You may have heard about the recent discovery of an Earth-sized planet that is not quite twice the mass of Earth. Remember, this isn’t an Earth-like planet because it is much too close to the star it orbits to be habitable. I happened to find this announcement especially interesting because I had just attended a talk the day before where Arecibo’s role in exo-planet detection was mentioned. Pulsar B1257+12 was observed at Arecibo and, through close examination of the data, astronomers were able to determine that it hosted two planets, of a few Earth masses, in 1992. Since then, they have detected a third planet and a possible fourth. These extra-solar planets were discovered several years before the “first” exoplanet detection around 51 Pegasi in 1995. I think it’s worth remembering that the first exoplanets, and the first approximately Earth-mass planets, were discovered by the Arecibo radio telescope. Now, you aren’t going to be able to find habitable planets around a pulsar (at least, not habitable to any sort of life we know), but the fact that planets can exist there offers a lot of insight into theories of planet formation and stellar evolution.