K2-288Bb: A Small Temperate Planet in a Low-mass Binary System Discovered by Citizen Scientists

Observations from the Keplerand K2 missions have provided the astronomical community with unprecedented amounts of data to search for transiting exoplanetsand other astrophysical phenomena. Here, we present K2-288, a low-massbinary system (M2.0 ± 1.0; M3.0 ± 1.0) hosting a small (R_p = 1.9 R⊕), temperate (T_(eq) = 226 K) planetobserved in K2 Campaign 4. The candidate was first identified by citizen scientists using ExoplanetExplorers hosted on the Zooniverse platform. Follow-up observations and detailed analyses validate the planetand 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-masshost 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 (a_(proj) ~ 55 au), and further follow-up may provide insight into its formation and evolution. Additionally, its estimated size straddlesthe observed gap in the planetradius distribution. Planetsof this size occur less frequently and may be in a transient phase of radius evolution. K2-288 is the third transiting planet system identifiedby the ExoplanetExplorers program and its discovery exemplifies the value of citizen sciencein the era of Kepler, K2, and the Transiting ExoplanetSurvey Satellite.