Why are small terrestrial planets grouped together in the inner solar system, while giant planets are together in the outer solar system? And why only the small planets are made mostly of rock and metal? Why don’t we find planets made mostly of nitrogen or carbon? In short, why is our solar system the way it is? Let’s find out.
To answer these questions, we need to understand how the solar system was formed. We weren’t there for the formation of the solar system, but over the years we have found some clues. We need to piece these clues together to come up with a scenario that best explains the facts.
The solar system is more than just an arrangement in space. It is also an arrangement over time—vast amounts of time—which means that we need to look back in time. Traveling backwards in time allows us to explain the key features of the solar system that we have been ignoring or taking for granted.
We’ll do a thorough gathering of the clues available to us and see what needs explaining. We’ll try to make sense of the clues as we go along.
Clue No. 1: Orbital Plane and Orbital Direction
The first clue is that we need to notice that all the planets are in the same orbital plane, and they all orbit in the same, prograde direction around that plane. Not only that, the direction of their orbits is the same as the direction of the Sun’s rotation.
The best explanation for this set of observables is that all the planets formed the same way, out of a spinning disk of gas and dust that surrounded the proto-Sun, all rotating in the same direction. Planets formed from the disk by having material clump together through their mutual gravitational attraction to form larger and larger bodies. Eventually, they grew large enough to become planets.
Learn more about how the solar system family is organized.
Clue No. 2: Location and Composition of the Planets
The second clue comes from the location and composition of the planets. The rocky terrestrial planets are closer to the Sun than the giant planets. The gas giants are closer than the ice giants.
This can be explained by considering the temperatures in the protoplanetary disk. The Sun at the center would have been sending heat out through the disk, just like the Sun heats the solar system today. Regions closer to the Sun would have been much hotter than regions farther away. This gradient in temperature resulted in different materials condensing out of the gas into solid phases at different distances from the Sun.
At the earliest times, only materials with very high solidification temperatures, like metals and silicate rocks, could condense out of the gas in the inner solar system. Farther out, molecules with lower condensation temperatures could condense as well because temperatures were cooler. These included things like water, ammonia, and methane—what planetary scientists call ‘ices’.
These differences had two consequences. First, the primordial material that went into making the inner planets was different from the outer planets. Planets grew from gravitational interactions between smaller objects. If two objects were close enough to each other, they could merge into a larger object. This continued until planet-sized objects emerged and there wasn’t much stuff left around the individual planets to keep them growing.
But if only metals and rocks were available in the inner solar system, then the planets that formed in this region would be mostly metal and rock, just like we see. In the outer solar system, the planets were forming out of building blocks that had metal, rocks, and ices.
This leads us to the second consequence. Because ices also condensed in the outer solar system, the outer planets had more material available to make the planets. This means that the planets grew faster and larger in the outer solar system.
In fact, Jupiter and Saturn grew so fast that they were massive enough to start gravitationally attracting the hydrogen and helium in the disk. This made them grow into the gas giants we see today. The inner planets never grew large enough to attract a lot of the hydrogen and helium gas.
What about Uranus and Neptune? They have some hydrogen and helium gas, but nowhere near as much as Jupiter and Saturn. That can be explained by the fact that they were growing farther out in the disk, where the density of available material was lower and the orbital speeds for scooping anything up were slower.
We know from looking at other protoplanetary disks that it takes about 10 million years for gas to be blown away or accumulated in the planets. This suggests that Jupiter and Saturn must have reached their giant size in about 10 million years.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.
Clue No. 3: Architecture of the Solar System
A third sort of clue comes from the overall architecture of the solar system. Let’s start with the fact that there are only a handful of planets—8 of them. And each planet is relatively isolated in space. That is, the planets are few and far between, with the large planets especially far apart.
This can be explained by how planets grow through gravity. Consider a time when the disk was filled with bodies the size of boulders to houses. These are called planetesimals. Gravitational encounters between these bodies sometimes resulted in collisions.
A head-on collision that is slow enough might result in the two objects merging. A faster collision involving weaker bodies might result in the objects breaking apart. Stronger bodies that hit at an angle might rebound like billiard balls.
The specific result would depend on the speed of collision, the angle of the collisions, and on the structural properties of the planetesimals.
Those planetesimals that experienced more merging collisions would grow. And the bigger they got, the larger their gravitational force, so they could attract even more objects, and grow even faster. This is known as oligarchic growth because a few large bodies experienced runaway growth, as they managed to eat up all the mass in that area of the disk.
Eventually, the growing planetary embryos accreted all the material in their region of the disk, basically sweeping clear a ring-like portion of the disk. This is why the planets are relatively isolated in space. They each had their own feeding zone, so to speak.
Whenever growing planetesimals were especially close together, then it’s likely that they would have gravitationally interacted with each other to either merge, or cause one body to get flung far away.
When the feeding zone was cleared, the protoplanets stopped significant growth, leaving us with planets. That doesn’t mean there was no more growth or collisions. Indeed, collisions still happen today with every meteor impact. So, in a way, the planets are still growing.
Through this entire process, the smaller objects not accreted typically get a boost in speed that results in more elliptical orbits. But objects flung away could also eventually return and cause later collisions. The collisions just weren’t happening as frequently later on.
Learn more about how our Sun defines our solar system.
Clue No. 4: Asteroids, Comets, Kuiper Belt Objects and More
A fourth clue comes from the many objects that did not become planets. The solar system has many small bodies, like asteroids, comets, Kuiper belt objects, and the Oort cloud. These are planetesimal remnants that didn’t end up accreted into the planets. That’s why studying these small bodies is so important. They show us unused building blocks of the planets that have remained relatively unchanged.
Why didn’t these planetesimals become planets? In the case of the asteroid belt, the asteroids were in a region of space where growth into a larger body was made impossible because of repeated gravitational disruption by the mighty Jupiter.
The asteroids closer to Earth mostly come from bodies that were ejected out of the asteroid belt over time. The asteroid belt may offer us the closest thing to examples of the rocky planetesimals that formed the terrestrial planets.
As for the outer solar system in the Kuiper belt, beyond the orbit of Neptune, only a few Pluto-sized bodies are known to have grown in this region. Perhaps the Kuiper belt region was too sparsely populated, and orbital motions were too slow, to grow many larger bodies through gravity. Or, perhaps we’ll find more.
There may be additional, large Kuiper belt objects that we haven’t discovered yet. Like the possible ‘Planet 9’, which may be Neptune-sized, orbiting at far distances and affecting the orbits of small objects in the Kuiper belt.
In any case, the Kuiper belt offers us samples of the icy planetesimals that formed the outer planets and moons.
Then there’s the Oort cloud that holds the planetesimals that were flung to the far reaches of the solar system through gravitational encounters with the growing planetary embryos. These objects can have highly eccentric orbits, which causes some to visit the inner solar system, becoming long-period comets that visit us every so often.
But we should also keep in mind that the asteroids, Kuiper belt objects, and the Oort cloud have evolved over time, too. These former planetesimals have collided with each other. They’ve experienced space weathering from interactions with the solar wind. They’ve had their orbits altered from gravitational interactions with Jupiter and other bodies. And they’ve also experienced different temperatures since they formed.
This means we have to interpret small bodies today as evolved planetesimals, just like the planets today. They are not the same as they were 4.5 billion years ago.
Learn more about comets, the Kuiper belt, and the Oort cloud.
Clue No. 5: Interior Structure of the Planets
A fifth clue comes from looking at the layered interior structure of the planets. The planets are mostly differentiated into layers, with the densest components found deeper in their interiors.
That layered structure is explained by the fact that the collisions that occurred while planets were forming released a lot of energy. This energy heated the interiors of the proto-planets, and sometimes melted them. Once molten, denser material would sink to the center of the planet, causing the layered structure we typically see today.
For example, all the terrestrial planets have lighter rocky mantles surrounding their dense iron cores. And the giant planets have hydrogen and helium gas envelopes surrounding interiors of heavier elements.
Learn more about the Earth-Moon system.
Clue No. 6: Moon Systems of Planets
A sixth observable or clue to make sense of is that some planets have moon systems. The largest of those moons tend to orbit their planets in a disk in the same direction as the planets spin. That is, they kind of look like mini solar systems.
This suggests that a similar process that created the planets around the Sun created those moons around planets. If a planet is massive enough and was spinning, then gravity would have acted to accrete material into a disk surrounding the planet. Each proto-lunar disk could create moons the same way the protoplanetary disk created planets.
Clue No. 7: Signs of Collisions
A seventh clue about what happened comes from the signs of collisions which we see everywhere we look. We see giant impact craters on planets and moons, and these enormous craters suggest there were collisions between large bodies. We’ve also used collisions to explain solar system quirks like the Earth’s large moon, planetary rings, Mercury’s large core, and the spin axes of Uranus and, perhaps, Venus.
Common Questions about How the Solar System Was Formed
Yes, all the planets are in the same orbital plane, and they all orbit in the same, prograde direction around that plane.
At the time the solar system was forming, the Sun at the center would have been sending heat out through the protoplanetary disk, and regions closer to the Sun would have been much hotter than regions farther away. This gradient in temperature resulted in different materials condensing out of the gas into solid phases at different distances from the Sun. At the earliest times, only materials with very high solidification temperatures, like metals and silicate rocks, could condense out of the gas in the inner solar system. Hence, the terrestrial planets closer to the Sun and in the inner solar system are rocky.
The asteroids in the asteroid belt were in a region of space where growth into a larger body was made impossible because of repeated gravitational disruption by the mighty Jupiter. This is why these asteroids didn’t end up accreted into the planets.
The layered structure of planets is explained by the fact that the collisions that occurred while planets were forming released a lot of energy. This energy heated the interiors of the proto-planets, and sometimes melted them. Once molten, denser material would sink to the center of the planet, causing the layered structure we typically see today.