Celestia Forums

A theoretical class of planets not found in our solar system, formed when icy outer planets migrate inwards (like the 51 Pegasi-type planets are thought to have done) to the system's liquid water zone. Might be an interesting possibility?

http://arxiv.org/astro-ph/0308324

Ah, yes; the French have convinced me that waterworlds will be common throughout the galaxy.

there is no real reason why the planet has to migrate inwards, however-

if we imagine a large 2x Earth sized terrestrial forming at approximately the same orbital distance as the Earth (around a sun-like star) the amount of water falling onto the planet from comets can be quite considerable;
it is possible that all the Earth's water came from a class of comets that orbited near the orbit of Jupiter, most of which have now been flung into interstellar space by the gravity of that large planet (or impacted upon it).

This could result in a very wet planet, with an ocean tens of kilometers deep;
under the ocean there might be an ice-mantle, as the ice that forms at such great depths is denser than water.

Have a look at this;
http://xxx.lanl.gov/PS_cache/astro-ph/p ... 308159.pdf

in 42 simulations, the largest terrestrial planet they came up with was
3.85 Earth masses with a whopping 300 earth oceans full of water!

Such a world (with gravity of 2 gee) is an incredible place to
imagine.

Ok, I’m trying at the moment to produce an imaginary Waterworld as described by Leger and simulated by Raymond, Quinn and Lunine; this would have a mass of 3.85 Earths, a surface gravity of (?) approximately 2 gee.
Distance from the star would be the thermal equivalent of 0.96AU ;
I think I’ll make the central star a class K0, putting the true distance at about 0.7AU.
The water content of this world is 2.3%, equivalent to 352 Earth oceans; this would form an ocean perhaps a hundred km deep, with another 100 km of rock/ice mantle below it formed from something like ice-six (?) which is denser than water.
Not too clear on the exact pressure, and also the exact depths here; so I don’t know the type of ice involved at this depth.
Note that the depth of the ocean and thickness of the ice mantle would be considerably more on a superterrestrial waterworld of six earth masses as envisaged by Leger; but such a giant world was not produced in Raymond’s simulations, so I will leave that as a project for another day

The overall density of this world would be just a bit smaller than that of the Earth, due to the high water content
The core of such a planet could be metallic, surrounded by a silicate/metallic outer core, then the icy mantle; it seems possible than volcanic activity might not be entirely stifled by such a relatively thin icy mantle- perhaps a new type of water-ice volcanism might have developed…

The atmosphere would have originally been water vapour and NH3, destroyed by sunlight and hydrogen boil off to become nonbiogenic oxygen and nitrogen; I think CO2 and sulphur compounds may well have bubbled up from the bottom of the ocean as well; it seems that primitive life might develop with luck.
Anyone able to figure out the diameter of such a world? I think I could do it myself but my maths always goes adrift…

Well, to do it right would need all sorts of integrations across the various pressure/density relationships of the substances involved, as was performed in Leger et al's Icarus paper on "ocean planets", but I think we can do something on the back of an envelope that might not be too far astray.
Your planet is made up of 2.25e25kg rock and 5.3e23kg water. With an assumed density of 5500km/m^3 for rock and 1000kg/m^3 for water, that gives you a rock core of 9920km radius, and an overall radius of 10340km, as a starting point from which to insert jigger factors. Surface gravity of such an object is 14.3N/kg, rising to 15.2N/kg at the surface of the rock core. So not too far wrong if we assume about 1.5g across the whole water column. With such gravity, pressure rises by 1MPa for every 68m of descent through the water column, so we reach the critical threshold of 1.1GPa for the Ice-VI transition at a depth of 75km. Our liquid ocean is therefore 75km deep, has a volume of 1e20m^3, and a mass of 1e23kg.
From our total mass of 5.3e23kg of water, that leaves 4.3e23kg of Ice-VI, at a density of 1300kg/m^3: a volume of 3.3e20m^3. That's sufficient to cover the rock core to a depth of 270km.
So 9920km of rock and metal, 270km ice and 75km liquid water, giving an overall radius of 10265km, a surface gravity of 1.5g and a density of 5070kg/m^3. (The radius has been reduced from our starting radius because of the increased density of Ice-VI, which didn't feature in the original calcs.)
Of course all this assumes that the water and rock are completely partitioned, whereas we know that a fair amount of water would be contained within the rock - you could jigger the thickness of the mantle downwards to allow for this. You might also reduce the overall radius slightly to allow for the compressibility of water and ice, but this isn't going to be dramatic, given the relatively thin layer they occupy.

Re your non-biogenic oxygen - isn't it just going to react with the other atmospheric components to form various nitrogen oxides, and nitrates in the ocean? Nitrogen and oxygen are so cheerfully reactive, you'd really need to posit some sort of life to keep them driven away from their natural equilibrium state - photosynthetic organisms and denitrifying bacteria, perhaps. Or you could have a murky brown nitrous oxide/nitrogen dioxide atmosphere over a sea of nitric acid ...

Grant

Thank you Grant; I'll give you an acknowledgement when the page goes up...
my amateurish estimation of the diameter gave 19,800km, so I was quite a bit out from your 20,530km...

the nitrate compounds would need to be dissociated by biological action, but I was planning on including some photosynethetic and other active organisms- without them the sea would be a mixture of nitric and sulphuric acid, a very potent mixture...

Anonymous wrote:my amateurish estimation of the diameter gave 19,800km, so I was quite a bit out from your 20,530km...

I think we're within spitting distance of each other, given the uncertainties involved. We've come within less than 5%.

Grant

Great!
Another newbie to this forum, John Dollan, has already included the 'waterworld' type in his long-established Planetary Classification list (here, under Panthelassic type)
http://www.onewest.net/~dollan/ARCpclindex.html

Cheers, John!
steve

Panthelassa is now a fully colonised waterworld;
http://www.orionsarm.com/worlds/Panthalassa.html

thank you Grant for giving Mr Raymond's model a lot more detail (which I have incorporated as far as possible)

Is there any way of getting more from these worlds than just a small picture, and the descriptions seen on the data panel. Maybe watching them inside Celestia? I've just watched the 'Fading Suns' Celestia Mod, and that seems to be a good start (althought several errors still have to be removed). I'd like to see something similar for 'Orions Arm'.

Greets,

maxim

I'll certainly make that a project for the future;
the textures need a little work in the meantime.
one of the things that is good about Celestia is the chance to see the stars from another solar system; you can make up a whole new set of constellations to reflect local preoccupations for instance.

Sounds good!
I'm looking forward for it.

maxim

eburacum45 wrote:I'll certainly make that a project for the future;
the textures need a little work in the meantime.
one of the things that is good about Celestia is the chance to see the stars from another solar system; you can make up a whole new set of constellations to reflect local preoccupations for instance.

The density of the stars in stars.dat drops the further one is from Sol, so if your addon planetary system is far from Sol, side of the sky with Sol in it would look different from the other side. With the system I am working on, the 100th closest star is about 38 light years away. With Sol, it's about 18 light years. It's 100 light years away in Delphinus. The nearest stars, the beta and the two gamma Delphinus stars, happen to be the brightest along with Canpous.
So is it a good idea to add some stars?

Brendan

Brendan,

You could create a star catalog (.stc file) defining an appropriate selection of stars in the neighborhood.

Alternatively, you might try Pascal Hartmann's addon stars.dat which adds almost 2million stars from the Tycho database. It's downloadable at http://perso.wanadoo.fr/celestia.stars/index.html

Whoo! Thanks for the plug, Steve! And sorry for being so late in noticing and replying.... Besides, for some reason I never get an email notification for responses to my posts here...

BTW, the PCL will be applicable fully to OA as well, as soon as Alan gets it posted on the site.

...John...

eburacum45 wrote:Great!
Another newbie to this forum, John Dollan, has already included the 'waterworld' type in his long-established Planetary Classification list (here, under Panthelassic type)
http://www.onewest.net/~dollan/ARCpclindex.html

Cheers, John!
steve