Deep within the cores of stars and the atmosphere of giant gas planets, intense pressure converts atoms into an unusual state physicists call degenerate matter. A normal atom is made up of a nucleus of protons and neutrons surrounded by a cloud of electrons. In degenerate matter, atoms become so squished that the tidy collections of nuclei and electrons break down into a slurry of sub-atomic particles. Hundreds of miles within Jupiter and Saturn lies a thick layer of a type of degenerate matter known as metallic hydrogen.
Hydrogen on Earth exists as a gas, unless super-cooled into a liquid. You may have seen science class demonstrations with a different super-cooled gas, liquid nitrogen. The liquid would have evaporated continuously into a dense fog, like a b-movie special effect, and frozen anything that came into contact with it. Your teacher might have used liquid nitrogen to freeze bananas into such a brittle state that they could be shattered them with a hammer. Nitrogen atom are tiny, and like all elements that are gases at room temperature, it takes very little energy to make the atoms so agitated that they bounce around as a gas (instead of the relative stillness of liquid or solid matter). Liquid nitrogen requires temperatures below -300°F, and smaller atoms like helium and hydrogen need to be cooled even further (lower than -400°F) to become liquids. For reference, the coldest (naturally occurring) temperature ever recorded on Earth was about -129°F.
When you add in the intense pressure of gas planets, though, the rules change. And I mean INTENSE pressure. At sea level on Earth, you’d feel a pressure of one atmosphere, or atm. At the bottom of the Mariana trench, six and a half miles under the sea, the pressure reaches over 1000 atm. About 6000 miles deep inside Jupiter (still over 30,000 miles from the core), the pressure reaches two million atmospheres. At this pressure, hydrogen atoms are crammed close together into a liquid state (even though they’ve been heated to a temperature of 17,000°F). The pressure is so high that atoms are forced to stick together and they can’t move apart the way they would if you heated them that high on Earth.
Normally, there’s a limit to how dense you can pack atoms – they can’t get any closer than the distance between the nuclei and electrons. Imagine a group of people each spinning a ball on a string above their head. The people can’t get any closer than the length of the strings or the balls will collide – and nuclei can’t get any closer than their buzzing electron shells will allow. However, if you squish a bunch of ball-spinning people or atoms into a small space, they are forced into each other. Balls go bouncing off, and so do electrons.
Larger atoms, like iron, give up electrons all the time under normal temperature and pressure. They have plenty of electrons to spare, and the outermost electrons are far away (relatively speaking) from the charged protons in the nucleus. Positively charged protons in the nucleus attract the negatively charged electrons and hold them in the atom, but the distance between nucleus and electrons in larger atoms makes it easier for the electrons to go flying off to greet their neighbors. In fact, this is what makes an element conductive. The computer you’re reading this on right now depends on the fact that some elements’ atoms have no problem releasing electrons in a river of current. Insulators, on the other hand, hold tight to all of their electrons and prevent the flow of current. Hydrogen atoms only have one electron, and they are loath to give it up under normal conditions.
However, when smushed together by the crushing pressure of a gas giant’s atmosphere, hydrogen atoms are forced to release it into a big soupy mess of protons and electrons (hydrogen doesn’t have any neutrons). This hydrogen soup is called metallic hydrogen because it’s highly conductive, a key property of metals.
The currents that swirl around the metallic hydrogen seas in Jupiter and Saturn are what give these planets their magnetic fields (which on Earth is produced by a molten iron core). The magnetic field of Jupiter is so strong that it’s likely that hundreds of Earths could fit just in the metallic hydrogen layer of the planet.
Metallic hydrogen has only survived creation in the lab briefly and in tiny amounts, but physicists theorize that it could be used as a clean fuel or a superconductor if they could just get it to stick around. Liquid hydrogen is already used as a rocket propellant, and converting from liquid hydrogen to metallic hydrogen fuel would allow us to pack 30-40 times the amount of fuel in the same space. Years from now a metallic hydrogen engine could drive spacecraft out to Jupiter where they stop by for a quick refuel before heading out into the galaxy.