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Jupiter and Saturn formed from pebbles of ice

Amy Tyndall, News reporter
Aug 21, 2015, 19:02 UTC

Sen—A new study reveals that Jupiter and Saturn were probably born from the gradual accumulation of icy planetary pebbles, solving a long-standing issue regarding the timescale of their formation.

The current, standard model of planet formation describes how they are created out of a dusty, gaseous nebula that starts to collapse in on itself under the influence of gravity. The nebula material becomes increasingly compressed in the centre, which in turn causes a rise in temperature. The result is the formation of the hot core of a new star like our Sun. The rest of the surrounding dust and gas is in a constant state of motion, causing most of it to begin rotating along the same direction and eventually flatten into a thin disc known as a "protoplanetary disc"—a very good example of this is the HL Tauri system, first imaged by the Atacama Large Millimeter/submillimeter Array (ALMA) last year.

The disc material then starts to become clumpy. These clumps are still rotating in the same direction as the protoplanetary disc as a whole, causing them to sweep up more material as they move. As the clumps begin to stick together, their combined gravitational pull increases and, in turn, attracts more, larger clumps. This process is known as "accretion", and the size of the clump gradually grows until it eventually becomes large enough to form the rocky core of a baby planet, or "planetesimal".

In the inner part of the disc, most of the dust and gas has been swept up during the formation of the new star, leaving only rocky remnants behind. However, planetesimals located in the outer disc have more time to accrete extra gaseous, icy material, thanks to their position away from the vaporising heat of the star. This explains why, in our own Solar System which formed some 4.5 billion years ago, we can find small rocky planets like Mercury and Venus close to the Sun, whereas the larger gas giants like Saturn and Jupiter are much further away.

However, where this "core accretion" model falls short is in its timescale. Observations have shown that protoplanetary discs have a lifetime between 1-10 million years, meaning the gas giants, which mostly consist of hydrogen and helium, will also have formed in this time frame before the initial nebula dispersed. Their solid, rocky cores are around ten times the mass of the Earth, yet the Earth itself took anywhere from 30-100 million years to form. So the mystery is: just how did Jupiter and Saturn form within the short 10 million year time frame?

Researchers at Southwest Research Institute (SwRI), USA and Queen’s University, Canada, discovered that planetesimals can grow to the appropriate size if they gradually accrete "planetary pebbles"—small, icy objects about 30cm (1 ft) in diameter.

"Classical models of planet formation, where dust collides with dust to build stones, stones collide with stones to form boulders, then mountains, and so on, take too much time," SwRI Research Scientist Dr Katherine Kretke and co-author of the Nature paper explained to Sen.

"A few years ago, a team at Lund Observatory in Sweden (then graduate student Michiel Lambrechts and his advisor Anders Johansen) suggested that, in this model, the largest planets in the Solar System were formed not by Pluto-sized objects colliding with each other as in the classical model, but by the Pluto-sized objects capturing planetary pebbles. This is an entirely new picture of how giant planets formed. But they could not investigate this idea in a full simulation to see if something like the Solar System could actually emerge."

The team were successful in using computer modelling to replicate the entire process, showing that pebble accretion can indeed reproduce many of the features we see in the outer Solar System—namely a couple of giant planets (Jupiter and Saturn), some ice giants (Neptune and Uranus), and a population of small icy bodies beyond that (the Kuiper Belt).

"We used a computer code called LIPAD that can model the dynamical evolution of the Solar System," said Dr Kretke. "We can start from pebbles and planetesimals (objects the size of those in the asteroid belt) and track them as they scatter each other, collide and grow, all the way to giant planets."

Dr Kretke added: "We showed that, as long as pebbles are produced and captured over a long enough period of time then the biggest planetary embryos have time to gravitationally interact with their smaller competitors and scatter them away, so the biggest embryos can 'eat' as may pebbles as they like while their smaller competitors are effectively starved. This allows the biggest ones to grow large enough in plenty of time to capture their hydrogen envelopes and become the giant planets we see today.

"The fact that pebble accretion so naturally reproduces the outer Solar System while side-stepping long standing problems in planet formation theory really makes it seems as though we are on the right track in understanding the mysteries of the origin of our—and other—planetary systems."

The study, published in the journal Nature, is entitled "Growing the Gas Giant Planets by the Gradual Accumulation of Pebbles." The lead author is Dr Hal Levison, an Institute scientist in the SwRI Planetary Science Directorate, and is co-authored by Dr Kretke, and Dr Martin Duncan, a professor at Queen’s University in Kingston, Ontario.