Artwork of the central regions of a protoplanetary disc. Planets are formed inside giant discs of gas and dust called protoplanetary discs, or proplyds for short. They are a natural consequence of the star formation process, with the star forming at the centre and the planets gradually accreting from leftover materials in the disc. In this image we take a look inside a proplyd close to the newly forming star. The rocky materials in the disc are gathering into progressively larger bodies, and will one day merge into fully fledged worlds. Two large bodies are showing colliding. A collision such as this is thought to have occurred in Mercury's past, removing much of the planet's crust and leaving it with an oversized core for its size.