Many exoplanets discovered could be as much as 50% water but their size has an impact on what that means.
“This is water, but not as commonly found here on Earth,” said Dr. Zeng. “Their surface temperature is expected to be in the 200 to 500 degree Celsius range. Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath.” …
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The larger question is whether life can exist on so-called “water worlds” at all. As Scientific American explored earlier this year, the problem is what happens on a planet with enormous, deep oceans and very little land. At the bottom of the oceans, an ice layer forms due to the extreme pressure. This enormous ice layer would effectively seal off the bottom of the land from the actual water, cutting off the geological processes that are necessary for life to bloom in the deeps.On Earth, hydrothermal vents on the ocean floor provide the energy required to support thriving ocean ecosystems, even in the jet-black conditions of the deeps. Increase that depth by 20 times or more, and the ice layer that forms could simply choke off the possibilities. The weathering of land rocks provided an important source of minerals in Earth’s oceans, and that step would also be missing on a water planet. Joel Hruska, “Water Worlds May Be Common in the Universe, and That’s Bad News for Life” at ExtremeTech
In other words, Earth doesn’t just “have water”; it is fine-tuned for life. We need to find exoplanets that are likewise fine-tuned.
See also: At Scientific American: Maybe aliens live too fast or too slow for us to recognize
SETI reacts to the new study that says not to wait up for extraterrestrials
Researchers: We have dissolved the Fermi Paradox! (They’re NOT Out There)
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What becomes of science when the evidence does not matter?