What is the process behind planet formation?

It's awe-inspiring to consider that Earth and its neighboring planets were once mere specks of cosmic dust. Approximately 4.5 billion years ago, our sun was a young protostar, gathering mass through gravitational forces and igniting nuclear fusion within its core. Back then, the solar system didn't exist; instead, there was a vast, spinning cloud of particles known as the solar nebula.

To understand how remnants of gas and dust evolved into planets, astronomers have primarily examined the structure of our solar system. They've also observed distant, younger solar systems in various stages of formation.

As the sun formed, the leftover gas and dust flattened into a spinning protoplanetary disk. Within this disk, rocky particles collided, merging into larger bodies that grew by attracting more particles through gravity. These bodies condensed into planetesimals, which eventually collided to form the solid inner planets. Simultaneously, frozen gases accumulated into massive spheres, creating the outer gas giants.

What explains the formation of rocky planets near the sun and gas giants farther out? One hypothesis points to the solar wind, the constant stream of plasma emitted by stars. In the sun's early stages, this wind was significantly more powerful than it is now—strong enough to push lighter elements like hydrogen and helium away from the inner regions. As these elements reached the outer orbits, the solar wind weakened, allowing the gravity of the gas giants to capture them, resulting in their massive, gas-covered forms with solid cores of rock and ice.

This theory of planetary formation assumes gas giants always reside in a solar system's outer regions. However, in 1995, astronomers identified 51 Pegasi b, a "hot Jupiter" gas giant orbiting close to its star. This finding challenged existing theories, suggesting such planets form farther out and later migrate inward.

Scientists propose that such orbital migration, driven by gravitational interactions with other celestial bodies, could span hundreds of millions of years. This process would also obliterate any smaller planets in its trajectory.

As we uncover more about the architecture of other solar systems, we gain deeper insights into how our own planets came to be.