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We present new observations of the multi-planet system HIP~41378, a brightstar ($V$ = 8.9, $K_s$ = 7.7) with five known transiting planets. Observationsin Campaign 5 of the K2 mission showed multiple transits of two Neptune-sizedbodies and single transits of three larger planets ($R_P= 0.33R_J, 0.47R_J,0.88 R_J$). K2 recently observed the system again in Campaign 18. We observeone new transit each of two of the larger planets, HIP41378~d and~f, givingmaximal possible orbital periods of 1114 and 1084 days, respectively. Otherpossible periods include integer divisions of these maximum values down to alower limit of about 50 days. We use all available photometry to determine theeccentricity distributions of HIP41378~d \& f, finding that periods$\lesssim$300 days require non-zero eccentricity. We also perform a stabilityanalysis on the orbits of planets d and f to assess the likelihood of theirdifferent possible periods, finding that short periods (P $<$ 300 days) aredisfavored. In addition, we observe transits of the inner two planets b and cwith Spitzer/IRAC, which we combine with the new K2 observations of theseplanets to search for transit timing variations (TTVs). We find a linearephemeris for planet b, but see a significant TTV signal for planet c thatcould be induced by planet d, e, or f. The ability to recover the two smallerplanets with Spitzer shows that the several planets in this fascinating systemwill continue to be detectable with Spitzer, CHEOPS, TESS, and otherobservatories. This will allow us to precisely determine the periods of d andf, characterize the TTVs of planet~c, recover the transits of planet~e, andfurther enhance our view of this remarkable dynamical laboratory.

Revisiting the HIP41378 system with K2 and Spitzer