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Comic-style illustration showing a sun in the upper left corner, planets, black space, and rocky asteroids in the bottom left corner.

1.2. How did our Solar System form?

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1.1. Are we really made of star stuff?

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1.3

Grades K-2 or Adult Naive Learner

Do you know what a planet is? A planet is a big, round world, floating in space. It can be made mostly of rock or even mostly of gas, just like the air all around us.

You, me, and everyone we know lives on a planet called Earth. Our planet is in space and goes around the Sun. Now, did you know that the Sun is a star? Well, there are also seven other planets going around our star, the Sun. The Sun and the planets are part of what we call the Solar System.

The Solar System is really old. The Sun and all of the planets came from a big cloud of stuff in space. Do you know that raindrops come from clouds in the sky? Well, it turns out that stars and even planets can come from clouds in space. Our Sun came from the middle of a big cloud in space, and the planets of our solar system also formed from that same cloud, moving around the Sun in the same kind of pattern that they follow today.

Portrait photo of Comic-style illustration showing a sun in the upper left corner, planets, black space, and rocky asteroids in the bottom left corner.
The solar system formed 4.6 billion years ago from a collapsing molecular cloud, which then became a protoplanetary disk. Its center ignited into the Sun while surrounding material accreted into planets, moons, and smaller bodies.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

For us to learn about where we came from, we need to understand how our solar system formed.

The Sun and the planets and all of the asteroids and comets and other stuff in our solar system all formed from a really big cloud of gas and dust in space. There are clouds of gas and dust all around our galaxy. Sometimes these clouds can slowly turn into stars and planets when enough material is available and clumps together forming massive collections of ice and rock.

Do you know what kind of pattern the planets make when they go around the Sun? It kind of looks like a big circle, right? Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same way, with the Sun forming in the middle. That’s why we see the planets moving around the Sun the way that they do today! We call that pattern of how a planet moves around the Sun an “orbit.” Have you heard of anything else that has an “orbit”? Our Moon orbits around our Earth, just like our Earth orbits around our Sun, and our entire solar system is also orbiting around the galaxy. Orbits are really important for us to learn about if we want to know where we came from.

Portrait photo of Comic-style illustration showing a sun in the upper left corner, planets, black space, and rocky asteroids in the bottom left corner.
The solar system formed 4.6 billion years ago from a collapsing molecular cloud, which then became a protoplanetary disk. Its center ignited into the Sun while surrounding material accreted into planets, moons, and smaller bodies.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

Earth is the only world that we know of that has life. All of the plants and animals and microbes and other living things on Earth have evolved here. So, for us to understand where life as we know it came from, we need to understand where our planet came from.

The Sun and the planets and all of the other stuff in our solar system all formed from a really big cloud of gas and dust in space. We call such a cloud a “nebula” and more than one of them we refer to as “nebulae.” There are nebulae all around our galaxy, and it’s from these nebulae that stars and planets form. Nebulae are massive clouds of dust and debris in space and have all the ingredients to form stars and planets. When enough material is available, it begins to stick together forming a large mass. In time, the mass can grow large enough to form a planet or even a new star.

We currently think that our solar system formed from a large nebula, perhaps after the explosion of a nearby star. Some big stars can explode, something called a supernova, and that explosion has enough energy to make the gas and dust in nearby nebulae start swirling and spinning about. As this happened, it caused a lot of the material in the nebula to fall into its center, and that’s where the Sun started forming. Meanwhile, the rest of the gas and dust in the nebula began colliding and sticking together, making little pieces of metal and rock. Those small pieces then collided with each other, forming larger pieces, which then collided with each other to form even larger ones. These were young planets, and eventually, over a long time and through many, many collisions, our eight planets were formed – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

We call the pattern that the planets make when they go around the Sun an “orbit.” Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same direction as the orbits of the planets today, with the Sun forming in the middle and also spinning in the same direction. That’s why we see the planets moving around the Sun the way that they do today!

You might also know that the Moon orbits around Earth. For something to be a moon, it needs to be in orbit around a planet. One thing that makes a planet is that a planet has to be orbiting a star. But star systems also have orbits. They orbit around their entire galaxy. So, orbits are really important for us to learn about if we want to know where we came from.

Portrait photo of Comic-style illustration showing a sun in the upper left corner, planets, black space, and rocky asteroids in the bottom left corner.
The solar system formed 4.6 billion years ago from a collapsing molecular cloud, which then became a protoplanetary disk. Its center ignited into the Sun while surrounding material accreted into planets, moons, and smaller bodies.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

As the physical context for life as we know it, it is important to learn about Earth’s origins so we can understand life’s origins. Although life may exist in situations other than that of a planet orbiting a star, it makes sense to explore the phenomenon of planetary system formation as a context for the emergence and evolution of life.

The story of the formation of our solar system begins in a region of space of called a “giant molecular cloud”. You might have heard before that a cloud of gas and dust in space is also called a “nebula,” so the scientific theory for how stars and planets form from molecular clouds is also sometimes called the Nebular Theory. Nebular Theory tells us that a process known as “gravitational contraction” occurred, causing parts of the cloud to clump together, which would allow for the Sun and planets to form from it.

Before gravitational contraction, the majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang, with small amounts of heavier elements such as carbon and oxygen which were made via nucleosynthesis in prior generations of stars (see 1.1 above). The material in this giant cloud was not uniformly distributed – there were regions of higher density (more dust and gas within a specific volume of space) and regions of lower density (less gas and dust within that same volume).

Evidence from meteorites suggests that the energy produced by a nearby exploding star (a supernova) passed through a higher density region in the cloud and caused it to begin to swirl and twist about. This area of the cloud is sometimes called the pre-solar nebula (“pre” = before; “solar” = star or Sun). As molecules in the pre-solar nebula were swirling about, some of them started bumping into each other and sometimes would even stick together. As more and more of these clumps formed, gravity caused them to start sticking together and to fall into the center of the pre-solar nebula, which only caused gravity to pull even more of the material into the center of the cloud, and this is the process that’s referred to as gravitational contraction.

While all of this was happening, the action of molecules bumping into each other over and over slowly caused the pre-solar nebula to flatten into a spinning disk of dust and gas. This is sometimes called a circumstellar disk (“circum” = around; “stellar” = star) or protoplanetary disk (“proto” = first or before). Almost all of the material in the disk collected in the center, giving rise to the young Sun. However, some of the particles in the spinning disk began colliding with each other and sticking together, forming larger and larger fragments. The larger a fragment became, the more mass it had and therefore the more gravitational pull it exerted. Which in turn drew more and more material to it, and the larger it became, and so on. This process is called “accretion,” and resulted in the production of many planetesimals (small objects that build up into planets), and eventually, the planets themselves.

While the young Sun was starting to heat up in the middle of the protoplanetary disk, it warmed up the disk so much that nothing could stay solid really close to the Sun (it all melted). A little further out from the Sun, stuff like metal and rock was able to cool enough to make solid materials for forming the planets. But it was still so hot there that molecules that are often liquids or gases here on Earth (like water, ammonia, carbon dioxide and methane) couldn’t easily stick to the solid planet-forming materials. Those molecules could only really be added to planets that were a lot further from the Sun, where it was cold enough for them to clump together with the other solid stuff. This is why we have gas giant planets like Jupiter and Saturn which are very different from the rocky planets like Earth and Venus.

Portrait photo of Comic-style illustration showing a sun in the upper left corner, planets, black space, and rocky asteroids in the bottom left corner.
The solar system formed 4.6 billion years ago from a collapsing molecular cloud, which then became a protoplanetary disk. Its center ignited into the Sun while surrounding material accreted into planets, moons, and smaller bodies.
NASA/Aaron Gronstal

Storyline Extensions

The planets are named after stories from long ago:
Our planets are named Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Seven of the planets are named after gods from Roman mythology. These are Mercury, Venus, Mars, Jupiter, Saturn, and Neptune. However, Uranus is a name from Greek mythology (Uranus was the god of the sky). Also, the name for our planet, Earth, comes from Old English, and appears to have come from people who lived in Northern Europe long ago.

Our location in the galaxy:
Our Milky Way galaxy is really big! If we could travel outside of the galaxy and look back at it, it would look like a big disk of dust and gas and stars, with a big bulging sphere of stars near the middle. The disk of the galaxy is about 100,000 lightyears in diameter. That means that it takes light about 100,000 years to travel from one side to the other. Our little solar system (little in comparison to the galaxy, that is) lies about 30,000 lightyears from the center of the galaxy. Just as moons orbit around planets, and planets orbit around stars, star systems also orbit around the center of the galaxy. Our own solar system is traveling through the galaxy at over 500,000 miles per hour! And our very long orbit around the galaxy takes almost 250 million years! But we’re not alone out here. There are lots of other stars and other worlds in the galaxy. Our best estimates right now are that there are about 100-400 billion stars in the Milky Way. And, even though we’ve only just begun finding exoplanets, some astronomers believe there is evidence for more planets than stars in the milky way and other galaxies. That’s an awful lot of worlds!