New Model of Jupiter's Core Ignites Planet Birth Debate

Bruce Dorminey
for National Geographic News
December 4, 2008
Underneath its swirling cloud layers, Jupiter may harbor a solid core roughly equal in mass to 16 Earths—more than twice as large as previously believed.

That's the conclusion of a controversial new computer simulation that represents the first radical rethinking of the planet's core in nearly two decades.

The work has reignited debate among planetary scientists over how gas giants such as Jupiter first formed.

"The biggest surprise was the large core," said study leader Burkhard Militzer of the University of California, Berkeley.

"We concluded that the planet formed by core accretion," when colliding grains of dust, ice, and small planetary bodies meld to create planetary embryos and eventually fully formed planets.

Core of the Issue

Many scientists think core accretion is a good model for the birth of rocky terrestrial planets.

But it has been hard to apply to gas giants, which are so gassy and massive that simulations suggest they wouldn't have had enough time to grow as large as they are between their core formations and now.

One leading alternative theory, championed by Alan Boss of the Carnegie Institution in Washington, D.C., is disk instability.

This is when clumps of gases inside the disk of planet-forming material around a young star will cool and collapse to form gas giants.

But if Jupiter really has a much larger rock-ice core than thought, Militzer said, the accretion model becomes a better fit.

The study comes just about two years ahead of a recently approved NASA mission called Juno that may finally put an end to the decades-long dispute.

"The real surprise [from Juno] would be if Jupiter has no core at all," Boss said. "Both models for making Jupiter would be in trouble."

For their study, Militzer and colleagues used advanced computer simulations to model changes in temperature, density, and pressure all the way to Jupiter's deep interior.

The simulation also used data about Jupiter's size and gravity field obtained by previous spacecraft, noted co-author William Hubbard of the University of Arizona.

Details of the simulation were recently published in the Astrophysical Journal Letters.

Militzer thinks that after initial accretion, Jupiter's newly formed core swept through the early solar system and gathered most of the outlying gas left over from the formation of the sun about 4.6 billion years ago.

Based on this notion, it took a mere ten million years for Jupiter to achieve its current heft of 318 Earth masses, most of which is hydrogen and helium gases.

But co-author Hubbard is not ready to endorse any given formation theory.

"There's not agreement even among the model makers about what's going on with Jupiter," he said.

"I'm not wedded to the notion that Jupiter formed by core accretion. That's just a reasonable interpretation of our result.

"With [the Juno mission], I would like to get a diagnostic that shows the signature of a core, perhaps in the circulation of the planet, perhaps in the magnetic field," Hubbard added.

(Related: "Lightning Strikes, Changing Climate Revealed on Jupiter" [October 9, 2007].)

A "Quantum Leap"?

The Juno mission is scheduled for launch in 2011. Upon arrival at Jupiter in 2016, the solar-powered spacecraft will head into a highly elliptical polar orbit.

For roughly a year Juno will map Jupiter's gravitational and magnetic fields at high resolution while also peering deep into the planet's atmosphere.

(Read more about the Juno mission on the NatGeo News space blog.)

Adam Burrows, a Princeton University astrophysicist not affiliated with the mission, said Juno could represent a "quantum leap" in understanding giant planets in general.

Better data about Jupiter's core, for example, could give researchers insight into the formation of the more than 200 known gas giant planets that have been found circling other stars.

But Ravit Helled, a Juno science team member based at the University of California, Los Angeles, said Jupiter's core mass would have to be either very large or very small to reveal information about its formation.

Boss, of the Carnegie Institution, added that it's possible Jupiter's core has eroded away over time.

If so, information needed to solve the dilemma would have disappeared along with it.

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