Protoplanetary disc
A flattened, spinning disc of gas and dust around a young star, where planets form.
When a cloud of gas and dust collapses to form a star, it spins faster and flattens into a disc, just as spinning pizza dough spreads out. This protoplanetary disc feeds material onto the growing star while dust grains inside it stick together, eventually forming planetesimals and planets. Discs usually last only a few million years before the star's light and wind clear away the remaining gas.
Telescopes now let astronomers see such discs around other stars. A famous 2014 image of the young star HL Tauri showed bright rings separated by dark gaps, possibly cleared by forming planets. Our own Solar System grew from a disc like this about 4.6 billion years ago; the early disc around the Sun is often called the solar nebula.
Meteorites preserve material from our disc that you can hold. The carbonaceous chondrite Allende, which fell in Mexico in 1969, contains white calcium-aluminium-rich inclusions (CAIs), among the oldest solids known, about 4.567 billion years old, as well as chondrules: round droplets melted by brief heating events in the disc. In a cut chondrite, CAIs look like pale, irregular patches and chondrules like small round grains.
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