Some astronomers searching for extraterrestrial life have proposed that atmospheric NH₃ (ammonia) can serve as a biosignature gas—an indication that a planet harbors life. Jingcheng Huang, Sara Seager, and colleagues evaluated this possibility, finding that on rocky planets, atmospheric NH₃ likely couldn’t reach detectably high levels in the absence of biological activity. But the team also found that on so-called mini-Neptunes—gas planets smaller than Neptune but with atmospheres similar to Neptune’s—atmospheric pressure and temperature can be high enough to produce atmospheric NH_3.