Solar System History 101 | The planetary society
The shape of the planets
While the young Sun was still collecting material to begin fusing hydrogen, tiny dust particles in the disk around it randomly collided and stuck together, becoming objects within a few years. several hundred meters in diameter. This process continued for many thousands of years, forming mile-sized objects large enough to gravitationally attract each other. This led to more collisions and accretions, forming Moon-sized protoplanets in less than a million years.
In the inner, hotter part of the solar disk, the planets grew mainly from rocks and metals because it was too hot for water and other volatile substances – substances that evaporate at room temperature – condense. Until hundreds of these worlds collided and combined in the inner solar system for about 100 million years until only four major bodies remained: Mercury, Venus, Earth and Mars . The inner planets have not become as big as the outer planets because the percentage of rocks and metals available in the Universe – and therefore the raw materials of our solar system – is less than that of hydrogen, helium and volatiles like water ice.
Just past this point, we believe a Mars-sized planet collided with Earth. The resulting debris coalesced to form the Moon. Mercury may have experienced a high-velocity collision with another planet that stripped Mercury’s outer layer, which would explain why the planet’s core makes up so much of its volume. The resulting debris may have spread out into space instead of forming a moon.
In the outer, colder part of the disc, gases and water ice dominated. The weaker gravitational influence of the Sun in this region, combined with the presence of much more matter, allowed the protoplanets to grow faster and become large enough to attract light elements such as hydrogen and helium. Jupiter was formed less than 3 million years after the birth of the solar system, making it the oldest planet.
Saturn formed soon after, picking up less material since Jupiter engulfed so much of the outer disk. With little hydrogen and helium, the next planets to form – Uranus and Neptune – accumulated more ices like water and ammonia. That’s why we call them ice giants. Some simulations show that additional ice giants may have formed and were then pushed out of our solar system.
Jupiter did not allow planets to form in the asteroid belt because its gravity pulled in dozens of moon- and Mars-sized baby planets, causing them to collide and shatter with other bodies or leave the area. This process took a few tens of millions of years after Jupiter was formed, leaving the asteroid belt with only small bodies of rock, ice, and metal that collectively weigh less than 1% of Earth’s mass. Ceres, the largest object in the asteroid belt, is considered an outlier because it has a lot of organic material and water ice, which means it probably formed further away and then migrated into the belt.
Small worlds stick together
As the inner terrestrial planets formed, the baby planets beyond Neptune collided and stuck together to form planet-like worlds like Pluto and lumpy, icy bodies like Arrokoth. These objects formed what we now know as the Kuiper Belt, although the belt is much denser than it is today. Just as Earth’s Moon formed after Earth collided with another world, similar collisions in the Kuiper Belt have created moons, some of which are relatively large. This may have been the case with Pluto and Charon.
The huge mass of Jupiter attracted a dense disc of matter which eventually coalesced into 4 planet-like moons: Io, Europa, Ganymede and Callisto. Titan, Saturn’s moon, formed in the same way. Some moons of outer planets like Triton to Neptune may have been independent worlds captured by the gravity fields of the giant planets.
It was, to our knowledge, the end of the beginning. Planets and other small worlds did not push further as the strong solar wind from the young Sun blew most of the remnants of dust and gas into interstellar space.