Two Small Transiting Planets and a Possible Third Body Orbiting HD 106315

Abstract

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 ⊕ and 3.95 +0.42 −0.39 R ⊕ and orbital periods of 9.55 d and 21.06 d, respectively. A radial velocity (RV) trend of 0.3±0.1 m s −1 d −1 indicates the likely presence of a third body orbiting HD 106315 with period 160 d and mass 45M ⊕ . Transits of this object would have depths of 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 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.

Precise masses for the transiting planetary system HD 106315 with HARPS

“…Therefore, this star cannot be the source of the eclipses and has a negligible impact on the measurement of the planet radius (a correction of around 0.002%). Hence, our results confirm the non-detection of close-in companions to HD 106315 within 10 arcsec presented by Crossfield et al (2017), whose observations have a higher sensitivity closer to the target. We also exclude the existence of significant contaminating stars present at larger distances, but inside the K2 aperture.…”

“…The light curve is dominated by red noise due to granulation and we cannot find a clear rotation period. Moreover, the K2 light curve shows two sets of planetary signals that were previously reported by Crossfield et al (2017) and Rodriguez et al (2017). Planet b shows a transit depth of 297 ± 34 ppm, while planet c shows a transit depth of 944 ± 21 ppm.…”

“…Since the star is relatively hot and fast-rotating, measuring precise radial velocities is challenging (Rodriguez et al 2017). Crossfield et al (2017) performed a first radial velocity follow-up of this target with the HiReS spectrograph at the Keck telescope. They used exposure times of three to six minutes, and observed a large rms at the level of 6.4 m s −1 .…”

“…The inner planet, HD 106315b, has a period of 9.55 days and a radius of ∼ 2.4 R ⊕ , while HD 106315c, the outer planet, has a period of 21.05 days and a radius of ∼ 4.2 R ⊕ . Crossfield et al (2017) obtained radial velocities of HD 106315, which did not constrain the mass of either planet but revealed a long term trend that suggests the presence of an additional third body in the system. The host star, HD 106315, has V=9.0 mag, which is currently the third brightest planet host star found by K2.…”

“…Two teams simultaneously announced the discovery of two planets with small radii (Crossfield et al 2017;Rodriguez et al 2017). The inner planet, HD 106315b, has a period of 9.55 days and a radius of ∼ 2.4 R ⊕ , while HD 106315c, the outer planet, has a period of 21.05 days and a radius of ∼ 4.2 R ⊕ .…”