Question: I hope this isn't too broad.

I would like an Earth-like planet for November's [NaNoWriMo](http://nanowrimo.org/) that is just enormous, but plausible.

Then I started to get into material composition, proximity to the star, trying to achieve the right densities - well, you get the idea.

I'm trying to have 10x the radius of our current Earth, but with similar gravity (ugh) and at least somewhat similar climate and surface conditions; I do not need to have life on this planet, but minerals and liquid water are important to us. Not quite a Jupiter-sized planet, but much larger than ours. Other factors can be modified.

Answer: 
Let me say it outright from the start: you can't get a $10 \times R\_\oplus$ radius world to be inhabitable by anything resembling humans.

Before you despair, let me remind you that planetary surface increases with the square of the radius, and Earth is huge to begin with: 510 million square kilometers. So for twice the radius, you get 2.04 **billion** square km, and for three times the radius, you get 4.5 billion square km, nearly 10 times larger!

Imagine how different history would have been had the Earth been that large. The larger surface area might mean slower days with longer nights, or perhaps faster orbital rotation. The terror of the night would be prolonged, or storms might be more massive than anything we know here on Earth. The vast oceans would regulate and moderate climate more than on Earth with their vast heat capacity, while waves might build up over the ocean vasts to truly astonishing heights (100m or more). The atmosphere itself would likely be deeper and denser, and given the higher planetary mass, retain more of the primordial gases.

**So how do you get such a thing?** The most likely process would be to start from a [Chthonian planet](http://en.wikipedia.org/wiki/Chthonian_planet), have it drift away from the star and rebuild some of the atmosphere from comets and eruptions.
The real SFNal way to do this is simply to "pave over" Saturn. A "supra-mundane terraformed world of Saturn" essentially has the atmosphere of Saturn covered over with some material which provides a solid "floor" for people to sand and work on, support buildings and so on.

Materials like graphene or a "fabric" woven from Fullerine cables will provide the high strength to weight ratio needed to make this plausible, and while it may seem a bit like standing and working on a trampoline, the reality is the structure will be so vast that most flexing will be damped out and you and I would hardly feel anything.

Now the questions of atmosphere and climate will be more difficult to answer. An atmosphere can certainly be added to the outside of the supra-mundane world of Saturn, importing materials from the many moons of Saturn and importing extra nitrogen from the distant moons of Neptune and bodies from the Kuiper belt (if you can cover over Saturn, you can easily import materials from deep space), as well as oceans and soil to support whatever sorts of biospheres you desire. Since you are so far from the Sun, a system of mirrors will be needed in orbit to provide extra illumination. 

Weather and climate will be very different from that of Earth simply due to the effects of scale, and there will also be lots of variables based on how you compose the atmosphere, the percentage of land to ocean you choose to create and the details of how and where the platoons of mirrors orbit and illuminate the planet, so there will be a lot of handwaving that can be done to meet your needs as a story teller. I would expect that the early period where the atmosphere and biologicals are being added will be marked by violent discontinuities as equilibriums are established and then disrupted, so people living there might not settle on the supra-mundane world unit late in the building and seeding process.

Actively moving Saturn into a closer orbit around the Sun is also possible, but the amount of time and energy needed is "astronomical". If a sufficient number of asteroids, comets or similar sized bodies could be sent in looping orbits from the Kruiper belt to pass by Saturn, the planet could transfer some of its orbital energy to these bodies (speeding them up and flinging them from the Solar System) and gradually moving into a closer orbit around the Sun. This will require a lot of planning, as these bodies could crash into planets or spacecraft in the outer system, while as Saturn approaches Jupiter orbit, unexpected effects might arise out of gravitational interactions, resonance orbits between Saturn and the Asteroid belt and even the inner planets. It might be better to stick with mirrors.

So maybe our descendants in AD 2500 might be starting a project of this magnitude, it will certainly need a vast amount of resources and the ability to control vast amounts of matter and energy in order to do so.