What I would do is get the highest resolution data that you can find that would reasonably fit on your target client computer and then fill the rest in with procedural functions to get something that looks good as a player walks round. You can make large planets in high resolution, even earth sized as long as it's pseudo-random. The problem is trying to replicate existing terrain.

Gnollrunner said:
The problem is trying to replicate existing terrain.

I actually work on this. Result from erosion simulation over some simple initial noise:

The Gaea tool (has free version) takes one second to calculate something like that. So i think you could do it too as a background task to make procedural planets more natural and interesting, eventually.

Nice realtime demo - press c to drop water (source on github): https://lanlou123.github.io/Webgl-Erosion/​