[fgicwxyo] Increasing a planet's gravity

Problem: you've got a planet with gravity too weak to hold a desirable atmosphere.  Solution: increase gravity by increasing density (but constant mass), transmuting the planet's material into a denser material, like iridium.  It's terraforming with a Philosophers' Stone.

What's the least massive planet that can be converted this way?  The density of iridium increases with pressure, just like iron at the earth's core.