Hi,

I'm working on a story and am in love with the idea of a planet that has roughly four times the surface area of current Earth. So far I have concluded the Radius would need to be 12,500km (just under what I desire) and the mass would be 22.911187 * 10^24 kg to have almost exactly the same surface gravity as earth.

The main part of my story isn't the planet itself or the science that created the planet. However, being the geek that I am having the planet be some-what realistic would be amazing, of course if it can't work then it's not a big loss.

The problem I've encountered so far is the density. Having a planet that size would mean hiking across huge lands and countries made of... foam. I was wondering if there was any sufficient way to make the density be the same (as in, from the surface a human wouldn't know it was another planet... rocks, trees etc.).

I've thought about the core of the planet, maybe it being something other that Iron. I've done some reading and found that Gold or Platinum are extremely rare and I shouldn't count on a planet having something like that.

So now I turn to you with hopefully puppy dog eyes to help me out... if you can :thumbsup: at this point I'm beginning to feel the futility of my efforts.

Thanks guys!

By using a very small nickel iron core (<1,000 km), a very thick anorthosite crust and a basalt composition mantle I was able to come up with a surface gravity just 13% greater than the Earth. That should be manageable by terrestrial organisms. I omitted to save the spreadsheet, but I think the planetary mass was around 24 * 10^24 kg and the diameter as you specified.

That's right; with a smaller metallic core a planet with Earth-like gravity could be quite a bit larger. The largest terrestrial type planets with Earth-like gravity would probably be waterworlds, with very deep oceans and ice underneath that; but that would mean no land for your inhabitants to live on.
I recommend this paper
http://arxiv.org/abs/0704.3454
for some information about bulk properties of superEarths.

Incidentally, one way to get more land surface for a planet is to have smaller oceans; the Earth's surface is 70% water- by carefully distributing small continents throughout the oceans, you could create much more habitable area without causing too much of the surface to become uninhabitable desert.

What abouth a snowball super water world. A large waterworld conisting for 50% out of ice where live developed in a underground liquid ocean. Some of this life uses a heat source to burst out of the ice layer to form organic islands on a ice sea. Forming a organic permafrost layer of only a couple of meters over a large ice sea.

It's a very dry world yet there is plenty of plant life because the roots can reach the ice under it.

Could this exist?

What is the likelyhood of those planets existing?

Also.... how do you calculate those things, my geeky friends? :D

What abouth a snowball super water world. A large waterworld conisting for 50% out of ice where live developed in a underground liquid ocean. Some of this life uses a heat source to burst out of the ice layer to form organic islands on a ice sea. Forming a organic permafrost layer of only a couple of meters over a large ice sea.

It's a very dry world yet there is plenty of plant life because the roots can reach the ice under it.

Could this exist?
This sounds like a fascinating possibility.

One way to increase the effective temperature in the plant layer would be to use biologically grown greenhouses; the plants could grow transparent bubbles over themselves, filled with carbon dioxide from their respiration, to trap as much heat from the local star as possible.

These warm bio-greenhouses would need to be insulated from the ice below or they would melt holes in it and sink.

Maybe the trees on the islands produce some heat to get some of the ice liquid as a source of water while they also give of some heat to the surrounding place. And therefore create a sort of microclimate like you get around most city's where it's warmer inside the city then the urban area.
http://www.metoffice.gov.uk/education/secondary/students/microclimates/images/Londonheatisland.gif

then again maybe a large inner moon has broken up and rainend down onto the surface creating a partial organic soil on a ice subsurface (it worked for Iaptus (http://en.wikipedia.org/wiki/Iapetus_%28moon%29#Two-tone_coloration))

You could also do a large moon scenario. It would help with anything you're trying to say. For example, some of Jupiter's moons are tidally locked. I'm pretty sure some heat is created as a result of that.

I had a good link for this, but I'm not allowed to post them yet.

Also what is the story going to be on, if you decided.