Left column (top to bottom): Lomb-Scargle periodograms of the raw radial velocities (RV), the K2 photometry (Phot), the HARPS window function (WF), S index, Hα, full width half maximum (FWHM), and bi-sector inverse slope (BIS) time series. The orbital periods of K2-18b, K2-18c, the stellar rotation period, and its first harmonic are highlighted with vertical dashed lines. Right column: the false alarm probability as a function of normalized periodogram power for each time series. The FAP curve for the photometry spans very low power and is barely discernible in its subpanel. Suffice it to say that any signal with normalized power >10^-2 has a FAP ≪ 0.1%. The FAP curve for the S index exhibits FAP ≳ 30% for all power visible on its ordinate and therefore is only visible in the upper right of its subpanel.