What is a planetesimal?
A planetesimal is one of the small solid bodies, from about a kilometre to hundreds of kilometres across, that formed in the young Sun's disc of gas and dust and were the building blocks of planets.
Around 4.6 billion years ago, the newborn Sun was surrounded by a flattened disc of gas and dust called the protoplanetary disc. Tiny dust grains collided and stuck, forming fluffy clumps. Turning those clumps into solid bodies large enough to hold together by gravity is one of the trickiest steps to explain, but somehow, possibly through turbulence concentrating particles into dense clouds that collapsed, the disc produced bodies around a kilometre to a few hundred kilometres across. These are planetesimals, a word meaning "infinitesimal planets".
Planetesimals collided and merged, growing into larger protoplanets and eventually planets. Many did not end up in planets, and some survive today as asteroids, comets and objects in the Kuiper belt beyond Neptune. NASA's New Horizons spacecraft flew past one, the double-lobed Arrokoth, in 2019, revealing a pristine planetesimal that formed by a gentle merger.
Meteorites give us direct samples of planetesimals. The oldest dated solids, calcium-aluminium-rich inclusions found in chondrites such as Allende, are about 4,567 million years old and mark the starting point for Solar System history. Some planetesimals formed early enough to contain plenty of short-lived radioactive aluminium-26, whose decay heat melted them. They separated into metal cores and rocky mantles and crusts, a process called differentiation. Iron meteorites are pieces of such cores, while howardites, eucrites and diogenites come from the crust of Vesta, a surviving protoplanet.
Chondrites tell the other side of the story. Their parent bodies formed a little later or stayed smaller, so they never melted fully. They preserve chondrules, once-molten droplets, and the fine dust of the disc. By dating different meteorites, scientists can show that planetesimal formation began within the first few million years of the Solar System's history.