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Queen’s researcher finds new model of gas giant planet formation

Simulations lead to new understanding of early solar system.

Queen’s University researcher Martin Duncan has co-authored a study that solves the mystery of how gas giants such as Jupiter and Saturn formed in the early solar system.

In a paper published this week in the journal Nature, Dr. Duncan and co-authors Harold Levison and Katherine Kretke (Southwest Research Institute) explain how the cores of gas giants formed through the accumulation of small, centimetre- to metre-sized “pebbles.”

“As far as we know, this is the first model to reproduce the structure of the outer solar system – two gas giants, two ice giants, and a pristine Kuiper belt beyond Neptune,” says Dr. Duncan.

Image Courtesy of NASA/JPL-Caltech
This artist’s concept of a young star system shows gas giants forming first, while the gas nebula is present. Dr. Duncan and his co-authors at the Southwest Research Institute (SwRI) used computer simulations to determine how Jupiter and Saturn evolved in our own solar system. These new calculations show that the cores of gas giants likely formed by gradually accumulating a population of planetary pebbles – icy objects about a foot in diameter.

The “standard model” for planet formation states that these cores formed a slow procession of accretion. Small fragments, only micrometres across, would accumulate to form larger rocks. These rocks would then collide with other objects, combining and growing larger. These collisions must occur at a very precise angle and speed to allow the objects to combine. Too fast and they both shatter; too slow and accretion cannot occur. This process of collision and growth would continue until the core reached the mass necessary, approximately 10 times the mass of Earth, to begin collecting gasses and growing into the gas giant planets we see today.

However, Dr. Duncan points out, the gas disk from which the planets would have drawn their atmospheres would have only been present for one to 10 million years. This timeline poses a major challenge for the standard model. Given that Earth is believed to have taken between 30 and 100 million years to form under the standard model, another mechanism would have to explain how planetary cores formed.

“The model doesn’t seem to produce (cores) massive enough or quickly enough,” says Dr. Duncan.

The model created by Dr. Duncan and his team found that collisions and accumulation of the so-called pebbles would have allowed the cores to form much more rapidly. Through hundreds of computer simulations, each taking several weeks or longer to run, the team’s simulations were able to produce multiple cores within the predicted timeframe for the gas giants to form. The model also predicts the formation of one to four gas giant planets; consistent with what we see in the outer solar system.

This was a major breakthrough for Dr. Duncan and his team, as previous simulations under the standard model had only been successful in isolation without outside interference from other planets forming nearby. For the first time, the team was able to simulate the environment of the entire early solar system and successfully replicate what exists today.

“It is a relief, after many years of performing computer simulations of the standard model without success, to find a new model that is so successful,” says Dr. Duncan.

When asked what the next steps may be in proving his model, Dr. Duncan said he would like to explore the formation of the rocky planets of inner solar system like Earth. He also suggests studying some of the wide variety of exoplanets recently discovered to see if his model can remain consistent with new findings in other solar systems.

“A lot of our understanding of how planets are formed has been radically revised in view of these new observations,” says Dr. Duncan. “We’re finding amazing diversity in these systems, so it’s a very exciting time to pursue these investigations.

The full study has been posted on the Nature website.