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Tiny pebbles make the biggest planets

Researchers solve the mystery of how Saturn and Jupiter formed, which changes how we think of planetary formation as a whole.

Artist's concept of a very early solar system, complete with nebula. NASA/JPL-Caltech

Gas giants Saturn and Jupiter may have formed, not from large planetary cores, but from an accretion of smaller rocks, researchers have found. This model solves a long-standing mystery about how these planets formed, and other gas giants just like them.

The paper, authored by a team of researchers at the Southwest Research Institute in Boulder, Colorado, and Queen's University in Canada, has been published in the journal Nature.

The problem with the previous model of planetary formation for Saturn and Jupiter was the timescale. The two planets probably formed very early on in the life of the solar system, back before the dispersion of the nebula that birthed the sun.

This is because the gas disc that is usually responsible for the formation of gas giant planets typically only lasts a maximum of around 10 million years, which means Saturn and Jupiter had formed by the time the solar system was 10 million years old. The Earth, a terrestrial (rocky) planet, by contrast, took around 30 million years to form.

The previous model is called core accretion, in which a core of rock and ice forms via a process known as accretion. Similarly sized rocky objects accumulate, forming first mountains, which then merge together to create larger and larger objects, until a planetary core is formed. Then gas and dust is pulled into the planet and wraps around it, creating the gas giant.

But the cores of Saturn and Jupiter are enormous, far larger than Earth, roughly the size of Neptune and Uranus. If Earth took 30 million years to form, it's highly unlikely that the cores of Jupiter and Saturn, the first step in a process that took just 10 million years, could form so quickly.

"The timescale problem has been sticking in our throats for some time," Hal Levison, scientist in the SwRI Planetary Science Directorate and lead author of the paper, said in a statement. "It wasn't clear how objects like Jupiter and Saturn could exist at all."

The model developed by the team, which includes SwRI research scientist Katherine Kretke and Martin Duncan, a professor at Queen's University in Kingston, Ontario, solves this problem.

The model is called pebble accretion, and it involves much smaller pieces of rock, ranging from between one centimetre and one metre in size (0.4in to 3.3ft). These accumulate and eventually collapse together through the force of gravity. The resulting object then proceeds to attract more pebbles.

Although the process sounds slow, it's accelerated by the movement of gases in the solar nebula. These create winds, which can blow pebbles towards the accumulating core. But the pebbles have to form relatively slowly in order for the baby planets to interact with each other gravitationally; models have demonstrated if the pebbles form quickly, it results in hundreds of Earth-sized cores. Since we don't have hundreds of Earth-sized planets in the solar system, something else must have occurred.

"If the pebbles form too quickly, pebble accretion would lead to the formation of hundreds of icy Earths. The growing cores need some time to fling their competitors away from the pebbles, effectively starving them. This is why only a couple of gas giants formed," Kretke explained.

The model developed by Levison's team successfully demonstrated that the pebble accretion model with slow-forming pebbles typically results in the formation of one to four gas giants at a distance of at least five times the distance between the Earth and the sun. Like, for example, the solar system.

"As far as I know, this is the first model to reproduce the structure of the outer solar system, with two gas giants, two ice giants (Uranus and Neptune), and a pristine Kuiper belt," Levison said.