Rocky Planets May Have Started Forming Just 100 Million Years After the Big Bang

New computer simulations suggest that planetesimals, the solid precursors of planets such as Earth, could have begun forming only about 100 million years after the Big Bang, according to researchers led by Daniel Whalen of the University of Portsmouth. The universe is roughly 13.8 billion years old, so the finding places the possible start of rocky planet formation within less than 1 percent of cosmic history, far earlier than scientists traditionally assumed.

The Big Bang produced a universe made almost entirely of hydrogen and helium. Carbon, oxygen, iron, silicon and the other ingredients needed to build rocky planets had to be forged later, inside stars. The researchers modeled the effect of Population III stars, the earliest and often extremely massive generation of stars, some of which are thought to end their lives as pair-instability supernovae, explosions that completely disrupt the star and can eject more than 100 times the Sun's mass in freshly made heavy elements. That debris can raise the metallicity of nearby gas — the abundance of elements heavier than hydrogen and helium — high enough for solid grains, and eventually planetary material, to form.

In the simulations, run in part by Whalen's PhD student Chris Jessop, enriched gas collapsed under gravity to form a star about 70 percent as massive as the Sun, surrounded by a rotating protoplanetary disc, the same basic structure that produced the planets of our own solar system. Within that disc, the researchers found several Earth-masses' worth of planetesimals accumulating at roughly 0.5 to 1 astronomical unit (AU) from the star, about the same distance that separates Earth from the Sun.

The disc also held substantial water, only a few times less than the water available when the solar system itself formed, the researchers report. The planetesimals in the simulation formed inside the disc's water snowline, the zone too warm for abundant water ice to persist, so they would initially have been dry. The researchers note that planets forming there could still gain water later from leftover material during planet-building, the process thought to have supplied much of Earth's own water.