I've created a map editor for a game I've been developing. The maps (planets) are made using a form of meta ball editing. Basically, in the end the Marching Cubes algorithm is used to get my final model.

Here's an example:

In the game however, when planets are or a largish size, as you would expect, I start having quite a lot of lag. So what I was wondering, is are there any good libraries out there that could be used to remove clusters or triangles that are basically flat and turn them into a single triangle?

I'm not after simply reducing the count, I want the same amount of detail, but to merge triangles where their normal is practically the same.

Also curious, as to how you would go about having more/less detail as you approach/leave areas of a planet. Obviously chunks isn't the easiest method when the world's a sphere.

The project is in C#.