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We observed Kepler-421 during the anticipated third transit of the snow-lineexoplanet Kepler-421b in order to constrain the existence and extent of transittiming variations (TTVs). Previously, the Kepler Spacecraft only observed twotransits of Kepler-421b leaving the planet's transit ephemeris unconstrained.Our visible light, time-series observations from the 4.3-meter DiscoveryChannel Telescope were designed to capture pre-transit baseline and the partialtransit of Kepler-421b barring significant TTVs. We use the light curves toassess the probabilities of various transit models using both the posteriorodds ratio and the Bayesian Information Criterion (BIC) and find that a transitmodel with no TTVs is favored to 3.6-sigma confidence. These observationssuggest that Kepler-421b is either alone in its system or is only experiencingminor dynamic interactions with an unseen companion. With the Kepler-421bephemeris constrained, we calculate future transit times and discuss theopportunity to characterize the atmosphere of this cold, long-period exoplanetvia transmission spectroscopy. Our investigation emphasizes the difficultiesassociated with observing long-period exoplanet transits and the consequencesthat arise from failing to refine transit ephemerides.

No Timing Variations Observed in Third Transit of Snow-Line Exoplanet Kepler-421b