What is the small body from which a planet originated in the early stages of the formation of the solar system?

Planetesimals: How Tiny Space Rocks Grew into Entire Planets

Ever wonder how planets like Earth actually came to be? It wasn’t some instant, “poof, there it is” kind of thing. Instead, our solar system’s planets—and all the others out there—started small, really small, as in, planetesimals. Think of them as the cosmic LEGO bricks of the solar system.

So, picture this: about 4.6 billion years ago, a massive cloud of gas and dust went kerplunk, collapsing under its own gravity . Most of that stuff ended up becoming our Sun, but the leftovers? They swirled around the newborn star, flattening into a protoplanetary disk—a cosmic pancake of potential . Inside this disk, tiny dust grains, like the ashes of long-dead stars, started bumping into each other. And here’s the cool part: they stuck together . Slowly but surely, these clumps grew, eventually forming planetesimals, ranging from the size of a pebble to small mountains .

These planetesimals were basically little balls of rock, ice, and metal, hanging out in the protoplanetary disk . And they’re the key ingredients for making planets . The prevailing theory, called the planetesimal hypothesis, says that planets are born from these humble beginnings . It all starts with those cosmic dust bunnies colliding and sticking, gradually snowballing into planetesimals . Once a planetesimal gets to be about a kilometer across, its own gravity kicks in, pulling in even more material and accelerating its growth into a protoplanet . Talk about a growth spurt!

Now, these protoplanets didn’t just sit there. They became cosmic vacuum cleaners, sweeping up smaller planetesimals in their paths . Over millions of years, they collided and merged, growing bigger and bigger, eventually becoming the planets we know and love (or, you know, tolerate) today .

The evidence for all this is scattered throughout our solar system. Take asteroids and comets, for example. These are basically leftover planetesimals, cosmic scraps from the early days . Then there are meteorites, those space rocks that sometimes land on Earth. Their composition, especially the presence of these little spherical things called chondrules, suggests they’re pieces of ancient planetesimals . And get this: most planets spin in the same direction they orbit the Sun, which makes sense if they formed from a swirling disk of planetesimals . Plus, all those craters you see on planets and moons? Those are the result of impacts from huge objects, some at least 100 kilometers across, proving that planetesimals were definitely a thing back in the day .

Of course, not every planetesimal made it into a planet. Some got ejected from the solar system entirely, becoming lonely wanderers in the galaxy. Others found a home in the asteroid belt, the Kuiper Belt, or even the distant Oort Cloud. These cosmic leftovers are like time capsules, giving us a peek into the conditions and ingredients of the early solar system .

Scientists are still digging into the details of planetesimal formation and planet building. They’re trying to figure out exactly how planetesimals formed in different parts of the protoplanetary disk, like near gas pressure bumps or at the “snow line,” where it’s cold enough for ice to form. And with new telescopes and instruments, we’re actually seeing the earliest stages of planet formation happening around other stars, watching those dust grains come together in real-time . It’s pretty mind-blowing stuff!

So, the next time you look up at the night sky, remember those tiny planetesimals. They may be small, but they’re the reason we’re all here. They’re the unsung heroes of planet formation, the humble building blocks that grew into the planets that make our solar system so fascinating.