New map hints at Venus's wet, volcanic past</> I felt plate tectonics was very likely a result of the Impact that formed the Moon since no model has predicted it's formation and both were events seemingly unique to Earth. I had never heard a theory addressing how common plate tectonics would be, but many comments suggesting it was needed to concentrate the ores needed by technological civilizations and/or the climatic stability resulting from Earth's carbon cycle. SeePlanetary habitability at wikipedia. If Venus also had plate tectonics then it probably is simply a function primarily of the mass of rocky worlds. One limit that is often left out is the mass at which a planet is lilely to be a water world. The more massive the planet the less surface to volume ratio it has and the deeper it's oceans will be. I doubt a water worlds will develope a technological civilization, although I believe it is possible to graft one onto a water based intelligence ala Brin and Niven.

Ignoring gravitational compression surface area (As)/ Volume (V) is proportional to As/Mass (M) which would be proportional to 1/R.

Average ocean depth (Ho) is proportional to to the mass of water in the planetoids forming a planet divided by the surface area of the final planet.

As a result, Ho is roughly proportional to xM/As where x is the mass fraction of water. If you hold the mass fraction of water constant the average ocean depth should increase in proportion to planetary radius R while mass increases in proportion to R cubed. The average depth of the ocean on Earth is about 5 miles and the Land is about a mile high. If the Earth was a perfectly smooth sphere the ocean would be about 4 miles deep. Gravity compresses mountains on larger worlds so average land masses above one mile will be less likely on larger worlds. Assuming a similar distribution of highs and lows though would require a 25% increase in average ocean depth to turn an Earthlike world into a waterworld. A 25% increase in radius would put a limit of a mere 2 Earth masses to become a water world with at most a few large islands. As a rough guess ETI is probably most likely on worlds 0.5 to 1.5 times as massive as Earth. Small land area greatly decreases the chances of the fluke niches and events that evolution of human intelligence. In order for the water mass fraction to vary greatly from Earth's the planet would probably have to be outside the habitable zone.

Regards,
Mark