For more clues, we can look at a young star, one that is just forming its system of planets.

A famous one is called Beta Painter. It is surrounded by a thin disk of dust, gas and little bits called planetesimals that range in size from grains of sand and large rocks to objects the size of mountains. Astronomers are pretty sure the disk formed from material left over when the star was born 20 million years ago.

Every star is born from a cloud of gas and dust that moves through space surrounded by other similar clouds. The force of gravity causes these clouds to tug on one another as they pass, which makes them slowly rotate.

Even when one of these clouds collapses to form a starit continues to rotate. The star forms, spinning at the center of a flat pancake of rotating gas and dust called a protoplanetary disk. All of it — the star, the gas, the dust — spins in the same direction.

Astronomers think that our solar system looked a lot like Beta Pictoris in its early years.

We think that inside the disk, the gas and dust can stick together in a process called accretion. As a baby planet starts to grow, it gets heavier, and its gravity attracts more and more little pieces.

When the baby planet gets massive enough, the force of gravity begins crushing it, making it denser. Because of the force of gravity, the planet spins faster, like an ice skater drawing in her arms to spin.

Rising pressure in the core then causes the core to melt. Denser materials sink toward the core, and lighter materials float to the planet’s surface. We end up with a planet with an iron core surrounded by rock, and maybe water and ice on the outer edges.

That’s what we see in our solar system.

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