How would could a planet with a hydrogen atmosphere exist?



I’ve been reading about earth likes and articles [such as this one]( have really piqued my interest – specifically the mentioning of planets that “could be a “Hycean” world with a hydrogen-rich atmosphere and a surface covered by ocean water.”

How could a planet with an atmosphere of flammable gas like Hydrogen not just become one big celestial Hindenburg? Why couldn’t the whole atmosphere just burn up if you were to light a match, or two rocks were to click together and spark?

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Hydrogen is only flammable if a suitable oxidizer is present. The high-oxygen atmosphere of the Earth is both very rare and relatively recent, and it’s made and maintained by photosynthetic life.

A hydrogen-rich atmosphere has no oxygen, and if a life form started producing it, it’d probably pretty quickly react with the hydrogen and make more water. Heck, it took billions of years of photosynthesis before oxygen started building up properly in the Earth’s atmosphere, because it got used up by reacting with everything else first. Before available oxygen, the oceans contained billions of tons of dissolved iron. When the oxygen came along, it all rusted. The rust, being insoluble in water, deposited as sediment. Lots of other minerals are oxides too, and only when the raw materials were depleted, could oxygen build up to levels useful for current life forms. By the way, this oxygen probably poisoned and killed most life that existed before it came along.

Fun fact: if we went to visit a hydrogen world, we could bring oxygen in tanks, flow it through a burner and light it. Flames work pretty much the same whether the fuel or the oxidizer is in the atmosphere. The youtube channel Cody’s Lab experimented with such reverse fire:

Hydrogen requires a sufficient source of oxygen to burn, if the atmosphere doesn’t have enough oxygen then it cannot sustain combustion. And if it has low oxygen it might be able to initially burn but it will use up the fuel quickly.