Pluto might be small, but it’s not boring. When the New Horizons spacecraft flew by in 2015, it found mountains up to 6 km high—made entirely of water ice. On Pluto, water ice is as hard as rock, because temperatures are below -220°C. The surface also has vast nitrogen glaciers, which flow like slow rivers of frozen air. So even at the edge of the solar system, there’s geological activity shaping an alien landscape.