If an element becoming colder slows the atoms down, which makes them more likely to be solid, than why are all the gas giants furthest from the sun?
In: Planetary Science
Temperature is not the only factor in states of matter. The other factor is pressure. You can look up a “phase diagram” to see how they relate. Something that is cold but low pressure may remain a gas, and something that is hot but high pressure may solidify.
Actually, there’s even another factor, which is probably more relevant here: each substance has different melting/boiling points. Liquid nitrogen is very “cold” to us, so why is it still liquid and not solid? Well, for a very simple reason: nitrogen freezes at an even colder temperature than that.
So in short: it’s cold by our standards, but not cold *enough* for the gases in those planets’ atmosphere, and the pressure is high but not high enough to squeeze them into a liquid or solid (though the core of Jupiter is in fact liquid because of the pressure).
The gas giants are made of a very specific couple of gases. Hydrogen and helium. To become a solid or liquid, not only must a gas slow down enough, it also has to stick together. Hydrogen and helium have near zero attraction to themselves or eachother, so they don’t become a liquid or solid until *very* low temperatures.
They’re also extremely abundant, which is why the gas giants are *giant* and mostly made of these gases. They’re just everywhere. The next question is, why is our dear Earth not also mostly hydrogen?
It does have a lot of hydrogen in it, but way less than the average across space, and this is because the sun gobbled up much of the hydrogen in the inner solar system and its radiation quickly blasts away anything left over. If we release helium into the air right now, it will float up near space and get blown away by the sun.
Hydrogen and helium are very light so tended to get blown outwards by solar wind, so there was just a lot more gas in the outer solar system where the gas giants formed. Temperature isn’t so important because even the furthest gas giant Neptune is above the temperature that hydrogen goes from gas to liquid.
Those gasses aren’t nearly cold enough to be solids.
At low pressures, liquids can’t exist. That’s why dry ice (solid CO2) sublimates rather than melts. There’s not enough pressure for it to be a liquid.
Saturn’s moon Titan has lakes of liquid methane on the surface because it is cold enough and has enough pressure to keep it liquid.
The gas giants are mostly hydrogen, which doesn’t become solid at all as far as we can tell under any temperature or pressure. We believe that inside the gas giants, eventually, you will reach an ocean of liquid hydrogen due to the immense pressures.
Now, why are all the gas giants far away from the Sun? Early on in the solar system, when the sun first switched on, the radiation pressure of the light from the sun was able to push all the light gas particles out to the outer solar system, but the heavier dust particles didn’t get pushed out, so the rocky planets were made of this dust pressed together, and the gas giants were made of all of these gasses clumping together.
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