Ttyjukl

I am creating a speculative evolution project for school (and partially for leisure) and I need a planet for my organisms to live on. I have a rough idea of what it may be like but I'm no expert in planetology and other planet-related sciences.

My planet has/is:

• A dense atmosphere


• Larger than Earth


• Only 16% of the planet's surface is water


• Most of the land is covered in thick rainforests


• 150% more water than Earth


• Most of the planet's water is in the atmosphere


• 34% oxygen ratio


• The water in the atmosphere is water vapor

I need advice on how to turn this into a plausible world capable of sustaining life.

Evapotranspiration, because the planet is covered in vegetation the rate of water moving from the ground/rivers/oceans into the atmosphere can be accelerated if the rate at which that vegetation gives up water to the atmosphere is high enough. This creates a world of thick mists and clouds because the atmosphere is permanently at or above it's water vapour saturation.

Speaking in terms of chemistry the liquid and gaseous phases of water are in dynamic equilibrium. In order for most of the water to be water vapor, the world would have to be incredibly hot.

The other option would be to have an incredibly low pressure which would not be consistent with your dense atmosphere.

Neither of these options seems plausible.

Now if you wanted a world with significantly more gaseous water than Earth but still within reason, I would suggest a mixture of increasing the temperature, by moving the planet closer to the sun, and decreasing the pressure, by decreasing the presence of other gases in the atmosphere. This would allow for a consistently high concentration of water vapor comparable to a foggy day on Earth.

You want a planet like Teneba. Hal Clement had great skill in creating worlds with unusual chemistry. In his novel, the combination of mass, temperature and pressure results in a world very close to water's critical point.

https://en.wikipedia.org/wiki/Close_to_Critical

A dense atmosphere - as Jackom5 has clarified, atmospheric pressure and temperature determine water's preferred state. We want to keep the planet cool enough to support life as we know it, so let's interpret "dense" as "dense-looking". The atmosphere appears dense because it is full of water vapor. When actually measured, it is pretty close to Earth's norms in order to not challenge the planet's life-friendly status. If you need it to really be denser, then that just raises the necessary air temperature. Higher temperatures are not a deal breaker, but given that Earth-borne thermophiles haven't evolved above bacterial size, it might limit the varieties of organisms which can live on your foggy world.

Larger than Earth - does not necessarily mean higher gravity. You can have your bigger planet, but let's make it less dense than Earth, so that our style of living beings could walk around in the fog and so that some of that life could fly. Flying through fog must be cool.

Only 16% of the planet's surface is water and 150% more water than Earth - is not a problem. If we make your oceans incredibly deep with steep cliffs instead of gradually sloping floors, they can hold almost half of a greater than Earth supply of water while taking up a lot less surface space than our oceans do.

Most of the land is covered in thick rainforests - as Ash offered, this can only help with the real challenge which we will be getting to soon.

34% oxygen ratio - okay, but it will make stuff a lot more combustible. Good thing that there is a lot of moisture in the air or your rain forests would live in constant jeopardy from wild fire.

The water in the atmosphere is water vapor - isn't really an issue. I'm not a scientist, but I'm pretty sure the only way that water can be in an atmosphere is as water vapor.

Which leaves the real challenge of your post...

Most of the planet's water is in the atmosphere

Others have already written about why this is difficult on a life bearing planet. Environments where water boils are not conducive to living organisms which prefer to keep the water inside them in its more useful liquid form. As previously stated, if Earth's thermophiles are to be trusted, high temperatures also seem to have a negative effect on the upper evolutionary threshold.

So let's make the high humidity a comparatively recent occurrence. Let's let the planet's life evolve in a nearly perfect earth equivalent environment; with clear skies, lots of sunshine and whatever else it was that made the miracle happen here on Earth. Let's let that life crawl out of the seas and start living on dry land.

...then let's boil the oceans!

Massive undersea volcanoes open at the bottom of those incredibly deep seas and start filling the planet's atmosphere with steam. Let's turn the heat down a little. We don't want to boil our young life. Just keep those oceans simmering slowly, while the life evolves and adapts to their changing world.

Give the recipe a billion years or so, with the water constantly steaming off of the oceans surfaces, riding gentle winds till they cool, condensing as dew, then gathering into puddles which grow into ocean-bound streams.

Your world is now permanently fog bound, muggy and slippery. Not my first choice for a vacation spot, but the life that could evolve there should be very interesting!