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The habitable zone doesn't mean "anything inside supports life, everything outside doesn't".

It specifically means "the light from the planets sun is in the range to support surface-level life".

If a distant moon in our solar system has a liquid ocean that A) doesn't mean it's water or anything else sane and B) that habitable zones are poorly defined.

Liquid oceans generated by orbital- or geothermal activity aren't covered in the habitable zone definition and aren't very useful as they usually require a parent body to provide energy to heat the ocean (jupiter for example) but just not enough to boil it off the moon.

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