“Planetary Pebble” Model May Solve Mystery of Jupiter and Saturn’s Formation

October 26, 2015 | Sarah Tse

Circumstellar Disk
Photo credit: NASA / JPL-Caltech / T. Pyle (SSC)

A new model for the formation of large gas giants squashes a bug that had pestered prevailing theories.

Astronomers have long pondered the puzzle of how the planets formed. After the sun’s birth 4.5 billion years ago, a disk of gas and dust remained for about 1 to 10 million years. Within that nursery-like disk, dust slowly accumulated into larger clumps that eventually combined into the planets we know today.

Although Jupiter and Saturn are the largest planets in the solar system, astronomers believe they formed much more quickly than their smaller brethren. The widely-accepted core-accretion model for gas giant formation dictates that these planets had to first develop their cores of ice and rock before they could gain their gassy shrouds. Only then could the cores pull gases from the residual nebula surrounding the young sun.

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But that means the cores must have formed within 10 million years, or else the gases would have dissipated by modern times.  However, the probability of massive “planetesimals” (objects smaller than planets) colliding and combining is too low for assembly of the core to have occurred in such a short time. Scientists estimate that the process of combining rocks into mountains, mountains into asteroids, and so on, would take at least 30 million, if not 100 million years just to build an Earth-sized mass. According to this theory, Jupiter and Saturn, which have at least 10 times the mass of Earth, shouldn’t exist at all. So how did they grow so large so quickly?  

A new model proposed by scientists at the Southwest Research Institute suggests that the cores of the gas giants assembled via gradual accumulation of icy planetary pebbles about a foot in diameter. As the pebbles clump together, these masses collapse into bodies big enough to establish their own orbits. But loose pebbles can’t move as freely through the gassy nebula surrounding the sun. Whereas two similarly-sized objects might not be able to pull each other together, one larger object can easily disrupt pebbles out of their orbit.

And what kept the pebbles from simply coming together into hundreds of Earth-sized rocks? The computer simulation showed that as the cores grew larger, they could essentially bully their runty rivals out of the way as their gravitational fields interacted. Those remaining in the planetary nursery could hog all the pebbles they needed to grow big and strong. Scientists calculated that this process could have formed Jupiter- and Saturn-sized cores well within 10 million years.

This theory then makes room for the Nice model, which suggests that the outer planets interacted with each other to nudge their orbits further and further from the sun. The researchers will continue to run computer simulations to see how this new pebble-accretion process works with existing data from other solar systems. For example, the recently discovered planet 51 Eridani B closely resembles a younger version of Jupiter; further analysis of its behavior may confirm this model of planetary formation. Pebble-accretion explains our system’s formation so much more neatly than previous theories, and it can transform the way astronomers currently understand planet formation.

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