Jupiter has a rocky core that is more than twice as large as previously thought, researchers announced today.

Burkhard Militzer, a geophysicist at the University of California, Berkeley, and his colleagues ran computer simulations to look at conditions inside Jupiter on the scale of individual hydrogen and helium atoms. Particularly, the researchers examined the properties of hydrogen-helium mixtures at the extreme pressures and temperatures that occur in Jupiter's interior.

With information gleaned from these simulations, the researchers developed another computer model. They found Jupiter's core is an Earth-like rock that's 14 to 18 times the mass of Earth, or about 5 percent of Jupiter's total mass. Previous studies suggested the core was only seven Earth masses or that Jupiter had no core at all.

Militzer's team found the planet's core is made up of layers of metals and rocks, along with methane ice, ammonia ice and water ice. Above this layer, they suspect an atmosphere of mostly hydrogen and helium. A metallic ball of iron and nickel, just like Earth's core, probably lies at the center of Jupiter's rocky core, they said.

"Our simulations show there is a big rocky object in the center surrounded by an ice layer and hardly any ice elsewhere in the planet," Militzer said. "This is a very different result for the interior structure of Jupiter than other recent models, which predict a relatively small or hardly any core and a mixture of ices throughout the atmosphere."

So Jupiter's interior would resemble that of Neptune and Uranus, which appear to have a rocky core surrounded by icy hydrogen and helium, but without the gas envelope of Jupiter and Saturn.

The large, rocky core implies that as Jupiter and other giant gas planets formed 4.5 billion years ago, they grew through the collision of small rocks that formed cores that captured a huge atmosphere of hydrogen and helium.

"According to the core accretion model, as the original planetary nebula cooled, planetesimals collided and stuck together in a runaway effect that formed planet cores," Militzer said. "If true, this implies that the planets have large cores, which is what the simulation predicts. It is more difficult to make a planet with a small core."

An alternative theory has the gas giants collapsing from a cloud of gas and dust, much like a star does.

The research, which is published in the Nov. 20 issue of the journal Astrophysical Journal Letters, was funded by NASA and the National Science Foundation.

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