In the story of the three bears, Goldilocks was choosy about which of three bowls of porridge she ate from (first was too cold, second was too hot, third was just right) which of three chairs she sat in (first too high, second too broad, third just right), and which of three beds to sleep in (too soft, too hard, just right).
Just last week lexicographer Ben Zimmer wrote a piece in the Wall Street Journal called "How Goldilocks Moved to Space and the World of Economists" which includes:
- Goldilocks principle
- Goldilocks problem
- Goldilocks planet
- Goldilocks zone
- Goldilocks economy
- Goldilocks theory
- Goldilocks market
Goldilocks was chosen because a habitable planet is not too close or too far from its star. For example, Venus is too hot, Mars is too cold, but Earth is just right.
This is a noun modifier where the noun Goldilocks is acting like an adjective and modifying a noun like planet.
It's probably widespread enough to be added to almost any noun with similar qualities to those listed: one that's not too much this, not too much that, but is just right.