Birth of a giant

Jupiter owes its present size and mass to the location where it formed roughly 4.6 billion years ago. It was the first planet to form in the protoplanetary nebula surrounding the newborn Sun – perhaps just a million years after the Sun itself and tens of millions of years before the rocky inner planets. Evidence suggests that the giant outer planets formed through a process known as pebble accretion, in which huge piles of small pebbles orbiting through the gas of the nebula became unstable and abruptly collapsed under their own gravity to form large protoplanets.

Jupiter formed beyond the frost line of the early Solar System – a region where volatile, easily melted icy chemicals were able to remain frozen. As a result, this region was the richest part of the nebula. Closer in, the Sun’s heat caused ices to evaporate, and its solar wind drove them outwards; further out, material naturally became more thinly spread. Nevertheless, as the proto-Jupiter’s gravity rapidly pulled in surrounding gas and ice over the next few million years, the growing planet was in a race against the solar wind blowing gases out of its grasp. Quite what form the early core took remains uncertain – not least because of recent discoveries about Jupiter’s deep interior.

“Jupiter was the first planet to form in the protoplanetary nebula”

1 The start of a system

Dust grains in orbit around our young star began to coalesce into planetesimals.

2 Growing into planets

The sizes of these planetesimals began to increase, moving in orbits to form planetary embryos.

3 The making of a gas giant

Before the gas in the disc disappeared with the solar wind, the planetesimals dragged gas envelopes onto themselves, giving them puffy layers.

4 Planetary scatter

Some of the gas giants scattered. Due to their sheer size they accreted the remaining planetesimals and embryos.