Dr. Paul M. Sutter is an astrophysicist working at Stony Brook University (New York, USA). He hosts Ask a Spaceman and Space Radio and is the author of the book How to Die in Space. He provided insights into the process of Solar System formation on Space.com’s expert forum.
According to Dr. Paul M. Sutter, we are currently located about 4.5 billion years into the life of the Sun, with a series of planets and smaller objects orbiting it. The formation of the Solar System remains a challenging puzzle for modern astronomy. Initially, the universe had very little; it consisted of stars formed from the breaking down of nebulae, which were gaseous and dusty clouds. Astronomers refer to this as the “pre-solar nebula,” which no longer exists today.
However, a nebula on its own cannot collapse to form a Solar System without something to trigger its motion. Thus, there was a supernova explosion, with shockwaves tearing through the pre-solar nebula, initiating its contraction. The supernova released a significant amount of radioactive elements, leading to the transformation from nebula to the Solar System. Over millions of years, this nebula contracted and cooled, eventually reaching a point where a pre-sun was surrounded by a thin disk of gas and dust, spinning rapidly.
4.5 billion years ago, our Sun was not as bright as it is today; it was compact and very hot. Although extremely hot, the Sun had not reached the critical density and temperature required to sustain nuclear fusion in its core. During this embryonic stage, planets began forming through a slow process. Close to the young Sun, the intense heat and light prevented anything other than rock from holding together; the rocky fragments started melting and gaseous elements such as hydrogen and helium evaporated. The surviving rocky fragments gradually reconnected, bonding together to form larger objects.
Eventually, over time, these rocky fragments fused together to form planets. Earth experienced a collision with an object roughly the size of Mars, and the debris from that collision eventually became our Moon.
Astronomers speculate that the four giant planets in the solar system: Jupiter, Saturn, Uranus, and Neptune, originally formed much closer to each other than they are today, and interactions with the remaining debris around them caused their orbital migration.
In the first scenario, Jupiter and Saturn moved inward towards the Sun, causing Uranus and Neptune to be pushed outward. In the second scenario, the outer worlds “played” with another giant planet and eventually pushed it out entirely. In the final scenario, Jupiter wandered near the orbit of Mars before jumping out, disrupting the serene orbits of the remaining outer worlds.
Astronomers believe that the migration of the outer planets created a period of intense interactions with comets and asteroids within the Solar System around 4 billion years ago. The shifting of the giant worlds disrupted all remaining matter in the Solar System, either sending them to safe cold outskirts or causing trouble for the rocky planets.