Jupiter's Eroding Core: Large Exoplanets Have no Cores

A new study indicates that the hydrogen and helium gases that made Jupiter a gas giant are destroying the planet's very core, leading astronomers to believe that most massive extrasolar planets have no cores at all and changing the view scientists have long held of these distant worlds.

Jupiter has been called a gas giant because it consists mostly of hydrogen and helium surrounding a central core of iron, rock, and ice.

The core, which weighs roughly 10 times as much as Earth, is a small component in a planet that weighs 318 Earths.

These same gases are causing the solid rock in Jupiter's core to dissolve into liquid, the researchers said.

Planetary scientists Hugh Wilson and Burkhard Militzer of the University of California, Berkeley, performed quantum mechanical calculations on the outcome if magnesium oxide (MgO), which is a key ingredient in the rock of Jupiter's core, is submerged in a hydrogen-helium fluid at the planet's heart.

According to the researchers, with MgOs high solubility, the core's temperature, which is hotter than the sun at approximately 16,000 degrees Kelvin, will make the solid rock in Jupiter's core melt into liquid.

In a paper submitted to Physical Review Letters, the scientists said that although the exact rate of erosion is unknown, it is also calculated that the ice in the core also dissolves, so Jupiter's present core may not be as large as it was when the planet formed.

While the new findings are important, a planetary scientist, Jonathan Fortney, said the big question is whether the convection in Jupiter's interior is vigorous enough to dredge up dissolved core material and toss it into the hydrogen-helium envelope.

Fortney said that if this was the case, then Jupiter's core could be smaller today than it was at birth; if not, the dissolved rock and ice will simply remain at Jupiter's center although the boundary between the core and mantle may not be so distinct.

"I think we've made much more progress in the past year than people had made in the previous 20 years," said Fortney, adding that those calculations have implications far beyond Jupiter since many of the planets orbiting other stars are more massive than Jupiter, so their cores are even hotter.

"For these planets, core erosion would be faster," says Militzer, which could support the theory that gas giants several times heavier than Jupiter might be completely coreless.

In 2016, NASA's Juno spacecraft will start orbiting Jupiter, which could provide data on the planet's interior by measuring its gravitational field.