The masses , atmospheric makeups , spin-orbit alignments , and system architectures of extrasolar planets can be best studied when the planets orbit bright stars .
We report the discovery of three bodies orbiting HD 106315 , a bright ( V = 8.97 mag ) F5 dwarf targeted by our K2 survey for transiting exoplanets .
Two small , transiting planets have radii of 2.23 ^ { +0.30 } _ { -0.25 } R _ { \oplus } and 3.95 ^ { +0.42 } _ { -0.39 } R _ { \oplus } and orbital periods of 9.55 d and 21.06 d , respectively .
A radial velocity ( RV ) trend of 0.3 \pm 0.1 \mathrm { m~ { } s } ^ { -1 } \mathrm { ~ { } d } ^ { -1 } indicates the likely presence of a third body orbiting HD 106315 with period \gtrsim 160 d and mass \gtrsim 45 M _ { \oplus } .
Transits of this object would have depths of \gtrsim 0.1 % and are definitively ruled out .
Though the star has v \sin i = 13.2 km s ^ { -1 } , it exhibits short-timescale RV variability of just 6.4 \mathrm { ~ { } m~ { } s } ^ { -1 } , and so is a good target for RV measurements of the mass and density of the inner two planets and the outer object ’ s orbit and mass .
Furthermore , the combination of RV noise and moderate v \sin i makes HD 106315 a valuable laboratory for studying the spin-orbit alignment of small planets through the Rossiter-McLaughlin effect .
Space-based atmospheric characterization of the two transiting planets via transit and eclipse spectroscopy should also be feasible .
This discovery demonstrates again the power of K2 to find compelling exoplanets worthy of future study .