So, astronomers calculated that a planet twice the size of Earth would simply start to accumulate gas and eventually turn into a small gas giant.
Enter Kepler-10c, which is 2.3 times the size of Earth and is emphatically not a gas giant. (And yes, they've also eliminated it being the core of a dead gas giant that somehow lost its astmosphere).
So, it shouldn't exist. And it even more shouldn't exist because it is 10 billion years old and formed before we believe there to have been enough rock to make any kind of earth-like planet.
Now I want to go poke around and see what's with this planet. It's a bit too big to be a Death Star...