Planetesimal
One of the countless small bodies, from about a kilometre to a few hundred kilometres across, that collided and merged to build the planets.
In the early Solar System, dust grains in the disc around the young Sun clumped into pebbles, and pebbles gathered into bodies large enough for gravity to hold them together. These planetesimals were the building blocks of planets. Many collided and merged into larger embryos and, in the end, the planets themselves. Others survived and are still with us as asteroids and Kuiper Belt objects.
Meteorites are our physical samples of planetesimals. Chondrites come from bodies that were never fully melted, while iron meteorites and achondrites come from planetesimals that melted and separated into core, mantle and crust. Dating shows that some of these melted bodies formed within a few million years of the oldest solids in the Solar System, so planetesimal formation was remarkably quick.
A striking look at an untouched planetesimal came on 1 January 2019, when the New Horizons spacecraft flew past Arrokoth in the Kuiper Belt. About 36 kilometres long, it is made of two flattened lobes joined gently together, just as models of slow collapse in a cold disc predict. Planetesimals are recognised from spacecraft images, orbits and the meteorites they leave behind.
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