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: 
An interesting idea would be to have a hollow planet... An old dr.who episode features a pretty awesome plot, where the villains control a giant pirate ship planet that is hollow and can mine other planets by teleporting around them
A cache of negative matter
--------------------------

Everything is simple except for making a giant planet with similar gravity to Earth's... so add some unobtanium, which conveniently explains why the planet's minerals are such a valuable resource.

Perhaps the planet has a cache of *[negative matter](https://en.wikipedia.org/wiki/Negative_mass)* inside it. This is *not* antimatter, but a hypothetical form of exotic matter that has negative gravity, which repels mass instead of attracting it. Such material is necessary for stable [wormholes](https://en.wikipedia.org/wiki/Wormhole) and [one possible FTL warp drive](https://en.wikipedia.org/wiki/Alcubierre_drive), so if those exist in your universe I would imagine negative matter to also exist, and moreover be very valuable.

There are no known particles with negative mass, but they are theoretically possible according to the current understanding of physics.

Your planet would have to be almost exactly half normal matter and half negative matter, with only an Earth's-worth of regular matter more. However, it should be reasonably stable; negative matter is still attracted by normal gravity, so you don't have to worry about the planet spontaneously exploding or negative crystals falling into the sky as soon as they are mined.