  Observations from the Kepler and K2 missions have provided the astronomical
community with unprecedented amounts of data to search for transiting
exoplanets and other astrophysical phenomena. Here, we present K2-288, a
low-mass binary system (M2.0 +/- 1.0; M3.0 +/- 1.0) hosting a small (Rp = 1.9
REarth), temperate (Teq = 226 K) planet observed in K2 Campaign 4. The
candidate was first identified by citizen scientists using Exoplanet Explorers
hosted on the Zooniverse platform. Follow-up observations and detailed analyses
validate the planet and indicate that it likely orbits the secondary star on a
31.39-day period. This orbit places K2-288Bb in or near the habitable zone of
its low-mass host star. K2-288Bb resides in a system with a unique
architecture, as it orbits at >0.1 au from one component in a moderate
separation binary (aproj approximately 55 au), and further follow-up may
provide insight into its formation and evolution. Additionally, its estimated
size straddles the observed gap in the planet radius distribution. Planets of
this size occur less frequently and may be in a transient phase of radius
evolution. K2-288 is the third transiting planet system identified by the
Exoplanet Explorers program and its discovery exemplifies the value of citizen
science in the era of Kepler, K2, and the Transiting Exoplanet Survey
Satellite.
