How are planets born?

Credit: ESO/L. Calçada

To build a planetary system, all you need is a cloud of gas and dust surrounding a young star, and lots of patience. Gravity does the rest.

Inside information: A protostar is surrounded by a thin, hot inner disc of gas and a thick, cooler outer disc that also contains lots of dust particles.
Credit: ESO/L. Calçada
Edge-on disc in the Orion Nebula
Side view: A dusty protoplanetary disc surrounding a newborn star in the Orion Nebula is seen edge-on in this Hubble Space Telescope image.
Credit: J Bally (University of Colorado) and H. Throop (SWRI)

Stars are born from collapsing clouds of gas and dust. Most of this material ends up in the central star, but a few percent or so form a flat, rotating disc around the star. Over a few million years, small dust particles within this disc grow into centimetre-sized pebbles — the first building blocks of planets. 4.6 billion years ago, our own planetary system grew from such a disc. Because of their planet-forming potential, these discs are known as protoplanetary discs, or proplyds for short.

ALMA image of the protoplanetary disc around HL Tauri
Bull’s eye: Rings and gaps in the observation of the disc surrounding the young star HL Tauri hint at the existence of planets in the making.
Credit: ALMA (ESO/NAOJ/NRAO)
Binary potential: Two solar systems in the making? Each star in the young binary HK Tauri is surrounded by its own protoplanetary disc (artist’s impression).
Credit: R. Hurt (NASA/JPL-Caltech/IPAC)

Starting with tiny snowflakes, you can make larger snowballs and giant snowmen. Likewise, small particles of gas and dust can grow into planets.