The planet is also among the first batch of exoplanets discovered with direct imaging. Usually, astronomers just look at stars for a while and wait for the exoplanet to pass in front, like a distant eclipse. Direct imaging finds planets by detecting the heat the planets themselves give off instead. The thing is, direct imaging can only see big, hot planets far from their host star, since the filtering used to get rid of all the non-planet light might also filter out smaller closer planets, and colder planets won't emit enough infrared light. Even at four times Jupiter's mass, HD 131399Ab is still one of the lightest planets found with direct imaging.