Did Jupiter Form From Pebbles?
Jupiter is gargantuan. With a mass two and a half times greater than all of the other planets combined and a volume equivalent to 1,321 Earths, the gas giant commands respect. There are some who even credit Jupiter with shaping the Solar System as we know it, altering the orbits of the lesser planets and cajoling careening asteroids.
Incredibly, despite Jupiter's massive influence and impressive resumé, the planet may have formed from mere pebbles just centimeters in size.
The pebble accretion model, as the idea is called, suggests that tiny objects first coalesce together due to drag then gravitationally collapse and form larger objects one hundred to one thousand kilometers in size. These larger objects, now referred to as planetesimals, than draw in all the remaining pebbles and become the cores of larger planets.
Simulations completed last year cast doubt on this interesting theory. They suggested that -- in the context of our solar system -- too many planetesimals would form -- as many as one hundred objects the size of Earth! Since our Solar System only contains eight planets and five recognized dwarf planets, this theory was mostly ruled out.
However, a new simulation carried out primarily by researchers at the Southwest Research Institute and published to the journal Nature suggests that if these pebbles form slowly enough, fewer large planetesimals will emerge.
But what's slow is still pretty fast on cosmic terms. According to the simulation, an object with one Earth-mass would grow in just 400,000 years. This object would then become the core of a gas giant, and start to accumulate dust and gas circulating throughout the nascent solar system.
The research fills in the blanks of how our neighborhood gas giants, Jupiter and Saturn, formed. Roughly 4.6 billion years ago, the Solar System was just a hot cloud of dust and gas. Eventually, some of the gas became so concentrated that it collapsed in on itself, forming the Sun. The rest of the material began to swirl and spin around the growing star, forming what's called a protoplanetary disc. In this roiling environment, pebbles joined together to form Jupiter's core, which then captured hydrogen and helium escaping the protoplanetary disc. Saturn's core likely formed a little bit later. When it did, it too began to snatch up the escaping gases. The rest is cosmic history.
Source: Harold F. Levison, Katherine A. Kretke & Martin J. Duncan. "Growing the gas-giant planets by the gradual accumulation of pebbles." Nature. 20 Aug 2015. doi:10.1038/nature14675