We've had proof of large rocky bodies for 4.5 billion years By large, I mean significantly larger than Earth. We know about rocky bodies up to one Earth mass.

We've had proof of large rocky bodies for 4.5 billion years By large, I mean significantly larger than Earth. We know about rocky bodies up to one Earth mass. We also know about low-density ice-gas bodies of 14 Earth masses or more. Bodies in between these two known types of planet, though, we're pretty ignorant about -- are they Earthlike or Uranuslike? Now, while N=1 makes for lousy statistics, we know that at least this one is Earthlike. We're actually still astonishingly ignorant about how planets form in the first place, so anything we can find out about the compositions of exoplanets is pretty valuable.