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Asteroids: What They Are and Where They Come From

Over a million asteroids are zipping around the Sun. Here’s what you should know about our distant neighbors.

Avery Hurt
ByAvery Hurt
Credit: Sergey Nivens/Shutterstock

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When we think of the solar system, we tend to think of the Sun and the nine planets that orbit it. But there’s a lot more orbiting the Sun than just planets (and dwarf planets — we see you, Pluto!)

Take asteroids, for example. Asteroids are the debris left over from the formation of the solar system. Four and a half billion years ago, our solar system was nothing more than a rotating cloud of gas and dust. When that cloud collapsed, possibly due to the shockwaves from a nearby exploding star, its enormous gravity pulled in most of the surrounding material in an event so intense that hydrogen atoms fused into helium atoms.

Ninety-nine percent of the cloud’s material became part of a huge nuclear reactor that we now call the Sun. The remaining one percent began to coalesce into planets and settle into regular orbits. But not everything ...

  • Avery Hurt

    Avery Hurt

    Avery Hurt is a freelance science journalist who frequently writes for Discover Magazine, covering scientific studies on topics like neuroscience, insects, and microbes.

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