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The recent detections of two transit events attributed to the super-Earthcandidate K2-18b have provided the unprecedented prospect of spectroscopicallystudying a habitable-zone planet outside the Solar System. Orbiting a nearbyM2.5 dwarf and receiving virtually the same stellar insolation as Earth, K2-18bwould be a prime candidate for the first detailed atmospheric characterizationof a habitable-zone exoplanet using HST and JWST. Here, we report the detectionof a third transit of K2-18b near the predicted transit time using the SpitzerSpace Telescope. The Spitzer detection demonstrates the periodic nature of thetwo transit events discovered by K2, confirming that K2-18 is indeed orbited bya super-Earth in a 33-day orbit and ruling out the alternative scenario of twosimilarly-sized, long-period planets transiting only once within the 75-day K2observation. We also find, however, that the transit event detected by Spitzeroccurred 1.85 hours (7-sigma) before the predicted transit time. Our jointanalysis of the Spitzer and K2 photometry reveals that this early occurrence ofthe transit is not caused by transit timing variations (TTVs), but the resultof an inaccurate K2 ephemeris due to a previously undetected data anomaly inthe K2 photometry likely caused by a cosmic ray hit. We refit the ephemeris andfind that K2-18b would have been lost for future atmospheric characterizationswith HST and JWST if we had not secured its ephemeris shortly after thediscovery. We caution that immediate follow-up observations as presented herewill also be critical in confirming and securing future planets discovered byTESS, in particular if only two transit events are covered by the relativelyshort 27-day TESS campaigns.

Spitzer Observations Confirm and Rescue the Habitable-Zone Super-Earth K2-18b for Future Characterization