We report the detection of Kepler-47, a system consisting of two planets orbiting around an eclipsing pair of stars. The inner and outer planets have radii 3.0 and 4.6 times that of Earth, respectively. The binary star consists of a Sun-like star and a companion roughly one-third its size, orbiting each other every 7.45-days. With an orbital period of 49.5-days, 18 transits of the inner planet have been observed, allowing a detailed characterization of its orbit and those of the stars. The outer planet's orbital period is 303.2-days, and although the planet is not Earth-like, it resides within the classical "habitable zone", where liquid water could exist on an Earth-like planet. With its two known planets, Kepler-47 establishes that close binary stars can host complete planetary systems.
An accurate characterization of the known exoplanet population is key to understand the origin and evolution of planetary systems. The determination of true planetary masses through the radial velocity (RV) method is expected to experience a great improvement thanks to the availability of ultra-stable echelle spectrographs. We took advantage of the extreme precision of the new- generation echelle spectrograph ESPRESSO to characterize the transiting planetary system orbiting the G2V star K2-38 located at 194pc from the Sun with V~11.4. This system is particularly interesting because it could contain the densest planet detected to date. We carried out a photometric analysis of the available K2 photometric light curve of this star to measure the radius of its two known planets K2-38b and K2-38c with Pb=4.01593+/-0.00050d and Pc=10.56103+/-0.00090d, respectively. Using 43 ESPRESSO high-precision radial velocity measurements taken over 8 months along with the 14 previously published HIRES RV measurements, we modeled the orbits of the two planets through a Markov Chain Monte Carlo (MCMC) analysis, significantly improving their mass measurements. Using ESPRESSO spectra we derived the stellar paremeters, Teff=5731+/-66K, logg=4.38+/-0.11dex, and [Fe/H]=0.26+/-0.05dex, and thus the mass and radius of K2-38, Ms=1.03^+0.04^_-0.02_M_{sun}_ and Rs=1.06^+0.09^_-0.06_R_[sun}_. We determined new values for the planetary properties of both planets. We characterized K2-38b as a super-Earth with Rp=1.54+/-0.14R_{earth}_ and Mp=7.3^+1.1^_-1.0_M_{earth}_, and K2-38c as a sub-Neptune with Rp=2.29+/-0.26R_{earth}_ and Mp=8.3+/-1.3M_{earth}_. Combining the radius and mass measurements, we derive a mean density of rho_p_=11.0^+4.1^_-2.8_g/cm^3^ for K2-38b and rho_p_=3.8^+1.8^_-1.1_g/cm^3^ for K2-38c, confirming K2-38b as one of the densest planets known to date. The best description for the composition of K2-38b comes from an iron-rich Mercury-like model, while K2-38c is better described by an ice-rich model. The maximum collision stripping boundary shows how giant impacts could be the cause for the high density of K2-38b. The irradiation received by each planet places them on opposite sides of the radius valley. We find evidence of a long-period signal in the radial velocity time-series whose origin could be linked to a 0.25-3M_Jup_ planet or stellar activity.
This is the first in a series of papers in which we analyze medium-resolution spectra of over 400 K and M giants in Baade's Window. Our sample was selected from the proper motion study of Spaenhauer et al. (1992AJ....103..297S). We have measured radial velocities for most of the sample, as well as line-strength indices on the system of Faber et al. (1985ApJS...57..711F). We analyze the random and systematic errors in velocities and line strengths, and show that the bright (V<16.0) stars in our sample are predominantly foreground disk stars along the line of sight toward Baade's Window. We find that most of the bulge K giants have stronger Mg absorption at a given color than do stars in the solar neighborhood. If the K giants in our sample are moderately old, we suggest that on average they may have [Mg/Fe] approximately +0.3, consistent with the results of recent high-resolution spectroscopy in Baade's Window.
We present radial-velocity (RV) measurements for the K giant stars HD 25723, 17 Sco, 3 Cnc and 44 UMa, taken at the Lick Observatory between 2000 and 2011. The best Keplerian fits to the data yield minimum masses of 2.5MJup and 4.3M_Jup_ for the planets orbiting HD 25723 and 17 Sco, respectively. The minimum masses of an additional candidate around HD 25723, and of planet candidates around 3 Cnc and 44 UMa, would be 1.3M_Jup_, 20.7M_Jup_ and 12.1M_Jup_, respectively.
One of the best ways to improve our understanding of the stellar activity-induced signal in radial velocity (RV) measurements is through simultaneous high-precision photometric and RV observations. This is of prime importance to mitigate the RV signal induced by stellar activity and therefore unveil the presence of low-mass exoplanets. The K2 Campaign 7 and 8 fields of view were located in the southern hemisphere, and provided a unique opportunity to gather unprecedented simultaneous high-precision photometric observation with K2 and high-precision RV measurements with the HARPS spectrograph to study the relationship between photometric variability and RV jitter. We observed nine stars with different levels of activity, from quiet to very active. We first probed the presence of any meaningful relation between measured RV jitter and the simultaneous photometric variation, and also other activity indicators (such as BIS, FWHM, logR0'HK, and F8) by evaluating the strength and significance of the monotonic correlation between RVs and each indicator. We found that for the case of very active stars, strong and significant correlations exist between almost all the observables and measured RVs; however, when we move towards lower activity levels the correlations become random, and we could not reach any conclusion regarding the tendency of correlations depending on the stellar activity level. Except for the F8 whose strong correlation with RV jitter persists over a wide range of stellar activity level, and thus our result suggests that F8 might be a powerful proxy for activity-induced RV jitter over a wide range of stellar activity. Moreover, we examine the capability of two state-of-the-art modeling techniques, namely the FF' method and SOAP2.0, to accurately predict the RV jitter amplitude using the simultaneous photometric observation. We found that for the very active stars both techniques can predict the amplitude of the RV jitter reasonably well; however, at lower activity levels the FF' method underpredicts the RV jitter amplitude.
The detection of low-mass transiting exoplanets in multiple systems brings new constraints to planetary formation and evolution processes and challenges the current planet formation theories. Nevertheless, only a mere fraction of the small planets detected by Kepler and K2 have precise mass measurements, which are mandatory to constrain their composition. We aim to characterise the planets that orbit the relatively bright star K2-138. This system is dynamically particular as it presents the longest chain known to date of planets close to the 3:2 resonance. We obtained 215 HARPS spectra from which we derived the radial-velocity variations of K2-138. Via a joint Bayesian analysis of both the K2 photometry and HARPS radial-velocities (RVs), we constrained the parameters of the six planets in orbit. The masses of the four inner planets, from b to e, are 3.1, 6.3, 7.9, and 13.0M_{Earth}_ with a precision of 34%, 20%, 18%, and 15%, respectively. The bulk densities are 4.9, 2.8, 3.2, and 1.8g/cm^3^, ranging from Earth to Neptune-like values. For planets f and g, we report upper limits. Finally, we predict transit timing variations of the order two to six minutes from the masses derived. Given its peculiar dynamics, K2-138 is an ideal target for transit timing variation (TTV) measurements from space with the upcoming CHaracterizing ExOPlanet Satellite (CHEOPS) to study this highly-packed system and compare TTV and RV masses.
The bright M2.5 dwarf K2-18 (M_s_=0.36M_{sun}_, R_s_=0.41R_{sun}_) at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star's habitable zone; K2-18b. Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet's mass which is required to interpret atmospheric properties and infer the planet's bulk composition. We have obtained precision radial velocity measurements with the HARPS spectrograph. We then coupled those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a correlated stellar activity model based on Gaussian process regression. We measured the mass of K2-18b to be 8.0+/-1.9M_{sun}_ with a bulk density of 3.3+/-1.2g/cm^3^ which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction >~50%. We also find strong evidence for a second, warm super-Earth K2-18c (m_p,c_sin(i_c_)=7.5+/-1.3M_{sun}_) at approximately nine days with a semi-major axis ~2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with the orbit of K2-18b although only a small mutual inclination is required for K2-18c to miss a transiting configuration; |{Delta}i|~1-2{deg}. A suite of dynamical integrations are performed to numerically confirm the system's dynamical stability. By varying the simulated orbital eccentricities of the two planets, dynamical stability constraints are used as an additional prior on each planet's eccentricity posterior from which we constrain e_b_<0.43 and e_c_<0.47 at the level of 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which, along with its proximity to the solar system, makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.
In an earlier campaign to characterize the mass of the transiting temperate super-Earth K2-18b with HARPS, a second, non-transiting planet was posited to exist in the system at ~9-days. Further radial velocity follow-up with the CARMENES spectrograph visible channel revealed a much weaker signal at 9-days, which also appeared to vary chromatically and temporally, leading to the conclusion that the origin of the 9-day signal was more likely related to stellar activity than to a planetary presence. Here we conduct a detailed re-analysis of all available RV time-series -- including a set of 31 previously unpublished HARPS measurements -- to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin of the curious 9-day signal. We conclude that the 9-day signal is real and was initially seen to be suppressed in the CARMENES data due to a small number of anomalous measurements, although the exact cause of these anomalies remains unknown. Investigation of the signal's evolution in time with wavelength and detailed model comparison reveals that the 9-day signal is most likely planetary in nature. Using this analysis we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of m_p,b_=8.63+/-1.35 and m_p,c_sin(i_c_)=5.62+/-0.84 Earth masses. This work, along with the previously published RV papers on the K2-18 planetary system, highlights the importance of understanding the time-sampling and of modelling the simultaneous planet plus stochastic activity, particularly when searching for sub-Neptune-sized planets with radial velocities.
The subdwarf-B pulsator, KIC10553698A, is one of 16 such objects observed with one-minute sampling for most of the duration of the Kepler Mission. Like most of these stars, it displays a rich g-mode pulsation spectrum with several clear multiplets that maintain regular frequency splitting. We identify these pulsation modes as components of rotationally split multiplets in a star rotating with a period of ~41d. From 162 clearly significant periodicities, we are able to identify 156 as likely components of l=1 or l=2 multiplets. For the first time we are able to detect l=1 modes that interpose in the asymptotic period sequences and that provide a clear indication of mode trapping in a stratified envelope, as predicted by theoretical models. A clear signal is also present in the Kepler photometry at 3.387d. Spectroscopic observations reveal a radial-velocity amplitude of 64.8km/s. We find that the radial-velocity variations and the photometric signal have phase and amplitude that are perfectly consistent with a Doppler-beaming effect and conclude that the unseen companion, KIC10553698B, must be a white dwarf most likely with a mass close to 0.6M_{sun}_.
