Zodiacal Exoplanets in Time (Zeit). I. A Neptune-Sized Planet Orbiting an M4.5 Dwarf in the Hyades Star Cluster

Abstract

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 K2 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 K2-25ʼs 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 Kepler M dwarfs (12%-20%). We use the derived stellar density as a prior on fitting the K2 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 (3.43 0.31 0.95 -+ R ⊕ ) much larger than older Kepler planets with similar orbital periods (3.485 days) and host-star masses (0.29 M e ). 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 K2 will help confirm whether this planet is atypical or representative of other close-in planets of similar age.

K2 Discovers a Busy Bee: An Unusual Transiting Neptune Found in the Beehive Cluster

“…The slow rotation of this intermediate-age M dwarf is remarkable when considering its close-in planet (see Section 4) and may indicate differences in angular momentum evolution due to initial conditions, the primordial disk, planet formation, or planet migration. In contrast, the very similar Hyades M dwarf planetary system K2-25 is among the fastest rotating M dwarfs in that cluster with a period of ∼1.9 days (Douglas et al 2014;David et al 2016a;Mann et al 2016a). …”

“…The combined RV constraints from our multi-epoch HIRES observations are described further in Section 4.1. Note.References are: 1-EPIC Catalog, 2- Kraus & Hillenbrand (2007), 3-this work, 4-Pan-STARRS1 3π catalog (version PV3), 5-2MASS catalog, 6 -this work, using (Mann et al 2016a), 7-this work, using SED fitting from Obermeier et al (2016), 8-this work, using Newton et al (2015).…”

“…Initial transit surveys that focused on 47 Tuc (Gilliland et al 2000;Weldrake et al 2005), NGC 2301 (Howell et al 2005) and NGC 7789 (Bramich & Horne 2006), found no evidence for transiting planets. Since then, fourteen planets have been discovered in open clusters, namely in NGC 6811 (Meibom et al 2013), NGC 2423 (Lovis & Mayor 2007), M67 (Brucalassi et al 2014(Brucalassi et al , 2016, the Beehive (Praesepe) (Quinn et al 2012;Malavolta et al 2016), the Hyades (Sato et al 2007;Quinn et al 2014;David et al 2016a;Mann et al 2016a) and Upper Scorpius (David et al 2016b;Mann et al 2016b). All planets in M67, the planet in NGC 2423, one planet in the Hyades and the Praesepe planets were detected with the radial velocity (RV) method.…”

Rotation of Late-type Stars in Praesepe with K2

“…Libralato et al (2016) derived rotation periods as a step in searching the LCs for exoplanet transits, but they did not discuss the periods nor the shapes of the phased LCs. Mann et al (2016a) similarly derive rotation periods while searching for exoplanet transits; they illustrate the distribution of rotation periods as a function of M k and color, but do not otherwise discuss the periods or LC morphology. Finally, our group plotted an early version of the Praesepe period distribution in Stauffer et al (2016) in the context of comparing the Pleiades rotation periods to those in other clusters.…”

Iota Horologii Is Unlikely to Be an Evaporated Hyades Star

“…At the time of its discovery, this planet had the most Earth-like orbit known. Exoplanets in the Hyades appeared elusive (Paulson et al 2004) until recent discoveries (Quinn et al 2014;Mann et al 2016). If a true Hyades member, ι Hor would add significantly to the planet statistics of this cluster.…”