December 2, 2011 | 1
Gas giant planets are among the most beautiful and awe-inspiring worlds. In our own solar system we’ve long gazed at Jupiter’s extraordinary swirling atmosphere, where stormy circulations like the Great Red Spot persist for centuries. We’ve also been captivated by Saturn’s vast ring system, on average barely sixty or so feet in thickness but over 60,000 miles in width – racing around the planet at velocities as high as 40,000 miles an hour.
The major bulk of these planets, and their cousins around other stars, consists of primordial hydrogen and helium – vast envelopes of matter cocooning their cores and rendering them inaccessible to us. The pressures deep down in a planet like Jupiter can reach a hundred million times those on Earth’s surface, and temperatures can be tens of thousands of Kelvin. Little wonder that these worlds are their own category, their own species.
Something curious should also happen within the most massive planets. At such huge pressures, hydrogen becomes a liquid, even though temperatures are high. This by itself is not so extraordinary, but hydrogen is special, the simplest possible atom – one proton and one electron. As pressures increase, the very nature of liquid hydrogen changes. Its phase alters, and instead of being just a thick, gloopy, insulating layer within a planet, it should transform into a metal. The idea is that the electrons that are normally associated with single protons in hydrogen atoms become unbound, jumping across a band-gap of energy states, allowing for precisely the kind of easy electrical and thermal conduction in a metal (and some think even superconductivity). This metallic hydrogen can exist as a solid, a lattice-work of arrayed atoms, or as a liquid.
Remarkably, various experiments here on Earth have been able to begin to probe the extreme conditions of this alien state of matter, and seem to confirm precisely this kind of behavior above about 2.5 million atmospheres pressure. But what of real planets? Are there things about the special properties of metallic hydrogen that play a role in setting characteristics that we might actually observe in exoplanetary systems?
A new paper showed up this past week by Wilson and Militzer that suggests some intriguing things. In this work, the authors run computations to try to understand the behavior of normal solids (the kind of rocky metal oxides that may form the seed-like cores of almost all planets) when they’re dunked in an ocean of hot liquid metallic hydrogen. It turns out that compounds like magnesium-oxide (a representative “rocky” component) are highly soluble in this situation. So the implication is that, if a gas giant planet forms around a large rocky proto-planet (perhaps 10 times the mass of the Earth), the liquid metallic hydrogen that eventually envelops that core may also melt, or dissolve it. This could disperse all that element-rich material into the bulk of the planet, which – and here’s the punchline – means that bigger, hotter “super-Jupiter” exoplanets may appear more abundant in heavy elements to our astronomical instruments not because they’re getting polluted by in-falling stuff, but because they’ve digested their original cores.
And if that’s true, then anytime we see giant exoplanets which have more heavy elements floating in their upper atmospheres, we might actually have a new probe of their original recipe. Or to put it rather less delicately, we get to examine the scat from their last supper.
[With thanks to the lyrics of Modern English, "I Melt With You" from After the Snow, 1982]