Pluto is solid, made up of mostly rock and ice, and actually, Pluto is no longer considered a planet. It was reclassified as a dwarf planet about 8 years ago due mostly to its small size compared to the other 8 planets, and the discovery of several other massive rocky bodies in the solar system that are similar in size or larger than Pluto ( including Eris, Ceres, Haumea and Makemake).