Is there a second Earth out there in space? Our knowledge of planetary systems far, far away is increasing constantly, as new technologies continue to sharpen our gaze into space. To date, 3,700 planets have already been discovered outside our solar system. The planetary masses and radii of these exoplanets can be used to infer their mean density, but not their exact chemical composition and structure. The intriguing question about what these planets could look like is thus still open.