The European Space Agency’s COROT planet-hunter mission has identified a super-massive, high-density planet that is unlike any other exoplanet yet found. The newly discovered world COROT-exo-3b is a hot giant that circles its star once every four days, making it similar to many other ‘hot Jupiter’ giant planets already found in other star systems. What makes this one unique is its density. Based on its estimated size and mass, COROT-exo-3b would have to be over twice the density of lead. Nothing like this has ever been found before.

Gas giant planets more massive than Jupiter are common, and can range up to about 15 Jupiter masses before they begin to sluggishly fuse hydrogen in their cores to make helium – at which point they are considered brown dwarf stars. Brown dwarfs scale up from there, all the way to red dwarf stars and then ‘normal’ stars. COROT-exo-3b is different. This planet is above the mass threshold for a brown dwarf, but it’s ridiculously small for its mass – about the physical size of Jupiter.

What can COROT-exo-3b be made of? That depends on where it formed originally. It probably didn’t form as close as it is now to its star. Right now it careens around its primary in about 1/22 the time it takes the planet Mercury to complete an orbit around the Sun – which is a slightly smaller star than COROT-exo-3b’s primary. Most likely COROT-exo-3b formed farther out, and later migrated inward. That sort of thing happens. But the high bulk density of COROT-exo-3b suggests it’s not a typical hot Jupiter-type gas giant.

The star orbited by COROT-exo-3b contains roughly the same enrichment of heavy metals as the Sun, according to ESA astronomer Magali Deleuil at the Laboratoire d’astrophysique de Marseille. So how can a planet be that dense? COROT-exo-3b is close to the density of osmium, which at ~22.6 grams per cubic centimeter is the densest metal in the periodic table.

It could be that COROT-exo-3b is simply a giant Mercury; formed mainly of heavy elements having the highest boiling temperatures, as Mercury and the terrestrial planets formed around the Sun. When our inner planets formed they picked up mostly elements that were already condensed from gas to solid particles (rock and metal) before the Sun’s early, violent solar winds swept all the remaining gases out toward Jupiter and beyond. If COROT-exo-3b is a giant Mercury is would mostly be made of metal and rock, compressed by gravity to incredible densities in its seething core. With several Jupiters’ worth of heavy elements, including radioactive uranium, strontium, rubidium and potassium, the interior of COROT-exo-3b would be orders of magnitude hotter than Earth’s core. Heat flow from the interior would produce truly bizarre volcanic activity: cataclysmic eruptions spewing liquid steel in weird, flattened sheets due to the colossal gravity. Oceans of lurid magma, under a crushing atmosphere of… of what, exactly? Metal vapor? Ionized sodium?

The problem here is that to make a single planet out of metal and rock requires far more metal and rock than would be available, at least according to conventional models of solar system development. The planet probably contains all that stuff, but it would have to contain lots of more basic material, too, to add up to 20 Jupiters…. material like hydrogen and helium. But having a massive proportion of metal and rock would explain the size and the density of the planet, and also why it’s not fusing hydrogen in its core – because the core is iron, and iron cannot fuse outside of a supernova or a neutron star.

Either way, COROT-exo-3b is going to force a lot of people to start re-imagining solar system formation processes, as they try and figure out how this weird superplanet got there.