Person:

Charbonneau, David

M-dwarf stars – hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun – are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers1,2: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star3. The nearest such planets known to transit their star are 39 parsecs away4 , too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities5−11. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.

Rotation is a directly observable stellar property, and it drives magnetic field generation and activity through a magnetic dynamo. Main-sequence stars with masses below approximately 0.35 ${M}{\odot }$ (mid-to-late M dwarfs) are fully convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insight into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth Project, we measure rotation periods for 387 nearby, mid-to-late M dwarfs in the northern hemisphere, finding periods from 0.1 to 140 days. The typical rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5%–1%. We find no period–amplitude relation for stars below 0.25 ${M}{\odot }$ and an anticorrelation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly rotating stars without Hα emission that nevertheless have strong photometric variability. We use parallaxes, proper motions, radial velocities, photometry, and near-infrared metallicity estimates to further characterize the population of rotators. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the solar neighborhood. We use the W space velocities and established age–velocity relations to estimate that stars with P < 10 days have ages of on average <2 Gyr, and that those with P > 70 days have ages of about 5 Gyr. The period distribution is dependent on mass: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods, and the gap between the fast and slow rotators is larger for lower masses. Our data are consistent with a scenario in which these stars maintain rapid rotation for several gigayears, then spin down quickly, reaching periods of around 100 days by a typical age of 5 Gyr.

The MEarth Project is a photometric survey systematically searching the smallest stars nearest to the Sun for transiting rocky planets. Since 2008, MEarth has taken approximately two million images of 1844 stars suspected to be midto-late M dwarfs. We have augmented this survey by taking nightly exposures of photometric standard stars and have utilized this data to photometrically calibrate the MEarth system, identify photometric nights, and obtain an optical magnitude with 1.5% precision for each M dwarf system. Each optical magnitude is an average over many years of data, and therefore should be largely immune to stellar variability and flaring. We combine this with trigonometric distance measurements, spectroscopic metallicity measurements, and 2MASS infrared magnitude measurements in order to derive a color-magnitude-metallicity relation across the mid-to-late M dwarf spectral sequence that can reproduce spectroscopic metallicity determinations to a precision of 0.1 dex. We release optical magnitudes and metallicity estimates for 1567 M dwarfs, many of which did not have an accurate determination of either prior to this work. For an additional 277 stars without a trigonometric parallax, we provide an estimate of the distance assuming solar neighborhood metallicity. We find that the median metallicity for a volume limited sample of stars within 20 parsecs of the Sun is [Fe/H] = −0.03 ± 0.008, and that 29 / 565 of these stars have a metallicity of [Fe/H] = −0.5 or lower, similar to the low-metallicity distribution of nearby G-dwarfs. When combined with the results of ongoing and future planet surveys targeting these objects, the metallicity estimates presented here will be important in assessing the significance of any putative planet-metallicity correlation.

Kepler-20 is a solar-type star (V = 12.5) hosting a compact system of five transiting planets, all packed within the orbital distance of Mercury in our own solar system. A transition from rocky to gaseous planets with a planetary transition radius of ~1.6 ${R}{\oplus }$ has recently been proposed by several articles in the literature. Kepler-20b (${R}{p}$ ~ 1.9 ${R}{\oplus }$) has a size beyond this transition radius; however, previous mass measurements were not sufficiently precise to allow definite conclusions to be drawn regarding its composition. We present new mass measurements of three of the planets in the Kepler-20 system that are facilitated by 104 radial velocity measurements from the HARPS-N spectrograph and 30 archival Keck/HIRES observations, as well as an updated photometric analysis of the Kepler data and an asteroseismic analysis of the host star (${M}{\star }$ = $0.948\pm 0.051$ ${M}{\odot }$ and ${R}{\star }$ = $0.964\pm 0.018$ ${R}{\odot }$). Kepler-20b is a ${1.868}{-0.034}^{+0.066}$ ${R}{\oplus }$ planet in a 3.7 day period with a mass of ${9.70}{-1.44}^{+1.41}$ ${M}{\oplus }$, resulting in a mean density of ${8.2}{-1.3}^{+1.5}$ ${\rm{g}},{\mathrm{cm}}^{-3}$, indicating a rocky composition with an iron-to-silicate ratio consistent with that of the Earth. This makes Kepler-20b the most massive planet with a rocky composition found to date. Furthermore, we report the discovery of an additional non-transiting planet with a minimum mass of ${19.96}{-3.61}^{+3.08}$ ${M}{\oplus }$ and an orbital period of ~34 days in the gap between Kepler-20f (P ~ 11 days) and Kepler-20d (P ~ 78 days).

NASA’s Kepler mission has provided several thousand transiting planet candidates during the four years of its nominal mission, yet only a small subset of these candidates have been confirmed as true planets. Therefore, the most fundamental question about these candidates is the fraction of bona fide planets. Estimating the rate of false positives of the overall Kepler sample is necessary to derive the planet occurrence rate. We present the results from two large observational campaigns that were conducted with the Spitzer Space Telescope during the the Kepler mission. These observations are dedicated to estimating the false positive rate (FPR) amongst the Kepler candidates. We select a sub-sample of 51 candidates, spanning wide ranges in stellar, orbital and planetary parameter space, and we observe their transits with Spitzer at 4.5 µm. We use these observations to measures the candidate’s transit depths and infrared magnitudes. An authentic planet produces an achromatic transit depth (neglecting the modest effect of limb darkening). Conversely a bandpass-dependent depth alerts us to the potential presence of a blending star that could be the source of the observed eclipse: a false-positive scenario. For most of the candidates (85%), the transit depths measured with Kepler are consistent with the transit depths measured with Spitzer as expected for planetary objects, while we find that the most discrepant measurements are due to the presence of unresolved stars that dilute the photometry. The Spitzer constraints on their own yield FPRs between 5-40%, depending on the KOIs. By considering the population of the Kepler field stars, and by combining follow-up observations (imaging) when available, we find that the overall FPR of our sample is low. The measured upper limit on the FPR of our sample is 8.8% at a confidence level of 3σ. This observational result, which uses the achromatic property of planetary transit signals that is not investigated by the Kepler observations, provides an independent indication that Kepler ’s false positive rate is low.

We recently demonstrated sub-m/s sensitivity in measuring the radial velocity (RV) between the Earth and Sun using a simple solar telescope feeding the HARPS-N spectrograph at the Italian National Telescope, which is calibrated with a green astro-comb. We are using the solar telescope to characterize the effects of stellar (solar) RV jitter due to activity on the solar surface with the goal of detecting the solar RV signal from Venus, thereby demonstrating the sensitivity of these instruments to detect true Earth-twin exoplanets.

Kepler-93b is a 1.478 ± 0.019 R⊕ planet with a 4.7 day period around a bright (V = 10.2), astroseismically characterized host star with a mass of 0.911 ± 0.033 M and a radius of 0.919 ± 0.011 R. Based on 86 radial velocity observations obtained with the HARPS-N spectrograph on the Telescopio Nazionale Galileo and 32 archival Keck/HIRES observations, we present a precise mass estimate of 4.02±0.68 M⊕. The corresponding high density of 6.88±1.18 g cm−3 is consistent with a rocky composition of primarily iron and magnesium silicate. We compare Kepler-93b to other dense planets with well-constrained parameters and find that between 1 and 6 M⊕, all dense planets including the Earth and Venus are well-described by the same fixed ratio of iron to magnesium silicate. There are as of yet no examples of such planets with masses > 6 M⊕. All known planets in this mass regime have lower densities requiring significant fractions of volatiles or H/He gas. We also constrain the mass and period of the outer companion in the Kepler-93 system from the long-term radial velocity trend and archival adaptive optics images. As the sample of dense planets with well-constrained masses and radii continues to grow, we will be able to test whether the fixed compositional model found for the seven dense planets considered in this paper extends to the full population of 1–6 M⊕ planets.

We report the first planet discovery from the two-wheeled Kepler (K2) mission: HIP 116454 b. The host star HIP 116454 is a bright (V = 10.1, K = 8.0) K1 dwarf with high proper motion and a parallax-based distance of 55.2 ± 5.4 pc. Based on high-resolution optical spectroscopy, we find that the host star is metal-poor with [Fe/H] = −0.16±0.08 and has a radius R = 0.716 ± 0.024 R and mass M = 0.775±0.027 M. The star was observed by the Kepler spacecraft during its Two-Wheeled Concept Engineering Test in 2014 February. During the 9 days of observations, K2 observed a single transit event. Using a new K2 photometric analysis technique, we are able to correct small telescope drifts and recover the observed transit at high confidence, corresponding to a planetary radius of Rp = 2.53 ± 0.18 R⊕. Radial velocity observations with the HARPS-N spectrograph reveal a 11.82 ± 1.33 M⊕ planet in a 9.1 day orbit, consistent with the transit depth, duration, and ephemeris. Follow-up photometric measurements from the MOST satellite confirm the transit observed in the K2 photometry and provide a refined ephemeris, making HIP 116454 b amenable for future follow-up observations of this latest addition to the growing population of transiting super-Earths around nearby, bright stars.

Detection and characterization of potentially habitable Earthsize extrasolar planets is one of the major goals of contemporary astronomy. By applying the transit method to very low-mass M-dwarfs, it is possible to find these planets from the ground with present-day instrumentation and observational techniques. The MEarth project is one such survey with stations in both hemispheres: MEarth-North at the Fred Lawrence Whipple Observatory, Mount Hopkins, Arizona, and MEarth-South at Cerro Tololo InterAmerican Observatory, Chile. We present an update on recent results of this survey, for planet occurrence rates, and interesting stellar astrophysics, for which our sample of 3000 nearby mid-to-late M-dwarfs has been very fruitful. All light curves gathered during the survey are made publicly available after one year, and we describe how to access and use these data.

The hot Jupiter HD 209458b is particularly amenable to detailed study as it is among the brightest transiting exoplanet systems currently known (V-mag = 7.65; K-mag = 6.308) and has a large planet-to-star contrast ratio. HD 209458b is predicted to be in synchronous rotation about its host star with a hot spot that is shifted eastward of the substellar point by superrotating equatorial winds. Here we present the first full-orbit observations of HD 209458b, in which its 4.5 μm emission was recorded with Spitzer/IRAC. Our study revises the previous 4.5 μm measurement of HD 209458b's secondary eclipse emission downward by ~35% to $0.1391%^{+0.0072%}_{-0.0069%}$, changing our interpretation of the properties of its dayside atmosphere. We find that the hot spot on the planet's dayside is shifted eastward of the substellar point by 40fdg9 ± 6fdg0, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T bright = 1499 ± 15 K) and nightside (T bright = 972 ± 44 K) emission indicate a day-to-night brightness temperature contrast smaller than that observed for more highly irradiated exoplanets, suggesting that the day-to-night temperature contrast may be partially a function of the incident stellar radiation. The observed phase curve shape deviates modestly from global circulation model predictions potentially due to disequilibrium chemistry or deficiencies in the current hot CH4 line lists used in these models. Observations of the phase curve at additional wavelengths are needed in order to determine the possible presence and spatial extent of a dayside temperature inversion, as well as to improve our overall understanding of this planet's atmospheric circulation.