Solar System – WorldAtlas

What is the solar system? The solar system is basically a star that revolves around one or more planets associated with it by its gravitational pull. Our solar system is home to eight planets, many dwarf planets, more than 200 moons, and countless asteroids, comets, meteorites and other forms of planetary debris. For most of history, our solar system was thought to be the only one. It was generally believed that the process of planet formation was so rare that it would have been nearly impossible for any star that planets formed around it. However, in the past 50 years, science has gradually discovered how planets form around stars, and it turns out that planet formation is a natural part of star formation. In our galaxy, every star is likely to be accompanied by one or more planets. When we stare at the stars on a clear night, we don’t just see single stars; We are looking for thousands of solar systems.

How is the solar system formed?

Supernova
Illustration of a supernova formation in outer space

For the solar system to form, a star must first form. Every star, regardless of its size or mass, forms in much the same way: a large cloud of hydrogen gas clumps together and collapses under the influence of its own gravity. Stars are generally born within star-forming regions called nebulae. A nebula is a huge cloud of hydrogen gas along with small amounts of other elements and compounds. Star formation is usually triggered by some external process such as a nearby supernova, another star forming, or a galactic merger event. External processes like these can heat nearby hydrogen gas, causing it to clump together. As more hydrogen clumps together, its attractiveness increases. Once the hydrogen cloud becomes massive enough and hot enough, nuclear fusion occurs. Within the core of a forming star, individual hydrogen nuclei will collide and fuse, forming helium nuclei. This process releases a huge amount of energy, and the proto-star that formed will soon explode as a real star. However, not all of the material in the region will become part of the star. Some will fall into orbit and become a protoplanetary disk. Planetary stardust will form in the protoplanetary disk.

How do you move from star dust to planets? This process was and remains somewhat ambiguous. When a star forms, a giant disk of stellar material forms around it, a protoplanetary disk. Inside this rotating disk of stardust is where planets and their moons form. For decades, astronomers assumed that our sun was unique in that it was thought to be the only planetary star. This was because every process astronomers could devise was unlikely to happen around other stars. However, in 1996 astronomers announced the confirmation of the discovery of the first planet orbiting a star other than our sun. Over two decades of intense research about exoplanets, this discovery has been followed up, revealing that every star we can see is likely accompanied by its own planets.

In star-forming regions, telescopes have found and confirmed the existence of planet-forming disks around young stars. Although these discoveries confirmed that planets form around stars in discs of stellar matter, subsequent observations have raised more questions than they have answered. Notably, it turns out that protoplanetary disks tend not to exist for very long (astronomically speaking). Instead, on average, a protoplanetary disk will exist for only 10 million years before dissipating into surrounding space or falling into the forming star. This means that the Solar System should form within a window of 10 million years, which is a very short period, especially considering that many geological processes on the planets themselves can take longer than that to unfold. The problem now was to solve the mystery of how stardust could create an entire solar system in that window.

The current explanation for how planets form is relatively straightforward. It starts with tiny particles of dust in orbit around a star. The gravitational pull between these tiny particles is not enough to clump them together to form larger objects. Therefore, for the first large rocks to form (called minor planets), electrostatic force must hold them together. Dust particles will have an electric charge, thus negatively charged particles are attracted to positively charged particles and vice versa. However, the electrostatic force will not be enough once things get too big. Fortunately, we have the oomph to take over once that happens. Once things get big enough for gravity to become relevant, planetary growth becomes exponential as more massive objects collide and stick together, increasing their gravitational pull and pulling in more material. Over the course of millions of years, several planets will appear.

planets collision

collision
Planetary collision between Earth and an exoplanet

A large number of planets can consist of a single protoplanetary disk. Up to a hundred planets have formed around the sun in our solar system. The solar system currently has nowhere near this many planets, so what happened to them all? With so many planets, it was virtually impossible for anyone to find a stable orbit. The attraction between them was extremely chaotic. Some of these planets have been completely expelled from the Solar System, doomed to drift through the void of interstellar space forever. However, most of the planets collide with others. Depending on the size of the planets and the angle at which the impact occurs, the two worlds either destroy each other or fuse together to form a larger planet. In our solar system, every planet has likely had one or more collisions in the past. Earth is no exception. About 4.5 billion years ago, Earth collided with a world the size of Mars. The resulting collision nearly destroyed the young Earth, but fortunately, Earth survived and absorbed the smaller world. The debris from the crash entered Earth’s orbit and eventually formed the Moon.

Solar system terms and definitions

condition Definition / Explanation

Solar System

A star around which one or more planets revolve due to the force of gravity

Protostar

A star in its early stages of formation, before nuclear fusion ignited

protoplanetary disk

A massive disk of stellar material forms around a young star, the birthplace of planets

nebula

A nebula is a star-forming region that contains an abundance of different elements and compounds

nuclear fusion

The process by which two atoms fuse together to form a heavier atom. It is the process that powers the stars