As humanity has reached further into space, we've come to learn a lot more about the lifecycle of the solar system.
From a collapsing cloud of gas into a brand new star to an accretion disc with planets vacuuming up debris, to measuring how much gas the Sun's got left in the tank and calculating we've got about another 4.5 billion or so years left in this thing.
After that, the Sun will start to exhaust its fuel and our solar system will enter its long, terminal decline and eventual death.
We are talking about a cosmological timescale that, to us, is unfathomably long in concrete terms, so none of us will be around to see any of these events come to pass.
But what if we could take an eternal galactic bird's-eye view of our small plot of the galaxy from start to finish? What would that lifecycle look like? Let's find out!
Birth of the Sun
So, you can't have a solar system without at least one star in the middle, and ours got its start roughly 4.6 billion years ago as an incredibly massive, densely packed cloud of dust and hydrogen gas known as a molecular cloud.
A molecular cloud can consist of the remains of a far older star that burned through its fuel and blew off heavy metals, gases, and other building blocks of a solar system in either a spectacular supernova or as a more modest shedding of material.
It may have been another supernova nearby that triggered this cloud to collapse in on itself after a shockwave passed through, or the cloud could have collapsed under its own weight — but in either case, the collapsed material formed into a swirling solar nebula.
Gravity pulled more and more material into the center of the nebula where the gas condensed under great pressure. This was the first major critical point in the solar system's lifecycle where things could have gone sideways.
Without enough mass to generate the enormous internal pressure needed to jam the nuclei of two hydrogen atoms together to make helium — a process called nuclear fusion — things could have ended up much differently.
When there isn't enough mass to trigger nuclear fusion, you end up with a body called a Brown Dwarf, which is essentially a failed star. It's something akin to a super Jupiter, an enormous gas giant free-floating in space without a host star and insufficient internal nuclear reactions to generate energy, light, heat, and all the other good stuff we associate with stars.
Fortunately, our Sun had enough material so that its internal fusion got going, and it would go on to accrete about 99% of the available matter in the molecular nebula.
Accretion disk and planetary formation in the inner solar system
According to the disc accretion theory, almost immediately, what was leftover began to form a disc of material around the Sun, stretching to the Kuiper belt.
Throughout this disc, material brushed up against each other and eventually started to accrete into larger bodies a few kilometers wide known as planetesimals within the first 100 million years of the birth of the Sun.
Closer to the Sun, it was hot enough that certain elements and compounds known as volatiles, like water ice and ammonia, couldn't exist in liquid form, much less solid, and so remained in a gaseous state in the accretion disc.
Meanwhile, the Sun had begun to produce a steady flow of particles from its nuclear furnace and blow these out in all directions, something we call the solar winds.
These, in turn, pushed out the lighter, gaseous volatiles toward the outer part of the disc, leaving only the densest, rockiest material like metals and silicates in the inner part of the solar system (though a small portion of the lighter elements were accreting to the growing planetesimals).
As these smaller planetesimals in the inner disc accreted more material and grew to be hundreds of kilometers wide, they became large enough that their gravitational pull distributed their mass into a more spherical shape.
They also started to disrupt the motion of other nearby planetesimals, which led to an increase in collisions, which, over time, grew some of these planetesimals large enough that they were upgraded into protoplanets.
Being larger than the surrounding material, these protoplanets exerted a much larger gravitational pull and they quickly came to dominate any other material in their orbital path. This allowed these protoplanets to accrete smaller planetesimals into themselves rapidly, which led to their swelling in size over a very short period of time.
Soon, the force of their gravity started differentiating the layers of the planets as heavier elements like iron and nickel were pulled deeper into the interior while lighter elements like oxygen, silicon, and magnesium formed a layer called the mantle. The very outer edge of the protoplanets became a hardened, rocky crust that was rife with volcanic activity.
In at least one case, that of Earth and Theia, it's theorized that these protoplanets began to pull on each other and disrupt their orbits: About 4.5 billion years ago, when the Earth was still a molten, rocky wasteland dominated by volcanoes, it's speculated that a protoplanet, Theia, between the size of Mars and Earth collided with the Earth, breaking loose a large amount of material from both its own mantle and that of Earth's, and sending it all into orbit around the Earth.
It is believed by some astrophysicists that Theia impacted the Earth at a steep angle and not a glancing blow, sinking its own iron core into that of Earth's, where the two mixed to become a single core of iron. According to this theory, the largely silicate mantles of the two protoplanets also mixed and became one.
Meanwhile, the mostly silicate ejecta from the impact formed a disk of material around the Earth and, just like the Sun's protoplanetary accretion disk, material in the disk started to coalesce into ever-larger pieces that would eventually make up the Moon.
It's suspected that Venus may have also suffered similar collisions, but as one of only two planets in our solar system to not have a moon of its own, this is still a great deal of debate on this since it is theorized that such a collision would almost certainly produce a moon similar to our own.
The disc accretion model does have some issues, which other models, such as the disc instability model and the pebble accretion model attempt to address. But disc accretion remains, at least for now, as the leading model.
Planetary formation in the outer solar system
Meanwhile, in the outer solar system, all of those volatiles that were being blown out of the inner solar system by the solar winds were passing what is known as the "frost line", an imaginary boundary far enough away from the Sun that these volatiles can condense into liquid and ice.
This hunk of icy material combined with other hunks of icy material to form larger bodies the size of asteroids, but smaller than planetesimals. There are theories about these icy bodies growing large enough though that they formed the core of gas giants like Jupiter, but it's likely that the core of the gas giants is made of a fuzzy soup of iron and silicate material mixing around in an ocean of hydrogen and helium liquid.
What we do know is that almost as soon as the solar system started coalescing, the first planet out of the gate was Jupiter. As the largest planet in the solar system, it is mostly made of the same material as the Sun, sucking up primordial gases in the earliest days of the solar system while the Sun was just starting to ignite into a star.
Jupiter, in fact, has about twice the mass of all the other planets in the solar system combined and is large enough that it creates a barycenter between itself and the Sun, that is, a center of gravity around which both bodies orbit, or a common center of mass.
Had things turned out a bit differently and Jupiter had enough mass to ignite the nuclear fusion of its hydrogen, it could have become a star in its own right and ours would have been a binary-star solar system rather than a single-star one.
This didn't happen though, and Jupiter's hydrogen is only able to condense into a liquid state deep in Jupiter's interior. The liquid hydrogen around Jupiter's core, in fact, is believed to be the largest "ocean" in the solar system.
The pressure keeping Jupiter's hydrogen in liquid form may also be stripping its hydrogen atoms of their electrons, a potential source of Jupiter's enormous magnetic field.
As mass increases though, so does the effect of gravity; so, as Jupiter soaked up gas and material from the protoplanetary accretion disk, there's reason to believe that its orbit could have been pulled closer to the Sun.
Had this gone on for long enough, Jupiter could have migrated all the way into the inner solar system and become a so-called Hot Jupiter. For the most part, Jupiter did not end up with this fate due to the intervention of Saturn, which formed near Jupiter just in time to exert a restraining pull on it and keep it from migrating inward and wrecking whatever protoplanetary formation was starting to take place in the inner part of the solar system.
This restraining effect forced Jupiter to settle more or less into its present orbit and left the inner solar system to its own devices. However, Jupiter's gravitational pull is still enormous, and it has dozens of verified moons orbiting around it. While some of these could be the work of accretion, many are the result of gravitational capture.
Not much is known about the formation of the last three planets in the solar system, Saturn, Uranus, and Neptune, but there are a lot of things we can say about them.
In terms of one of the most famous features of our solar system, Saturn's rings are largely the remains of icy bodies ripped apart by the planet's tidal forces.
These are thought to be the scattered remains of comets that came too close to Saturn's gravity well and were shredded as a result; the remains of shattered moons that got captured in Saturn's gravitational pull; and other material and dust blown out of the inner solar system that Jupiter didn't suck up.
Saturn is largely made of the same material that Jupiter is — hydrogen and helium — and a recent examination of its ring system revealed a rippling in its so-called D-ring that researchers have been able to use as a form of seismograph for the planet as a whole, revealing a core made of liquid hydrogen and helium, and containing chunks of solid material like iron and silicates.
It is likely, then, that the other gas giants have a similar internal composition to a degree.
While not as spectacular, all of the gas giants have rings, though those of Jupiter, Uranus, and Neptune are too faint to see.
Out beyond Neptune is the Kuiper Belt, the last vestiges of the accretion disk that formed the solar system. Including bodies as large as the dwarf planet Pluto, the Kuiper Belt is almost a slow-motion replay of the early formation of the terrestrial planets on the inner portion of the solar system.
When New Horizons passed the Kuiper Belt object Arrokoth on New Year's Day, 2019, it beamed back pictures of a pair of large semispherical bodies that had fused themselves together over time, likely after a collision at some point in the not-too-distant past.
This provided evidence for our theories about early terrestrial planet formation, but more research needs to be done before we can say so definitively.
Present Day Recap
This brings us more or less to the present day, where everything orbits the way it "should" and life has bloomed on at least one world. There may also be the potential for life on a few moons orbiting Jupiter and Saturn — but it will be a long time before we are in a position to verify or rule this out.
The Sun is well into its main sequence stage of development where it will remain for a few billion years to come. By and large, the eight planets of our solar system have cleared the proverbial gutters of their orbits, so little else remains besides a relatively small belt of asteroids between Mars and Jupiter.
In the furthest reaches of the Kuiper belt, where material like Arrokoth (formerly nicknamed "Ultima Thule") continue to slow-walk the planetesimal formation process, Pluto and other dwarf planets like Eris, Haumea, and Makemake continue their reign over the most distant stretch of the known solar system.
And, somewhere out there in the trans-Neptunian regions of the solar system, the mysterious Planet Nine, about 10 times the mass of Earth compressed to about four times its size, might be lurking, disturbing the trajectories of Kuiper belt objects and making its presence felt even though it has never been seen and its existence is still hotly debated.
This is more or less where we are, but it is just the beginning of what we expect to happen in the next 5 to 8 billion years, and even longer.
Stay tuned for the second half of our lifecycle of the solar system series, where we explore how we expect our solar system to die.