Water Worlds May be Too Wet for Life
Ocean planets may not be able to maintain a climate for life
These are planets, roughly similar in size to the Earth, but with no land, and with oceans perhaps more than 100km deep. They've found that on worlds like this, a unique form of "hot ice" forms at the extreme pressures under their oceans, which seals off the planetary core from the ocean and atmosphere. This prevents the formation of the kind of carbon cycle that exists on Earth, which exchanges CO2 between the rocky core, the ocean and the atmosphere.
This carbon cycle works to regulate temperatures on Earth, preventing things from getting too hot or too cold. But on a water world, this "thermostat" can't exist, and without perfect conditions, these planets will become super-hot or freeze solid, and thus be much less hospitable for life.