A massive collision, nuclear ignition and a collapsing cloud of swirling particles. Our solar system’s formation was one of the coolest, craziest and luckiest (for us!) events in our neighbourhood of the Milky Way galaxy.
But when did the solar system form exactly? And how do we know the timing if we weren’t there to see it?
So, When did the Solar System Form?
Our solar system formed around 4.5682 billion years ago from a molecular cloud of gas and dust swirling around the young sun. This cloud collapsed from either its own gravity or the shockwaves of a nearby supernova, forming a solar nebula – a spinning disk of material swirling around the sun, according to the widely accepted Nebular Hypothesis.
Over time, the debris in the solar nebula started to clump together. Eventually, they smashed into each other, collecting into bigger and bigger clumps called planetesimals – the process known as accretion.
All the planets in our solar system were made from the accretion of these clumps and planetesimals colliding over millions of years. For more on the processes involved, see exactly how the solar system formed.
How do we know that’s when the solar system formed?
We normally say the solar system formed 4.5–4.6 billion years ago, and the Earth itself at 4.5 billion years ago (see when did Earth form and how Earth formed.). But how do we know that?
There have been numerous studies throughout the years to gauge the solar system’s age. From radiometric dating on the oldest known rocks on Earth (zircon crystals from Western Australia). To dating moon rocks and meteorites from the Sahara desert (see the ASU study here) or observing other stars and inferring our sun’s age from its rate of nuclear reactions (more about observations at OpticalMechanics).
And they all point to the Earth, moon and meteors all forming at the same time around 4.5682 billion years ago (often rounded to 4.6bn years ago). The science is extremely fascinating. Read about when the Earth formed and how do we know when the Earth formed.
Another fascinating topic though, is why it formed in the first place. See, our solar system is quite far from active star-forming regions of the Milky Way. So there must be a story behind how our sun formed in the first place…
When and why did the Sun form?
From observing new stars being born in other galaxies, we deduce that the sun and the dust cloud that make up the solar system formed at roughly the same time.
The problem is if you look at where we are in the Milky Way galaxy, all the way out on the Orion Arm or Orion Spur, you’ll see we’re a long way away from the denser star-forming inner regions of the galaxy.
So scientists think we must have either once been a lot closer to the centre of the Milky Way, or our galaxy must have collided with a smaller dwarf galaxy – an event that’s known to spark star formation.
In 2018, researchers spotted some 30,000 “alien” stars in our galaxy that moved differently and are made of different “stuff” to stars of the Milky Way. They behaved much like we’ve seen when galaxies collide, so researchers using the European Space Agency’s Gaia telescope showed that there are indeed dwarf galaxies colliding with the Milky Way every now and again.
One of those dwarf galaxies, Sagittarius, seems to have smashed through our galaxy’s disk a few times in the past 7 billion years. And one of those collisions lines up quite well with the formation of our sun.
We think it might have sparked our sun’s formation because when galaxies collide, it ignites or speeds up star formation in the galaxy – making it totally possible for new stars to form, even out here in the Styx of the Milky Way.
Ah, but there’s a problem with the inner planets
Another fascinating puzzle around the solar system’s formation is that there’s something wrong with the current state of our inner planets.
See, if you look at how the solar system formed, you’ll see why it shouldn’t be possible for Earth to have water (or at least so much of it). See, the early solar nebula pushed all water content into the outer solar system, leaving the inner one full of dry, rocky material. So how did the water get all the way back here? See exactly how Earth formed.
Another weird thing is why Mars is so small. The planets are supposed to get bigger as we go away from the sun. And the first 3 do – Mercury is the smallest, Venus is much bigger, Earth is even bigger and Mars should be even bigger than Earth. But it’s not. Something stopped Mars from growing and caused our solar system to grow weirdly.
What’s more, our asteroid belt contains an eclectic mix of materials, many of which shouldn’t be there at all, if our solar system just normally formed out of clumps of stuff collecting together in various places around the sun.
So what gives?
The Grand Tack hypothesis
Researchers found that the only way our solar system made sense is if big old Jupiter didn’t form where it is now (or didn’t stay in one place, at least). The theory goes that Jupiter formed in the outer solar system, and migrated inwards (preventing Mars’ growth and pulling exotic asteroid material from the outer solar system inward into the inner solar system), before migrating back outwards again to where it is today.
In fact, we think that there was a point where all or most of the gas giants migrated, which partially explains another anomaly – Neptune’s weird tilt and crazy orbital pattern. More in how the solar system formed.
Wait, what is the solar system again?
Our solar system is made up of the 8 planets (sorry Pluto!), the over 200 moons and countless planetesimals, rocky bodies and asteroids that circle our sun.
We have the sun (our star) in the middle, and Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune orbit it. That’s basically our little solar neighbourhood, and the whole setup is between 4.5 and 4.6 billion years old and part of an even bigger Milky Way galaxy.
Oh, and it’s pretty unique because, though there are likely other forms of life out in the universe somewhere, ours is the only place we have 100% de facto evidence that life exists.
See exactly how Earth formed.
Watch: Some Awesome Documentaries on When the Solar System formed
Just click play, hit full screen if you want to and check out hours’ worth of awesome documentaries on our solar system’s formation right here…
The Whole History of the Earth and Life
One of my personal favourites, this non-commercial, mainly YouTube-only documentary is a project by Japanese creator @KarouGreenEmerald, based on the research findings of the Tokyo Institute of Technology’s Professor Shigenori Maruyama and the Hadean Bioscience Project, which proposes some new theories on the formation of Earth, development of life, evolution as well as some future-casting.
Birth of the Solar System
Episode 18 of National Geographic’s Naked Science Season 4, which aired on November 6, 2007. (I can’t find it available for purchase anywhere.)
History of the Earth
National Geographic’s 2011 Yavar Abbas documentary gives a pretty exciting overview of how the solar system and Earth may have formed. (Again, no purchase links for this title.)
TDC’s Formation of the Solar System
For a much shorter video, this one sums up some theories on solar system formation in 6 minutes, focusing on information from the Bennu asteroid, which NASA recently managed to sample.
@SEA’s The Birth of the Solar System
Independent content creator, Sea’s overview of solar system formation is relaxing, well researched and filled with extra insights and titbits.
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