William D. Cochran

University of Texas at Austin, Austin, Texas, United States

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Publications (191)1152.98 Total impact

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    ABSTRACT: \We present the sixth catalog of Kepler candidate planets based on nearly 4 years of high precision photometry. This catalog builds on the legacy of previous catalogs released by the Kepler project and includes 1493 new Kepler Objects of Interest (KOIs) of which 554 are planet candidates, and 131 of these candidates have best fit radii <1.5 R_earth. This brings the total number of KOIs and planet candidates to 7305 and 4173 respectively. We suspect that many of these new candidates at the low signal-to-noise limit may be false alarms created by instrumental noise, and discuss our efforts to identify such objects. We re-evaluate all previously published KOIs with orbital periods of >50 days to provide a consistently vetted sample that can be used to improve planet occurrence rate calculations. We discuss the performance of our planet detection algorithms, and the consistency of our vetting products. The full catalog is publicly available at the NASA Exoplanet Archive.
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    ABSTRACT: The first discoveries of exoplanets around Sun-like stars have fueled efforts to find ever smaller worlds evocative of Earth and other terrestrial planets in the Solar System. While gas-giant planets appear to form preferentially around metal-rich stars, small planets (with radii less than four Earth radii) can form under a wide range of metallicities. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the Universe's history when metals were far less abundant. We report Kepler spacecraft observations of KOI-3158, a metal-poor Sun-like star from the old population of the Galactic thick disk, which hosts five planets with sizes between Mercury and Venus. We used asteroseismology to directly measure a precise age of 11.2+/-1.0 Gyr for the host star, indicating that KOI-3158 formed when the Universe was less than 20% of its current age and making it the oldest known system of terrestrial-size planets. We thus show that Earth-size planets have formed throughout most of the Universe's 13.8-billion-year history, providing scope for the existence of ancient life in the Galaxy.
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    ABSTRACT: The Kepler mission discovered 2842 exoplanet candidates with 2 years of data. We provide updates to the Kepler planet candidate sample based upon 3 years (Q1-Q12) of data. Through a series of tests to exclude false-positives, primarily caused by eclipsing binary stars and instrumental systematics, 855 additional planetary candidates have been discovered, bringing the total number known to 3697. We provide revised transit parameters and accompanying posterior distributions based on a Markov Chain Monte Carlo algorithm for the cumulative catalogue of Kepler Objects of Interest. There are now 130 candidates in the cumulative catalogue that receive less than twice the flux the Earth receives and more than 1100 have a radius less than 1.5 Rearth. There are now a dozen candidates meeting both criteria, roughly doubling the number of candidate Earth analogs. A majority of planetary candidates have a high probability of being bonafide planets, however, there are populations of likely false-positives. We discuss and suggest additional cuts that can be easily applied to the catalogue to produce a set of planetary candidates with good fidelity. The full catalogue is publicly available at the NASA Exoplanet Archive.
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    ABSTRACT: The chemical composition of stars hosting small exoplanets (with radii less than four Earth radii) appears to be more diverse than that of gas-giant hosts, which tend to be metal-rich. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the Universe's history when metals were more scarce. We report Kepler spacecraft observations of Kepler-444, a metal-poor Sun-like star from the old population of the Galactic thick disk and the host to a compact system of five transiting planets with sizes between those of Mercury and Venus. We validate this system as a true five-planet system orbiting the target star and provide a detailed characterization of its planetary and orbital parameters based on an analysis of the transit photometry. Kepler-444 is the densest star with detected solar-like oscillations. We use asteroseismology to directly measure a precise age of 11.2+/-1.0 Gyr for the host star, indicating that Kepler-444 formed when the Universe was less than 20% of its current age and making it the oldest known system of terrestrial-size planets. We thus show that Earth-size planets have formed throughout most of the Universe's 13.8-billion-year history, leaving open the possibility for the existence of ancient life in the Galaxy. The age of Kepler-444 not only suggests that thick-disk stars were among the hosts to the first Galactic planets, but may also help to pinpoint the beginning of the era of planet formation.
    The Astrophysical Journal 01/2015; 799(2). DOI:10.1088/0004-637X/799/2/170 · 6.28 Impact Factor
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    ABSTRACT: We present an in-depth analysis of stellar activity and its effects on radial velocity (RV) for the M2 dwarf GJ 176 based on spectra taken over 10 years from the High Resolution Spectrograph on the Hobby-Eberly Telescope. These data are supplemented with spectra from previous observations with the HIRES and HARPS spectrographs, and V- and R-band photometry taken over 6 years at the Dyer and Fairborn observatories. Previous studies of GJ 176 revealed a super-Earth exoplanet in an 8.8-day orbit. However, the velocities of this star are also known to be contaminated by activity, particularly at the 39-day stellar rotation period. We have examined the magnetic activity of GJ 176 using the sodium I D lines, which have been shown to be a sensitive activity tracer in cool stars. In addition to rotational modulation, we see evidence of a long-term trend in our Na I D index, which may be part of a long-period activity cycle. The sodium index is well correlated with our RVs, and we show that this activity trend drives a corresponding slope in RV. Interestingly, the rotation signal remains in phase in photometry, but not in the spectral activity indicators. We interpret this phenomenon as the result of one or more large spot complexes or active regions which dominate the photometric variability, while the spectral indices are driven by the overall magnetic activity across the stellar surface. In light of these results, we discuss the potential for correcting activity signals in the RVs of M dwarfs.
    The Astrophysical Journal 01/2015; 801(2). DOI:10.1088/0004-637X/801/2/79 · 6.28 Impact Factor
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    ABSTRACT: We have used high-cadence radial velocity measurements from the Hobby-Eberly Telescope with published velocities from the Lick 3 m Shane Telescope, combined with astrometric data from the Hubble Space Telescope (HST) Fine Guidance Sensors to refine the orbital parameters of the HD 128311 system, and determine an inclination of 55.degrees 95 +/- 14.degrees 55 and true mass of 3.789(+0.924)(-0.432) M-JUP for HD 128311 c. The combined radial velocity data also reveal a short period signal which could indicate a third planet in the system with an M sin i of 0.133 +/- 0.005 M-JUP or stellar phenomena. Photometry from the T12 0.8 m automatic photometric telescope at the Fairborn Observatory and HST are used to determine a photometric period close to, but not within the errors of the radial velocity signal. We performed a cross-correlation bisector analysis of the radial velocity data to look for correlations with the photometric period and found none. Dynamical integrations of the proposed system show long-term stability with the new orbital parameters of over 10 million years. Our new orbital elements do not support the claims of HD 128311 b and c being in mean motion resonance.
    The Astrophysical Journal 11/2014; 795(1). DOI:10.1088/0004-637X/795/1/41 · 6.28 Impact Factor
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    ABSTRACT: Hot Jupiter systems provide unique observational constraints for migration models in multiple systems and binaries. We report on the discovery of the Kepler-424 (KOI-214) two-planet system, which consists of a transiting hot Jupiter (Kepler-424b) in a 3.31-d orbit accompanied by a more massive outer companion in an eccentric (e=0.3) 223-d orbit. The outer giant planet, Kepler-424c, is not detected to transit the host star. The masses of both planets and the orbital parameters for the second planet were determined using precise radial velocity (RV) measurements from the Hobby-Eberly Telescope (HET) and its High Resolution Spectrograph (HRS). In stark contrast to smaller planets, hot Jupiters are predominantly found to be lacking any nearby additional planets, the appear to be "lonely" (e.g. Steffen et al.~2012). This might be a consequence of a highly dynamical past of these systems. The Kepler-424 planetary system is a system with a hot Jupiter in a multiple system, similar to upsilon Andromedae. We also present our results for Kepler-422 (KOI-22), Kepler-77 (KOI-127; Gandolfi et al.~2013), Kepler-43 (KOI-135; Bonomo et al.~2012), and Kepler-423 (KOI-183). These results are based on spectroscopic data collected with the Nordic Optical Telescope (NOT), the Keck 1 telescope and HET. For all systems we rule out false positives based on various follow-up observations, confirming the planetary nature of these companions. We performed a comparison with planetary evolutionary models which indicate that these five hot Jupiters have a heavy elements content between 20 and 120 M_Earth.
    The Astrophysical Journal 09/2014; 795(2). DOI:10.1088/0004-637X/795/2/151 · 6.28 Impact Factor
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    ABSTRACT: Kepler-91b is a rare example of a transiting hot Jupiter around a red giant star, providing the possibility to study the formation and composition of hot Jupiters under different conditions compared to main-sequence stars. However, the planetary nature of Kepler-91b, which was confirmed using phase-curve variations by Lillo-Box et al., was recently called into question based on a re-analysis of Kepler data. We have obtained ground-based radial velocity observations from the Hobby-Eberly Telescope and unambiguously confirm the planetary nature of Kepler-91b by simultaneously modeling the Kepler and radial velocity data. The star exhibits temporally correlated noise due to stellar granulation which we model as a Gaussian Process. We hypothesize that it is this noise component that led previous studies to suspect Kepler-91b to be a false positive. Our work confirms the conclusions presented by Lillo-Box et al. that Kepler-91b is a 0.73+/-0.13 Mjup planet orbiting a red giant star.
    The Astrophysical Journal 08/2014; 800(1). DOI:10.1088/0004-637X/800/1/46 · 6.28 Impact Factor
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    ABSTRACT: Transiting planets around rapidly rotating stars are not amenable to precise radial velocity observations, such as are used for planet candidate validation, as they have wide, rotationally broadened stellar lines. Such planets can, however, be observed using Doppler tomography, wherein the stellar absorption line profile distortions during transit are spectroscopically resolved. This allows the validation of transiting planet candidates and the measurement of the stellar spin-planetary orbit (mis)alignment, an important statistical probe of planetary migration processes. We present Doppler tomographic observations which provide a direct confirmation of the hot Jupiter Kepler-13 Ab, and also show that the planet has a prograde, misaligned orbit, with lambda = 58.6 +/- 2.0 degrees. Our measured value of the spin-orbit misalignment is in significant disagreement with the value of lambda = 23 +/- 4 degrees previously measured by Barnes et al. (2011) from the gravity-darkened Kepler lightcurve. We also place an upper limit of 0.75 solar masses (95% confidence) on the mass of Kepler-13 C, the spectroscopic companion to Kepler-13 B, the proper motion companion of the planet host star Kepler-13 A.
    The Astrophysical Journal 06/2014; 790(1). DOI:10.1088/0004-637X/790/1/30 · 6.28 Impact Factor
  • Marshall C. Johnson, William D. Cochran
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    ABSTRACT: We present Doppler tomographic observations of the transiting planet Kepler-13b (aka KOI-13b), a highly inflated hot Jupiter orbiting the Teff =8500 K primary of a hierarchical triple stellar system. As the planet transits the rapidly rotating host star, it successively blocks regions of the stellar disk with different radial velocities, causing a ``bump'' in the stellar spectral line shape, which we resolve spectroscopically. The manner in which this perturbation moves across the stellar line during the transit gives information on the relative alignment between the stellar spin and planetary orbital angular momentum vectors. This is a powerful statistical probe of planetary migration processes, as the expected spin-orbit misalignment distributions from dynamically cold migration (disk interactions) and dynamically hot migration (planet-planet scattering, Kozai cycles) are significantly different: the former will lead to primarily aligned orbits; the latter to a wide distribution. Doppler tomography also promises to be a powerful technique for confirming transiting planet candidates around rapidly rotating stars like Kepler-13A, which are not amenable to radial velocity follow-up and thus are currently a poorly sampled region of parameter space.
    Proceedings of the International Astronomical Union 06/2014; 8(S299):295-296. DOI:10.1017/S1743921313008661
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    ABSTRACT: Approximately half of the extrasolar planets (exoplanets) with radii less than four Earth radii are in orbits with short periods. Despite their sheer abundance, the compositions of such planets are largely unknown. The available evidence suggests that they range in composition from small, high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick hydrogen and helium gas envelopes. Here we report the metallicities (that is, the abundances of elements heavier than hydrogen and helium) of more than 400 stars hosting 600 exoplanet candidates, and find that the exoplanets can be categorized into three populations defined by statistically distinct (∼4.5σ) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (radii less than 1.7 Earth radii), gas dwarf planets with rocky cores and hydrogen-helium envelopes (radii between 1.7 and 3.9 Earth radii) and ice or gas giant planets (radii greater than 3.9 Earth radii). These transitions correspond well with those inferred from dynamical mass estimates, implying that host star metallicity, which is a proxy for the initial solids inventory of the protoplanetary disk, is a key ingredient regulating the structure of planetary systems.
    Nature 05/2014; 509(7502):593-5. DOI:10.1038/nature13254 · 42.35 Impact Factor
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    ABSTRACT: The Kepler mission has discovered over 2500 exoplanet candidates in the first two years of spacecraft data, with approximately 40% of them in candidate multi-planet systems. The high rate of multiplicity combined with the low rate of identified false-positives indicates that the multiplanet systems contain very few false-positive signals due to other systems not gravitationally bound to the target star (Lissauer, J. J., et al., 2012, ApJ 750, 131). False positives in the multi- planet systems are identified and removed, leaving behind a residual population of candidate multi-planet transiting systems expected to have a false-positive rate less than 1%. We present a sample of 340 planetary systems that contain 851 planets that are validated to substantially better than the 99% confidence level; the vast majority of these have not been previously verified as planets. We expect ~2 unidentified false-positives making our sample of planet very reliable. We present fundamental planetary properties of our sample based on a comprehensive analysis of Kepler light curves and ground-based spectroscopy and high-resolution imaging. Since we do not require spectroscopy or high-resolution imaging for validation, some of our derived parameters for a planetary system may be systematically incorrect due to dilution from light due to additional stars in the photometric aperture. None the less, our result nearly doubles the number of verified exoplanets.
    The Astrophysical Journal 02/2014; 784(1). DOI:10.1088/0004-637X/784/1/45 · 6.28 Impact Factor
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    ABSTRACT: The K2 mission will make use of the Kepler spacecraft and its assets to expand upon Kepler's groundbreaking discoveries in the fields of exoplanets and astrophysics through new and exciting observations. K2 will use an innovative way of operating the spacecraft to observe target fields along the ecliptic for the next 2-3 years. Early science commissioning observations have shown an estimated photometric precision near 400 ppm in a single 30 minute observation, and a 6-hour photometric precision of 80 ppm (both at V=12). The K2 mission offers simultaneous observation of thousands of objects at a precision far better than is achievable from the ground. Ecliptic fields will be observed for approximately 75-days enabling a unique exoplanet survey which fills the gaps in duration and sensitivity between the Kepler and TESS missions, and offers pre-launch exoplanet target identification for JWST transit spectroscopy. Astrophysics observations with K2 will include studies of young open clusters, bright stars, galaxies, supernovae, and asteroseismology.
    Publications of the Astronomical Society of the Pacific 02/2014; 126(938). DOI:10.1086/676406 · 3.23 Impact Factor
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    ABSTRACT: We report on the masses, sizes, and orbits of the planets orbiting 22 Kepler stars. There are 49 planet candidates around these stars, including 42 detected through transits and 7 revealed by precise Doppler measurements of the host stars. Based on an analysis of the Kepler brightness measurements, along with high-resolution imaging and spectroscopy, Doppler spectroscopy, and (for 11 stars) asteroseismology, we establish low false-positive probabilities for all of the transiting planets (41 of 42 have a false-positive probability under 1%), and we constrain their sizes and masses. Most of the transiting planets are smaller than 3X the size of Earth. For 16 planets, the Doppler signal was securely detected, providing a direct measurement of the planet's mass. For the other 26 planets we provide either marginal mass measurements or upper limits to their masses and densities; in many cases we can rule out a rocky composition. We identify 6 planets with densities above 5 g/cc, suggesting a mostly rocky interior for them. Indeed, the only planets that are compatible with a purely rocky composition are smaller than ~2 R_earth. Larger planets evidently contain a larger fraction of low-density material (H, He, and H2O).
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    ABSTRACT: We provide updates to the Kepler planet candidate sample based upon nearly two years of high-precision photometry (i.e., Q1-Q8). From an initial list of nearly 13,400 Threshold Crossing Events (TCEs), 480 new host stars are identified from their flux time series as consistent with hosting transiting planets. Potential transit signals are subjected to further analysis using the pixel-level data, which allows background eclipsing binaries to be identified through small image position shifts during transit. We also re-evaluate Kepler Objects of Interest (KOI) 1-1609, which were identified early in the mission, using substantially more data to test for background false positives and to find additional multiple systems. Combining the new and previous KOI samples, we provide updated parameters for 2,738 Kepler planet candidates distributed across 2,017 host stars. From the combined Kepler planet candidates, 472 are new from the Q1-Q8 data examined in this study. The new Kepler planet candidates represent ~40% of the sample with Rp~1 Rearth and represent ~40% of the low equilibrium temperature (Teq<300 K) sample. We review the known biases in the current sample of Kepler planet candidates relevant to evaluating planet population statistics with the current Kepler planet candidate sample.
    The Astrophysical Journal Supplement Series 12/2013; 210(2). DOI:10.1088/0067-0049/210/2/19 · 14.14 Impact Factor
  • William D. Cochran, M. Endl, E. J. Brugamyer, P. J. MacQueen
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    ABSTRACT: We report the detection of Jovian mass planets orbiting three nearby metal-poor thick disk stars. These discoveries were all made using precise radial velocity measurements from the High Resolution Spectrograph of the Hobby-Eberly Telescope. All of the planets are of Jovian mass or larger, with orbital periods ranging from about a year to over six years. HIP 14342 shows two planetary companions with orbital periods near a 2:1 resonance. The other planets detected orbit HIP 13366 and HIP 109384. All three of these stars are kinematic members of the galactic "thick disk", which is a population of stars with a larger vertical scale height and a larger velocity dispersion that the thin disk to which the Sun belongs. The thick disk stars are of lower total metallicity than the Sun, and are also chemically different than thin disk stars, having the abundances of their alpha-capture elements (e.g. O, Ne, Mg, Si, S, Ca, Ti) enhanced by 0.2 to 0.4 dex over those of thin disk stars of the same [Fe/H]. The majority of planets found among stars with [Fe/H] < ~-0.2 orbit thick disk stars, even though thin disk stars significantly outnumber thick disk stars in this metallicity range. Thus, the enhanced abundance of the alpha-capture elements, which are also key elements in the chemistry of planet-forming materials, may be responsible for the large fraction of low-metallicity thick-disk stars with planetary companions.
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    ABSTRACT: We announce the discovery of a ~2 Jupiter-mass planet in an eccentric 11-year orbit around the K7/M0 dwarf GJ 328. Our result is based on 10 years' worth of radial velocity (RV) data from the Hobby-Eberly and Harlan J. Smith telescopes at McDonald Observatory, and from the Keck Telescope at Mauna Kea. Our analysis of GJ 328's magnetic activity via the Na I D features reveals a long-period stellar activity cycle, which creates an additional signal in the star's RV curve with amplitude 6-10 m/s. After correcting for this stellar RV contribution, we see that the orbit of the planet is more eccentric than suggested by the raw RV data. GJ 328b is currently the most massive, longest-period planet discovered around a low-mass dwarf.
    The Astrophysical Journal 07/2013; 774(2). DOI:10.1088/0004-637X/774/2/147 · 6.28 Impact Factor
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    ABSTRACT: We present the detection of five planets-Kepler-62b, c, d, e, and f-of size 1.31, 0.54, 1.95, 1.61 and 1.41 Earth radii (R⊕), orbiting a K2V star at periods of 5.7, 12.4, 18.2, 122.4, and 267.3 days, respectively. The outermost planets (Kepler-62e and -62f) are super-Earth-size (1.25 < planet radius ≤ 2.0 R⊕) planets in the habitable zone (HZ) of their host star, receiving 1.2 ± 0.2 and 0.41 ± 0.05 times the solar flux at Earth's orbit (S⊙). Theoretical models of Kepler-62e and -62f for a stellar age of ~7 Gyr suggest that both planets could be solid, either with a rocky composition or composed of mostly solid water in their bulk.
    Science 04/2013; DOI:10.1126/science.1234702 · 31.48 Impact Factor
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    ABSTRACT: Since the discovery of the first exoplanets, it has been known that other planetary systems can look quite unlike our own. Until fairly recently, we have been able to probe only the upper range of the planet size distribution, and, since last year, to detect planets that are the size of Earth or somewhat smaller. Hitherto, no planets have been found that are smaller than those we see in the Solar System. Here we report a planet significantly smaller than Mercury. This tiny planet is the innermost of three that orbit the Sun-like host star, which we have designated Kepler-37. Owing to its extremely small size, similar to that of the Moon, and highly irradiated surface, the planet, Kepler-37b, is probably rocky with no atmosphere or water, similar to Mercury.
    Nature 02/2013; 494(7438):452-4. DOI:10.1038/nature11914 · 42.35 Impact Factor
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    ABSTRACT: Planets embedded within dust disks may drive the formation of large scale clumpy dust structures by trapping dust into resonant orbits. Detection and subsequent modeling of the dust structures would help constrain the mass and orbit of the planet and the disk architecture, give clues to the history of the planetary system, and provide a statistical estimate of disk asymmetry for future exoEarth-imaging missions. Here we present the first search for these resonant structures in the inner regions of planetary systems by analyzing the light curves of hot Jupiter planetary candidates identified by the Kepler mission. We detect only one candidate disk structure associated with KOI 838.01 at the 3-sigma confidence level, but subsequent radial velocity measurements reveal that KOI 838.01 is a grazing eclipsing binary and the candidate disk structure is a false positive. Using our null result, we place an upper limit on the frequency of dense exozodi structures created by hot Jupiters. We find that at the 90% confidence level, less than 21% of Kepler hot Jupiters create resonant dust clumps that lead and trail the planet by ~90 degrees with optical depths >~5*10^-6, which corresponds to the resonant structure expected for a lone hot Jupiter perturbing a dynamically cold dust disk 50 times as dense as the zodiacal cloud.
    The Astrophysical Journal 01/2013; 764(2). DOI:10.1088/0004-637X/764/2/195 · 6.28 Impact Factor

Publication Stats

6k Citations
1,152.98 Total Impact Points

Institutions

  • 1980–2015
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 2012
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, Maryland, United States
    • San Diego State University
      • Department of Astronomy
      San Diego, CA, United States
  • 2010
    • SETI Institute
      Mountain View, California, United States
  • 2004
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States