ZODIACAL EXOPLANETS IN TIME (ZEIT). I. A NEPTUNE-SIZED PLANET ORBITING AN M4.5 DWARF IN THE HYADES STAR CLUSTER

<jats:title>ABSTRACT</jats:title> <jats:p>Studying the properties of young planetary systems can shed light on how the dynamics and structure of planets evolve during their most formative years. Recent <jats:italic>K</jats:italic>2 observations of nearby young clusters (10–800 Myr) have facilitated the discovery of such planetary systems. Here we report the discovery of a Neptune-sized planet transiting an M4.5 dwarf (K2-25) in the Hyades cluster (650–800 Myr). The light curve shows a strong periodic signal at 1.88 days, which we attribute to spot coverage and rotation. We confirm that the planet host is a member of the Hyades by measuring the radial velocity of the system with the high-resolution near-infrared spectrograph Immersion Grating Infrared Spectrometer. This enables us to calculate a distance based on K2-25's kinematics and membership to the Hyades, which in turn provides a stellar radius and mass to ≃5%–10%, better than what is currently possible for most <jats:italic>Kepler</jats:italic> M dwarfs (12%–20%). We use the derived stellar density as a prior on fitting the <jats:italic>K</jats:italic>2 transit photometry, which provides weak constraints on eccentricity. Utilizing a combination of adaptive optics imaging and high-resolution spectra, we rule out the possibility that the signal is due to a bound or background eclipsing binary, confirming the transits’ planetary origin. K2-25b has a radius (<jats:inline-formula> <jats:tex-math> <?CDATA ${3.43}_{-0.31}^{+0.95}$?> </jats:tex-math> <?MML <mml:math> <mml:msubsup> <mml:mrow> <mml:mn>3.43</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.31</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.95</mml:mn> </mml:mrow> </mml:msubsup> </mml:math>?> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522308ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> <jats:italic> R</jats:italic> <jats:sub>⊕</jats:sub>) much larger than older <jats:italic>Kepler</jats:italic> planets with similar orbital periods (3.485 days) and host-star masses (0.29 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>). This suggests that close-in planets lose some of their atmospheres past the first few hundred million years. Additional transiting planets around the Hyades, Pleiades, and Praesepe clusters from <jats:italic>K</jats:italic>2 will help confirm whether this planet is atypical or representative of other close-in planets of similar age.</jats:p>