Solar nebula, planet accretion and formation theories – this is how the solar system formed in 6 out-of-this-world steps
How do you go from a cloud of spinning dusk and rock around a newborn star to our majestic planets, moons and comets? Well, science has some pretty cool theories and observations about how our solar system came to be.
How the solar system formed (step by step)
Step 1: The Sun and Solar Nebula
It all started about 4.6 billion years ago with a giant cloud of gas and dust, known as the solar nebula. Triggered perhaps by the shockwave from a nearby supernova, this cloud began to collapse under its own gravity.
See more details on when our solar neighbourhood formed and how we know when it formed.
As it collapsed, the centre of the cloud became increasingly dense and hot, eventually forming our Sun. The birth of the Sun was a pivotal moment, as its gravitational pull began to shape the rest of the nebula.
Step 2: The Protoplanetary Disk & Snow Line
Around the newborn Sun, the remaining cloud flattened into a spinning protoplanetary disk. This disk, composed of gas and dust, was the birthplace of the solar system’s planets.
Within this disk, zonal distribution occurred – closer to the Sun, the heat vaporised lighter elements and especially water, leaving behind drier materials to form rocky planets. Further out beyond the Snow Line, where it was cooler, ice, gas and water-rich molecules prevailed, setting the stage for hydrogen-rich gas giants.
Step 3: Accretion & Planet Formation
Within the protoplanetary disk, particles of dust and gas began to collide and stick together, forming clumps. These clumps gradually grew, a process known as accretion, leading to the formation of planetesimals – the building blocks of planets.
Over time, these planetesimals collided and combined to form the planets we know today – the rocky inner planets: Mercury, Venus, Earth and Mars; and the gaseous outer planets: Jupiter Saturn, Uranus and Neptune. See when did Earth form?
Also see exactly how Earth formed.
Note: as the planetesimals became bigger, these collisions would have become more awesome – just imagine two Mars-sized “planets” smashing into each other and fusing. It would have been the most amazing fireworks spectacular in history.
Step 4: The Planets, Comets & Asteroids
As the planets formed, they left in their wake smaller bodies – the comets and asteroids.
Comets formed in the outer regions of the solar system and, as such are rich in hydrogen – they tend to be icy remnants of the solar system’s formation. Asteroids tend to be a bit rockier and drier – like the inner planets – and they’re primarily found in the asteroid belt between Mars and Jupiter.
It’s almost as if comets and asteroids are the bits and pieces left over, the parts that didn’t clump together into the planets. Mars is supposed to be a lot bigger than it is today – we think that Jupiter’s gravity prevented it from forming properly: the Asteroid Belt is literally the parts of Mars that never fused.
What’s more, the Asteroid Belt and Kuiper Belt today are pretty good analogues for what most of the protoplanetary disk looked like 4.5–4.6bn years ago,
Step 5: Planetary Migration
An intriguing aspect of this formation process is planetary migration. Theorists suggest that the giant planets, like Jupiter and Saturn, didn’t form in their current locations. They might have migrated and moved around in the early solar system, due to gravitational interactions with the disk of material and other planetary bodies. That would have significantly influenced the solar system’s architecture.
For example: If the solar system was zonally distributed by water content (see Step 2 above), then how can a dry, rocky planet like Earth have so much water on it? (See exactly how Earth formed.)
Of course, it was delivered here by comets from the outer solar system. But what would have caused the comets to do that? One theory is that a big gas giant like Jupiter or Saturn must have pulled the comets inward as they migrated. That’s perhaps why we still see some comets with weird orbits that bring them from way out in the cool depths of the outer solar system to pretty much on our doorstep every now and again.
Step 6: The Late Heavy Bombardment
Science agrees the solar system began forming 4.6 billion years ago, with proto-Earth being born at least 4.5bn years ago (see when the Earth formed). But, studying the craters on the moon and extrapolating that data for Earth, scientists believe that proto-Earth might have only been about a tenth of its size when it originally formed.
Around 4 billion (The Hadean) to 3.8 billion (The Archean) years ago, the solar system experienced the Late Heavy Bombardment. This was a period of intense asteroid and comet impacts. (Perhaps this was when Jupiter pulled the comets into the inner solar system?)
This epoch played a crucial role in shaping the surfaces of the inner planets. And it may have been pivotal in delivering water and organic compounds to early Earth.
In fact, from what we know of planet formation, all of Earth’s metallic materials would have sunk into the core when Earth formed. Meaning that there should be no iron or gold or platinum for us to mine. But there is, which means all the metal we’re mining today didn’t come from the Earth. It was likely all delivered here during the Late Heavy Bombardment. Probably a good amount of our water was delivered then, too.
How do we know that’s how the solar system formed?
Since no one was there to witness it, all our theories about how the solar system formed come from proxies. For example, we infer most info on formation from actually observing other solar systems at various life stages with telescopes.
Next, we sample and study all the rocks we can find. The earth, of course, but we also dated and studied rocks from the moon, asteroids and comets, to get an idea of their age and composition. There’s some more interesting info from the free text on the geosciences here.
Get the whole story about how we know when the Earth formed.
Wait, what is the solar system again?
Our solar system comprises 8 planets (sorry Pluto!), over 200 moons and countless planetesimals, rocky bodies, comets, and asteroids that circle our sun.
We have the sun (our star) in the middle. It’s orbited by Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Next, there’s an asteroid belt just beyond Mars, the Kuiper belt just beyond Neptune (where planetesimal Pluto is). And the giant spherical Oort Cloud just beyond that.
That’s our little solar neighbourhood, and the whole setup is between 4.5 and 4.6 billion years old and part of the even bigger Milky Way galaxy.
Watch: Some Awesome Documentaries on How 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…
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 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.)
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.
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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