The Solar System: Planets and Formation Explained

Milky Way over New Zealand's Southern Alps. Seen from Bruce Bay, West Coast. — The night sky over New Zealand's Southern Alps gives a spectacular view of the Milky Way, the galaxy in which our own solar system resides. Mike Mackinven / Getty Images

Our planet Earth is part of a solar system that consists of eight planets orbiting a giant, fiery star we call the sun. For thousands of years, astronomers studying the solar system have noticed that these planets march across the sky in a predictable way. They've also noticed that some move faster than others — and some seem to be moving backward.

But we're getting ahead of ourselves. Let's go back to how this solar system started.

How Our Solar System Formed

Around 4.6 billion years ago, the early solar system began to take shape from a massive cloud of gas and dust known as the solar nebula. Triggered by an external force — possibly a nearby supernova — the nebula collapsed under the force of gravity and started spinning, due to the conservation of angular momentum.

In the center of the spinning cloud, a protostar formed, growing hotter and denser over time. As the surrounding material began to stick together through accretion, small dust grains collided and clumped into larger bodies called planetesimals. These planetesimals further merged and collided, forming protoplanets that grew in size and mass.

All this time, the Sun also grew bigger and brighter because it was collecting more and more matter. It became the dominant force in the solar system, accounting for the vast majority of the solar system's mass — more than all the planets, asteroids and comets put together.

As the protoplanets continued to gather material, their interiors heated up and underwent differentiation, with denser materials sinking to their cores and lighter materials rising to their surfaces. This process led to the formation of the rocky terrestrial planets (more on those a little later).

The Sun's intense radiation and solar wind cleared away the remaining gas and dust, but only up to a certain distance. Further out, where it was cooler, gas and ice could remain in a gaseous state, resulting in the formation of gas giants like Jupiter and Saturn. Even further, ice giants Uranus and Neptune acquired their atmospheres and icy mantles.

Contextualizing the Sun

The sun (which, incidentally, is only a medium-size star) is larger than any of the planets in our solar system. Its diameter is 1,392,000 kilometers (864,949 miles). Earth's diameter is only 12,756 kilometers (7,926 miles) — meaning more than one million Earths could fit inside the sun.

The large mass of the sun produces an enormous gravitational pull that keeps all the planets of the solar system in their orbits. Even dwarf planet Pluto (formerly the ninth planet outright), which is six billion kilometers (3,728,227,153 miles) away, is kept in orbit by the sun.

Planets in Our Solar System

Each planet in our solar system is unique, but they all have a few things in common, too. For example, every planet has a north and a south pole. These points are in the center of the planet at its ends.

A planet's axis is an imaginary line that runs through the center of the planet and connects the north and south poles. The imaginary line that runs around the planet at its middle (like your waist) is called its equator. While every planet rotates on its axis, some planets rotate quickly and some rotate slowly. The time that it takes for a planet to rotate once on its axis is its rotation period.

As each planet in our solar system rotates on its axis, it also revolves around the sun. The time that it takes for a planet to make a complete revolution around the sun is the planet's year. The path that the planet follows around the sun is called its orbit.

The main asteroid belt between Mars and Jupiter also divides our solar system into the inner and outer solar system. Here's a bit about each of the eight planets, in order of their distance from the sun.

Terrestrial Planets

The inner solar system consists of four rocky planets: Mercury, Venus, Earth and Mars, located closest to the Sun. These inner planets have solid surfaces, sloped terrains and potential for secondary atmospheres.

Mercury, the smallest planet, orbits closest to the Sun.
Venus has a thick, toxic atmosphere, making it the hottest planet.
Earth is the only known habitable planet with a breathable atmosphere, liquid water and a protective magnetic field.
Mars has a thin atmosphere and a desolate landscape.

Giant Planets

Jupiter, Saturn, Uranus and Neptune are the four giant planets known as Jovian planets, all consisting mainly of hydrogen and helium. These outer planets have rings, thick atmospheres and numerous moons. The gas giants (Jupiter and Saturn) have no solid surfaces and are larger than terrestrial planets like Earth. Uranus and Neptune, on the other hand, are classified as ice giants.

Jupiter, named for the Roman god, is the largest planet in our solar system.
Saturn is best known for its prominent ring system, but it also boasts more moons than all the other planets have combined.
Uranus is the only planet in our solar system that's so tilted, its equator is nearly at a right angle to its orbit.
Neptune is the only one of the eight major planets that you can't see in the night sky without a telescope.

Thanks to Hubble Space Telescope data, we know gas giants are not exclusive to our solar system; certain exoplanets outside our system also exhibit similar characteristics.

Dwarf Planets (Including Pluto)

Although we tend to think only about the sun and the planets when we consider our solar system, there are many other types of bodies that huddle around the sun along with Earth and its planetary brothers and sisters. These other celestial bodies include moons (and some of those moons have moons), comets, meteors, asteroids, plain old space dust and the much-debated dwarf planets.

Back in 2005, scientists discovered a far-flung body of rock and ice that they later called Eris. The fact that it was larger than Pluto and farther from the sun prompted existential questions around what, exactly, constitutes a planet. Was Eris the tenth planet in our solar system? If not, why could Pluto be a planet but Eris couldn't?

In 2006, the International Astronomical Union (IAU) determined that an object must meet the following criteria in order to qualify as a planet:

It has to orbit the sun directly — that means Earth's moon doesn't count, because it orbits our planet, not the sun.
It has to be large enough to be spherical in shape, due to its own gravity.
It must have "cleared its neighborhood," meaning that it's the dominant object in its orbit.

That last one is what got Pluto downgraded from 9th planet to the same category as Eris, Makemake, Ceres, Haumea and Orcus: dwarf planet.

Beyond Our Solar System

Past Neptune's orbit lies the Kuiper Belt, which contains icy bodies like Pluto and other Kuiper Belt objects. The solar system extends far beyond the planets, with objects like the Oort Cloud, a vast collection of icy bodies, marking its outer boundary.

Beyond this, we reach the heliopause, marking the boundary between our solar system and interstellar space, which is the region between stars where only a few gas molecules and dust particles can be found per cubic centimeter.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Oh, Those Retrograde Planets

If you spend time observing the solar system, you will notice that some planets — especially Venus and Mercury — seem to move backward in the sky. These planets don't really move backward, but they appear to move backward because their position in relation to Earth is changing. It's the same thing that happens when your car passes another car on the highway. The car you are passing seems to be going backward, but it's only because your car has gone past it. This odd backward motion is called "retrograde motion." A planet can also have "retrograde rotation," which means it rotates in the opposite direction of its orbit. Most planets in our solar system have "prograde rotation," which means they rotate in the same direction as their orbits.