Planetesimal

Planetesimals are small, solid objects that scientists think exist in clouds of dust and gas around young stars. These objects are important because they help us understand how planets, including our own Earth, form.

In the early days of our Solar System, about 4.6 billion years ago, planetesimals were some of the first building blocks that came together to create larger planets. By studying them, scientists can learn more about the processes that led to the formation of our Solar System and others like it.

Planetesimals are found in areas called protoplanetary disks and debris disks, which surround stars and contain the materials needed to form new planets. They give us clues about the early stages of planet formation and the conditions needed for planets to develop.

Formation

Planetesimals are tiny pieces of dust and rock that helped make planets. Scientists think these tiny pieces bumped into each other and stuck together, growing bigger over time. When they got big enough, their own pull of gravity helped them grow even faster 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 either thrown far away, moved into distant paths, or crashed into larger objects 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 hold clues about how our Solar System was formed. Even though the 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 and doesn’t get pushed around much by the gas around it. 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 and what they’re made of.