into objects, each one about a kilometre (just over half a mile) or so across. These ‘planetesimals’ then collided, smashing into each other and bulking up until the planets finally emerged.
But, where we find the Solar System’s eight planets today isn’t where they started their lives. They’ve jostled and jockeyed for position, migrating from their birthplaces to settle into a more stable configuration. At least that’s according to the Nice model, a popular version of the Solar System’s origin story named after the city in France where it was devised.
As the largest planet, Jupiter had a particularly big role to play. Today, Jupiter shares its orbit around the Sun with a population of ancient rocks known as the Trojan asteroids. These asteroids appear to be quite diverse, with varying compositions.
This means it’s unlikely that they all formed together in the same spot. If the planets did shuffle around, they would have scattered asteroids and comets far and wide, from many different places, in the process. Jupiter’s intense gravity could have vacuumed up many of the stragglers. “We know very little about the Trojans,” Fitzsimmons says.
That may soon change, though, thanks to NASA’s Lucy mission, which is en route to the outer Solar System. The spacecraft is named after the 3.2 million-year-old fossils of a hominin found in Ethiopia, which, when they were discovered in 1974, were the oldest-known, human-like remains.
The choice of name underscores the fact that asteroids and comets are the fossils of the Solar System. Astronomers, like archaeologists, pore over ancient fragments in search of answers. Lucy launched in 2021 and should arrive at the Trojans in 2027.
With the data that Lucy collects, “we should be able to work out where the Trojans have been,” Fitzsimmons says. In turn, that could tell us how the Solar System’s early evolution forced them into their current position.
Stellar siblings
According to the Nice model, Jupiter moved slightly inwards towards the Sun, whereas the other three giant planets (Saturn, Uranus and Neptune) moved outwards. The outer trio then encroached upon the Kuiper Belt, an icy band of objects that includes dwarf planet Pluto.
The outward migrating planets, particularly Neptune, scattered smaller objects from the Kuiper Belt further out and off the main plane of planets, creating what’s known as the Scattered Disc.
This distant region is thought to be the source of short-period comets – those that tumble around the Sun every few decades or centuries (Halley’s Comet being the most famous example).
Except there’s a problem: “The predictions and the observations don’t match,” says Fitzsimmons. In other words, the Nice model alone can’t account for the way the Scattered Disc looks today.
A relatively new idea – one that’s gaining popularity – could yet save the day. Astronomers are confident that the Sun wasn’t born alone.
They think it had many siblings that have since drifted away. Just as our siblings help shape the people we become, so the Sun’s brothers and sisters may have contributed to the way the Solar System looks today.
The infant Solar System could have experienced one or more stellar fly-bys as the Sun’s siblings drifted away – other stars buzzing past us at thousands or maybe even only hundreds of times Earth’s current distance from the Sun.
The gravitational pull of these stars could have upset the astronomical apple cart. “If the nascent Solar System got a little push with a close fly-by, that gives a much better fit to what we see now,” Fitzsimmons says.
In this way, the distribution of comets may hold the key to unlocking the secrets of the early Solar System. They may also be responsible for making it a suitable place for life, delivering the chemicals necessary for life to evolve on this planet.
For many years, comets were thought to have delivered water to the early Earth once it had cooled sufficiently after its fiery formation from colliding planetesimals.
However, astronomers are starting to change their minds. “The consensus is swinging towards asteroids,” Fitzsimmons says.
One of the major catalysts for the change of heart came when the European Space Agency’s (ESA) Rosetta mission visited the comet 67P/Churyumov-Gerasimenko a decade ago.
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Rosetta’s study of the comet found it contained a ratio of deuterium to ordinary hydrogen that’s three times higher than the ratio found on Earth. The result suggests comets like 67P may not have been responsible for delivering such chemicals to Earth.
But it’s not as clear-cut as that. Comet 67P has spent a long time in the inner Solar System. Interactions with the Sun have eroded its surface and could have changed its chemistry.
If we’re to really see what comets were like back when the Solar System formed, we need to study one that has never entered the inner Solar System.
That’s exactly the plan behind ESA’s Comet Interceptor mission, slated for launch in 2029. Its target hasn’t even been discovered yet. Astronomers are waiting to spot a pristine, incoming comet before they pounce.
“We’d see an object in a deep freeze from the time the giant planets formed,” says Fitzsimmons. As the pieces of our Solar System’s story begin to fall into place, attention is turning to how it compares to other planetary systems.
Strange and exotic systems
The last 25 years have seen an explosion of discoveries of exoplanets. The number of these planets orbiting other stars is fast approaching 6,000 and many are giving astronomers big surprises.
Before detecting these exoplanets, they expected to find solar systems that were broadly identical to ours, confirming their suspicion that ours is an average, or typical, solar system.
The emerging reality couldn’t be more different. Astronomers have found a celestial zoo of strange and exotic systems that wouldn’t look out of place in a science-fiction saga.
Systems with planets that orbit two stars, instead of one, or systems with planets orbiting so close to their central star that it probably rains liquid metal or jewels. Many of these other solar systems contain types of planets that we simply don’t have.
“The typical exoplanetary system has lots of in-between planets larger than Earth, but smaller than Neptune,” says Prof Karin Öberg, an astrochemist at Harvard University in the US. Then there are hot Jupiters – giant planets that have migrated so far inwards that they orbit their stars in days, sometimes even hours.
Astronomers have now discovered so many exoplanetary systems that they can begin to classify them, like biologists placing animals and plants into different species groups.
Last year, a team of Swiss astronomers identified four types of solar systems: ordered, anti-ordered, mixed and similar.
All the planets in a similar solar system have masses that are, unsurprisingly, similar, while in a mixed system, the masses vary from one planet to another.
Ordered solar systems (like ours) have planets that generally increase in mass the further out you go. In anti-ordered systems, it’s the other way around, with mass decreasing as you head outwards.
So which type of system is the most common? To date, no examples of anti-ordered systems have been found, but according to the Swiss study that came up with the classifications, around 80 per cent of systems can be classified as similar.
But just 1.5 per cent can be classified as ordered solar systems, like ours. It turns out our Solar System may very much be the exception, not the rule.
What’s more, it seems the seeds of our relative uniqueness were sown long ago, before there was even a collection of planets to orbit the Sun. It all comes down to the order in which the planets emerged from the disc of gas and dust. Once again, it’s Jupiter that appears to be the key.
The right kind of Jupiter
“One idea is that Jupiter formed early enough to stop the inflow of material into the innermost part of the Solar System,” says Öberg.
To play this role effectively, Jupiter also had to form in the perfect spot, close to a region astronomers refer to as the ice line. It’s the point at which temperatures drop sufficiently for gases such as water vapour, ammonia and methane to freeze into icy pebbles.
Like pebbles found on a beach, they’re typically a few centimetres (about an inch or two) across.
“As Jupiter feeds on those pebbles it creates a gap,” Öberg says. “It’s very difficult for other pebbles to cross that gap.” With the remaining pebbles stranded in the outer Solar System, the inner Solar System was starved of material from which to build large planets.
This potentially explains why we have an unusually ordered solar system apparently carved in two, with small planets close to the Sun and giant planets further out. Not many systems have the right kind of Jupiter.
Intriguingly, new observations by the James Webb Space Telescope (JWST) of the protoplanetary discs from which a system’s planets form, lend credence to this idea. In discs with large planets forming, there’s very little water vapour close to the star where an Earth-like planet could be forming.
The opposite is also true. “In discs where there are no gaps carved out by giant planets, we do see a huge amount of water in the innermost disc,” Öberg says.
Along with JWST, the other telescope improving our understanding of protoplanetary discs is the Atacama Large Millimeter Array (ALMA). A collection of 66 radio dishes scattered in the Chilean desert, it’s best suited for looking at the outer, cooler parts of protoplanetary discs.
“ALMA is really good at observing the organic molecules that are in pebbles,” says Öberg.
Given what we know about discs and how solar systems form, Öberg thinks one of the best places to find Earth-like planets could be around smaller stars such as red dwarfs.
But could asteroids and comets have delivered water and the building blocks of life from elsewhere in their discs? “The jury is still out,” Öberg says.
“I’m optimistic, but I do think we have a few more years of gathering data before that optimism is completely rewarded.” For one thing, the chemistry around these low-mass stars might be different from the chemistry we observe in our Solar System.
So, for now, we must wait. But continuing our work to understand how the Solar System formed will lead to more answers.
“If we manage to decipher what happened in our Solar System, those lessons might be applicable to other solar systems,” says Fitzsimmons. “Mother Nature is clearly very adept at making all sorts of planets,” he says.
Slowly, chapter by chapter, astronomers are piecing together the origin story of the Solar System, and with each new chapter we’re learning more about how we came to be in a position to wonder about it.
About our experts
Alan Fitzsimmons is a professor in the School of Mathematics and Physics at Queen's University Belfast and an expert in asteroid & cometary science.
An author of over 130 peer-reviewed publications, his research has focused on a broad range of topics like studying the first asteroid that's predicted to impact Earth, and investigating the first object from another star to visit the Earth,
Karin Öberg is a professor of Astronomy at the US University of Harvard. Specialising in astrochemistry, the goal of her research is to find out how the outcome of planet formation is affected by chemical processes.
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