Something a little different...

I'm an undergrad astronomer, and right now I'm working on a research paper for a planetary science class that you guys might find interesting:Consider this- a planet where the entire surface is occupied by a single global ocean.TLDR Visiting a water planet you'd be greeted with acidic oceans, scalding steam atmospheres, and mindboggingly deep oceans.The size of the oceans are pretty much just chance, the Earth is only .02% water by mass. A very slight variation of that number (with respect to the Earth's total mass) would result in the oceans being reduced to just a few lakes, or completely flooding the continental land masses.But, out past the asteroid belt, there are a bunch of comets and small moons with totally different bulk compositions, water mass fractions can run up to 40% of total mass. Its not too radical to consider an Earth sized planet made out past the asteroid belt, that is then brought inwards due to gravitational interactions, melting and resulting in something vaguely Earth-like.A world like this can hold on to its water for a surprisingly long time. Even a 1 Earth mass planet at Mercury's orbit would keep its water for billions of years. The planet effectively develops a massive steam atmosphere that is optically thick, insulating the oceans from the solar irradiation. Your ocean temp would shoot up to 647 Kelvin (705 F), becoming a supercritical fluid, somewhere between steam and boiling water.But what would a planet look like if it was more Earth like? Because of how humidity works, you'd be pretty close to 100% relative humidity everywhere, but if you had a background atmosphere of something neutral like nitrogen, you have to get up to around 350 kelvin (170 F) before your atmosphere becomes 20% water (at around 1 bar backgound pressure). Less than that it should look more familiar.But these planets are really unstable in the habitable zone. Because water is a greenhouse gas, moving in by 1/20th of an Earth-Sun distance causes a temperature change from 273 K to 373 K (31F to 211F). As more water evaporates, you retain more heat, evaporating more water, its a rapidly growing feedback loop. Additionally, the Earth stabilizes its CO2 content by the carbon-silicate cycle, which doesn't really work if your planet has no solid crust. This means that CO2 is sequestered in the ocean, making it pretty acidic (~6 to ~3 pH), and the ability of your ocean to store CO2 drops with temperature, giving another unstable feedback loop. This means that the vast majority of your worlds will be frozen out, or steam worlds, with tropical ocean getaways being a rarity.Within your oceans, pressure goes up with depth, but water has some funky high-pressure ices. This results in oceans about 100km deep (varying with surface temp and gravity), with sheets of ice, possibly hundreds to thousands of kilometers thick, surrounding a silicate core. You'd see high pressure ice even on those crazy steam worlds.Spooky, huh?