A Compact Multi-planet System with a Significantly Misaligned Ultra Short Period Planet

We report the discovery of a compact multi-planet system orbiting the relatively nearby (78 pc) and bright (K = 8.9) K-star, K2-266 (EPIC 248435473). We identify up to six possible planets orbiting K2-266 with estimated periods of P _b = 0.66, P _.02 = 6.1, P _c = 7.8, P _d = 14.7, P _e = 19.5, and P _.06 = 56.7 days, and radii of R _P = 3.3 R _⊕, 0.646 R _⊕, 0.705 R _⊕, 2.93 R _⊕, 2.73 R _⊕, and 0.90 R _⊕, respectively. We are able to confirm the planetary nature of two of these planets (d and e) by analyzing their transit timing variations ({m}_d={8.9}_-3.8^+5.7 {M}_\oplus and {m}_e={14.3}_-5.0^+6.4 {M}_\oplus ), confidently validate the planetary nature of two other planets (b and c), and classify the last two as planetary candidates (K2-266.02 and .06). From a simultaneous fit of all six possible planets, we find that K2-266 b’s orbit has an inclination of 75.°32 while the other five planets have inclinations of 87°-90°. This observed mutual misalignment may indicate that K2-266 b formed differently from the other planets in the system. The brightness of the host star and the relatively large size of the sub-Neptune sized planets d and e make them well-suited for atmospheric characterization efforts with facilities like the Hubble Space Telescope and upcoming James Webb Space Telescope. We also identify an 8.5 day transiting planet candidate orbiting EPIC 248435395, a co-moving companion to K2-266.