help

You would want to write: If it were far from the sun, the earth woule be much too cold to support living things.

I have corrected some other small errors in that sentence.

"Much too" is an intensifier--that means that it is a strong expression, stronger than simply using "too" alone.

"Too much" means more than is needed, or more than is acceptable. It almost always refers to a predicate nominative or the subject of a sentence, such as: "His wild ideas were too much for me." In that case, "too much" refers to "wild ideas."