This plot shows exoplanets less than 20 Earth masses with known mass and radius, including error bars (light grey lines). Most of these are too hot for life. Only K2-3 d seems to be a rocky world in the habitable zone. The planets in this plot are color coded by their parent star spectral type. Lines for some densities and theoretical pure compositions of iron, rock, and water are shown for reference (Seager et al., 2017). Mars, Venus, Earth, Uranus, and Neptune are included for comparison. Data is from the NASA Exoplanet Archive. This is large plot intended for viewing at high resolutions, click to enlarge. Credit: PHL @ UPR Arecibo.