Do planetary pebbles solve gas giant mystery?

There is a discrepancy between current theories as to how the gas giants in our Solar System formed and the time it takes for the accretion of rocky cores to occur. New research may have solved the conundrum.

Astronomers using NASA's Spitzer Space Telescope have gathered the most detailed data yet on a gap in a protoplanetary, or planet-forming, disk surrounding a young star.

This artist's concept illustrates one interpretation of the data, which attributes the disk gap to planet formation. At the center lies a young star that is pulling in material from an inner disk of dust and gas. The gap between this inner disk and the thick outer disk is believed to be occupied by developing gas giant planets. The putative planets prevent the outer disk material from naturally falling in toward the star, thereby creating the gap.

The inner disk is roughly the size of our inner solar system, or the distance between the Sun and Jupiter. The gap would span orbits equivalent to those of Jupiter and Saturn. The Saturn-like rings around the planets hint that they are very young and still surrounded by debris left over from their own formation. (Note: the planets in this illustration are exaggerated in size.)

At the edges of the solar system, the thick disk is expected to coalesce into asteroids, comets and possibly more planets. The bipolar flow, or dim jets of material, shooting out of the star's north and south poles, is a characteristic typical of young stars that are not yet fully formed.

Artist’s concept of a young star system, showing the gas giants forming first. Credit: NASA/JPL-Caltech


Planetary pebbles may be the key to an enduring mystery as to how the gas giants in our Solar System formed over such a short period of time.

Jupiter and Saturn are thought to have accumulated their gases before the solar nebula dispersed at the beginning of our Solar System’s formation.

Observations of young star systems show that the gas discs that eventually form planets usually have lifespans of about 1 to 10 million years. In other words, the gas giants must have formed within this timeframe.

Earth, contrastingly, could have taken between 30 to 100 million years to form, begging the question as to how Jupiter and Saturn formed so quickly.

Gas giant planets are largely thought to form via accretion, whereby a planet-sized core of ice and rock forms first.

Gas and dust then attaches itself and the planet begins to grow.

However, the problem with this model is that in order to accumulate such a massive atmosphere, a core of roughly 10 times the size of Earth would be required.

Considering how long it took Earth to form, how could these cores have formed in such a short timespan?

A new study by researchers at the Southwest Research Institute (SwRI) in San Antonio, Texas and Queen’s University in Ontario, Canada used computer simulations to solve the mystery.

“The timescale problem has been sticking in our throats for some time,” says Dr. Hal Levison, an Institute scientist in the SwRI Planetary Science Directorate and lead author of the paper.

“It wasn’t clear how objects like Jupiter and Saturn could exist at all.

New calculations by the team show that the cores of Jupiter and Saturn could form well within the 10-million-year time frame if they grew by gradually accumulating a population of planetary pebbles – icy objects about a foot in diameter.

Recent research has shown that gas can play a vital role in increasing the efficiency of accretion.

So pebbles entering orbit can spiral onto the protoplanet and assimilate, assisted by a gaseous headwind.”

Co-author, SwRI Research Scientist Dr. Katherine Kretke, says:

“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.”