Let's talk about how the solar system formed from the proto-planetary disk. It took millions of years to form the solar system, as it took the sun to attain its main sequence stage. The sun and its family evolved together. But the foundations for the modern solar system emerged when the sun was still a protostar.
Though they are products of the same molecular gas cloud, planet formation and the sun's genesis took two totally different paths.
Sun took more of a top-down approach, from being a giant gas cloud to being carefully chipped down to form a glorious golden yellow star. And planet building is more of a bottom-up approach since they were built brick by brick from the proto-planetary disk.
We know the planets formed one molecule at a time through agglomeration and accretion processes. But even after decades of research, astronomers cannot agree on the timescales involved in the various stages, or on the sequence in which the events took place.
The beginning of our story is when condensation of the proto-planetary disk started. The same annoying phenomena that cause water molecules to cloud your reading glasses closely resembles the planet making process. It all begins in the solar nebula about 2,200,000 years ago.
This animation shows how material around a young star is shaped into planets over billions of years. Credits: NASA's Goddard Space Flight Center Video.
From rocky planetesimals to proto-planets:
Let's spin the clock backwards. The sun hasn't formed yet; the solar nebula is a hot, dense soup of unique elements. While gases like hydrogen, helium, carbon and oxygen were everywhere, heavier stuff such as silica, methane, water and ammonia were also abundant. Metals were present in traces here and there, but not uniformly distributed.
Places surrounding the young sun are a seething 2000 degree Celsius. Only the densest materials like iron can condense to form particles at this temperature.
Further out in the planetary disk, silicate particles (fundamental material in rocks and sand) condense and form dust like shreds. Far beyond, near the present location of Jupiter, ice fragments accumulated. Astronomers call this region the 'snowline'.
Beyond the snowline, the temperature dropped dead to a mere 70-degree Celsius, allowing only gases like methane and ammonia to condense and form ice crystals.
At this point, the proto-planetary disk is a swirling storm of iron, sand, ice dust and snow spinning at thousands of kilometres per second. At this nerve-racking speed, some shards fasten on to its neighbours through electrostatic forces. This process kept ongoing. In a few thousand years, millions of pebble-sized rocks formed throughout the disk.
The pebble-sized rocks collided and merged to form mountain-sized planetesimals in the next thousand years. These planetesimals grew larger by attracting more matter from the disk with their own gravity. As more and more material gathered up, protoplanets the size of our modern moon occupied the disk.
Further reading: Planets a brief introduction
Formation of the Gas giants and the asteroid belt:
We know there are eight planets in the solar system. But have you ever wondered which one of them came first?
Well, planets grew at different rates in the solar family, but Jupiter came first. The reason is so simple you'll be in awe. The snowline had a tonne of icy planetesimals, and what's ice if not sticky?
Take a fist full of sand and a fist full of ice and grip them. You will get your answer!
Yes, ice is twenty times stickier than silicate particles. Therefore, planetary agglomeration happened more readily and efficiently, where there was more ice. The first planet of the solar system, Jupiter, was born. It began as an enormous ball of ice and gas, about fifteen times larger than the modern earth.
Proto-Jupiter was a giant child that had nothing to do but eat. It swept in all the material and kept growing until it formed a clean orbit around the sun.
Jupiter kept on growing for another million years. Its growth stopped at 300 earth masses, not because it ran out of material but because the sun had reached the T-Tauri phase. The violent T-Tauri sun blew out strong solar winds that swept all the material out of Jupiter's path, preventing further growth.
A giant disk of gas and dust similar to the solar nebula but smaller surrounded Jupiter. The planetary giant bullied out the leftover planetesimals out of orbit, which formed the present-day asteroids.
Saturn formation closely resembles Jupiter. But as everything happened too far away from the sun, it made the process way too slow. The material got swept away in its earlier stages, hence the planet remains smaller than Jupiter.
Formation of the Ice giants and comets:
Three million years have passed since Jupiter and Saturn had emerged, but the proto-planetary disk is still active.
The terrestrial planets were still in their planetesimal stages, two frozen earth-sized kernels formed beyond Saturn. Neptune and Uranus ceased what little gas was available and established themselves as whole new planets in the next 10 million years. The leftover planetesimals got tossed beyond Neptune, making the modern comets.
The Oort cloud that envelopes the solar system and the Kuiper belt are the two comet reservoirs where countless comets orbit the sun.
According to the nebular hypothesis, the outer two planets may be in the "wrong place". Uranus and Neptune are in a region where the low density of the solar nebula and longer orbital times make their formation extremely unlikely. They must have formed in orbits close to Jupiter and Saturn, where more material was available and migrated outward to their current positions over hundreds of millions of years.
Formation of the Terrestrial planets:
The Terrestrial planets Mercury, Venus, Earth, and Mars are the latecomers to the party. While the gas planets and the ice giants formed within ten million years, the terrestrials the formation process took longer.
Once the early rocky planetesimals had emerged, they gravitationally attracted fragments of the nearby wreck. About one million years later, several large rocky, metallic protoplanets occupied the inner Solar Nebula. And by 10 million years these protoplanets had grouped through gravitation so that only four dominant spheres remained. These, at last, were the primitive terrestrial planets Mercury, Venus, Earth and Mars.
But they were only half the size they are today.
Tens of millions of years later, even after the sun had started the main sequence, the terrestrials kept growing. It took perhaps 100 million years for the terrestrial planets
to mop up the debris, double their masses and rise to their present diameters.
Now we know how the planets formed, but a few questions remain unanswered. How did the planets get their satellites? Why the terrestrial planets don't have regular satellites? How did Saturn grow a ring system like no other planet? And much more we will answer in the upcoming weeks.
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References and sources:
1. Formation and evolution of the Solar System From Wikipedia, the free encyclopedia
2. The story of the solar system, book by Mark A. Garlick