The Kepler space telescope (and several observatories on the ground) have pinpointed a plenitude of planets around other stars, but astronomers' knowledge of them remains fuzzy. A new European mission launching in five years will bring them into focus, figuring out their size, density and internal structure.
The new telescope is called CHEOPS, for CHaracterising ExOPlanets Satellite, although it is not shaped like a pyramid. Its targets will be nearby stars that are known to harbor planets. Like Kepler, it will use the transit method of hunting planets, looking for blips in star brightness to tell if something is orbiting around them. This will allow more accurate measurements of a given planet's radius.
Astronomers know the masses of several planets, partly through observations that measure how the planets affect the wobbling of their stars. Given a radius and a mass, you can figure out density, and this will give clues about the planet's internal makeup. This will help astronomers learn how other planets form, especially the rocky super-Earths.