Planetesimal

Planetesimals are small, solid pieces that scientists think are found in clouds of dust and gas around young stars. They are important because they help us learn how planets, including Earth, are formed.

In the early days of our Solar System, about 4.6 billion years ago, planetesimals were some of the first pieces that came together to make bigger planets. By studying them, scientists can learn more about how our Solar System and others were formed.

Planetesimals are found in areas called protoplanetary disks and debris disks around stars. These areas have the materials needed to make new planets. Planetesimals give us clues about the early steps of planet formation and what is needed for planets to grow.

Formation

Planetesimals are small pieces of dust and rock. They helped make planets by bumping into each other and sticking together. Over time, they grew bigger. When they got large enough, their own gravity pulled them together even faster, turning them into objects as big as moons. Some planetesimals broke apart in crashes, but a few grew into the planets we see today.

Planetesimals in the Solar System

About 3.8 billion years ago, after a time called the Late Heavy Bombardment, most planetesimals in our Solar System were moved to faraway places, changed their paths, or crashed into bigger objects. This happened because of the pull of giant planets like Jupiter and Neptune. A few planetesimals became moons, like Phoebe, which orbits Saturn.

Today, the planetesimals that remain are very important for science. They give us clues about how our Solar System was formed. Even though sunlight changes their outer layers, the inside of these objects stays almost the same as when it was first made. This makes each one like a "time capsule," showing us conditions from the early days of our Solar System. One of the most unchanged planetesimals visited by a spacecraft is Arrokoth, which is shaped like two parts stuck together.

Definition of planetesimal

The word planetesimal comes from the word infinitesimal and means a very small piece that can become part of a planet.

Scientists use this name for tiny objects that help make planets. Some also call leftover small objects from when planets were made, like asteroids and comets, planetesimals. Experts agreed in 2006 that a planetesimal is a solid piece in space that is big enough to stay together because of its own gravity. These pieces are usually bigger than about 1 kilometer across.

When these small pieces get big enough — around 100 to 1000 kilometers — they start pulling in more pieces faster and are called embryos or protoplanets. Today, some of these small bodies might be called things like Kuiper belt objects or trojan asteroids, depending on where they are.