Zodiacal Exoplanets in Time (ZEIT). VI. A Three-planet System in the Hyades Cluster Including an Earth-sized Planet

<jats:title>Abstract</jats:title> <jats:p>Planets in young clusters are powerful probes of the evolution of planetary systems. Here we report the discovery of three planets transiting EPIC 247589423, a late-K dwarf in the Hyades (≃800 Myr) cluster, and robust detection limits for additional planets in the system. The planets were identified from their <jats:italic>K2</jats:italic> light curves as part of our survey of young clusters and star-forming regions. The smallest planet has a radius comparable to Earth (<jats:inline-formula> <jats:tex-math> <?CDATA ${0.99}_{-0.04}^{+0.06}{R}_{\oplus }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaa9791ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>), making it one of the few Earth-sized planets with a known, young age. The two larger planets are likely a mini-Neptune and a super-Earth, with radii of <jats:inline-formula> <jats:tex-math> <?CDATA ${2.91}_{-0.10}^{+0.11}{R}_{\oplus }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaa9791ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${1.45}_{-0.08}^{+0.11}{R}_{\oplus }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ajaa9791ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, respectively. The predicted radial velocity signals from these planets are between 0.4 and 2 m s<jats:sup>−1</jats:sup>, achievable with modern precision RV spectrographs. Because the target star is bright (<jats:italic>V</jats:italic> = 11.2) and has relatively low-amplitude stellar variability for a young star (2–6 mmag), EPIC 247589423 hosts the best known planets in a young open cluster for precise radial velocity follow-up, enabling a robust test of earlier claims that young planets are less dense than their older counterparts.</jats:p>