Why does the..

like me to wonder: Is there something about how the solar system formed that kind of “baked in” that direction of spin?

Birth of a star

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 Pictoris. It is surrounded by a thin disk of dust, gas and little bits called planetesimals; they range in size from a grain of sand to rocks maybe up to the size of a mountain. Astronomers are pretty sure the disk formed from material left over when the star was born.

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 star, it 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 – is spinning 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 that, like an ice skater drawing in her arms to spin, the planet spins faster. Rising pressure in the core 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 on the outer parts stuff like water and ice.

That’s what we see in our solar system.

What if Earth didn’t spin?

Earth’s spin is important for life. It causes day and night. It’s also important for ocean tides. Without the daily ebb and flow of water, it’s possible life would never have emerged from the sea onto land.

So, astronomers believe Earth spins because the entire solar system was already rotating when Earth formed – but there are still a lot of questions about how planets’ spins change over time, and how spin affects the evolution of life. With more than 5,000 planets now known beyond the solar system, future scientists are going to be busy exploring. ■