c. 5 Billion BCE

Solar Nebula

Star formation is a messy process. As giant molecular clouds collapse, almost all of the cloud’s gas and dust eventually falls into the central protostar—almost. A tiny fraction of the gas and dust remains in orbit around the forming star, and as the whole system spins and cools, that residual cloud of debris slowly flattens into a disk of gas, dust, and (farther from the star) ice. During this phase of star formation, it appears that all young stars start out with an accompanying disk, often called a solar nebular disk.

The nebula from which our own Sun eventually formed probably started collapsing about 5 billion years ago, though the exact timing is uncertain. Observations indicate that sun-like stars typically take about 100 million years to form, and that nebular disks form in only about 1 million years around young stars. Once the disk is formed, it changes rapidly, with tiny dust and/or ice grains colliding, sticking to each other, and growing into marble-size particles, in a process (called accretion) that computer models indicate takes only a few thousand years. These small particles collide with others, sometimes sticking together, and the process appears to continue on in a poorly understood, runaway fashion until, within perhaps only a few more million years, planetesimals (kilometer-sized clumps of dusty, icy, rocky, and/or metallic grains) and then asteroids 100–1,000 kilometers in size have formed.

Solar nebular disks don’t seem to last long; most of the dust accretes or is dispersed within about 10 million years. Close to the star, it’s too warm for ice to condense, so the planetesimals are mostly rocky and too small to gravitationally hold on to much gas. Farther out, ice and dust can be accreted into larger planetesimals, with enough mass to accrete huge amounts of gas as well, eventually growing into “gas giant” planets. Exactly how such messy beginnings lead to such elegant planetary systems, and in such little time, is currently a topic of much debate and speculation among astronomers.

Space artist Don Dixon’s conception of the proto-Sun and its solar nebular disk, the spinning cloud of gas and dust and ice from which all our solar system’s planets, moons, asteroids, and comets formed.