274 research outputs found
Prevalence of Earth-size planets orbiting Sun-like stars
Determining whether Earth-like planets are common or rare looms as a
touchstone in the question of life in the universe. We searched for Earth-size
planets that cross in front of their host stars by examining the brightness
measurements of 42,000 stars from National Aeronautics and Space
Administration's Kepler mission. We found 603 planets, including 10 that are
Earth size (1-2 Earth-radii) and receive comparable levels of stellar energy to
that of Earth (within a factor of four). We account for Kepler's imperfect
detectability of such planets by injecting synthetic planet-caused dimmings
into the Kepler brightness measurements and recording the fraction detected. We
find that of Sun-like stars harbor an Earth-size planet receiving
between one and four times the stellar intensity as Earth. We also find that
the occurrence of Earth-size planets is constant with increasing orbital period
(P), within equal intervals of logP up to d. Extrapolating, one finds
of Sun-like stars harbor an Earth-size planet with orbital
periods of 200-400 d.Comment: Main text: 6 pages, 5 figures, 1 table. Supporting information: 54
pages, 17 pages, 3 tables. Published in the Proceedings of the National
Academy of Sciences available at
http://www.pnas.org/cgi/doi/10.1073/pnas.131990911
RadVel: The Radial Velocity Modeling Toolkit
RadVel is an open source Python package for modeling Keplerian orbits in
radial velocity (RV) time series. RadVel provides a convenient framework to fit
RVs using maximum a posteriori optimization and to compute robust confidence
intervals by sampling the posterior probability density via Markov Chain Monte
Carlo (MCMC). RadVel allows users to float or fix parameters, impose priors,
and perform Bayesian model comparison. We have implemented realtime MCMC
convergence tests to ensure adequate sampling of the posterior. RadVel can
output a number of publication-quality plots and tables. Users may interface
with RadVel through a convenient command-line interface or directly from
Python. The code is object-oriented and thus naturally extensible. We encourage
contributions from the community. Documentation is available at
http://radvel.readthedocs.io.Comment: prepared for resubmission to PAS
Identification and Removal of Noise Modes in Kepler Photometry
We present the Transiting Exoearth Robust Reduction Algorithm (TERRA) --- a
novel framework for identifying and removing instrumental noise in Kepler
photometry. We identify instrumental noise modes by finding common trends in a
large ensemble of light curves drawn from the entire Kepler field of view.
Strategically, these noise modes can be optimized to reveal transits having a
specified range of timescales. For Kepler target stars of low photometric
noise, TERRA produces ensemble-calibrated photometry having 33 ppm RMS scatter
in 12-hour bins, rendering individual transits of earth-size planets around
sun-like stars detectable as ~3 sigma signals.Comment: 18 pages, 7 figures, submitted to PAS
Data-driven Spectroscopy of Cool Stars at High Spectral Resolution
The advent of large-scale spectroscopic surveys underscores the need to develop robust techniques for determining stellar properties ("labels," i.e., physical parameters and elemental abundances). However, traditional spectroscopic methods that utilize stellar models struggle to reproduce cool (< 4700 K) stellar atmospheres due to an abundance of unconstrained molecular transitions, making modeling via synthetic spectral libraries difficult. Because small, cool stars such as K and M dwarfs are both common and good targets for finding small, cool planets, establishing precise spectral modeling techniques for these stars is of high priority. To address this, we apply The Cannon, a data-driven method of determining stellar labels, to Keck High Resolution Echelle Spectrometer spectra of 141 cool (< 5200 K) stars from the California Planet Search. Our implementation is capable of predicting labels for small (< 1 R_⊙) stars of spectral types K and later with accuracies of 68 K in effective temperature (T_(eff)), 5% in stellar radius (R_*), and 0.08 dex in bulk metallicity ([Fe/H]), and maintains this performance at low spectral resolutions (R < 5000). As M dwarfs are the focus of many future planet-detection surveys, this work can aid efforts to better characterize the cool star population and uncover correlations between cool star abundances and planet occurrence for constraining planet formation theories
A Plateau in the Planet Population Below Twice the Size of Earth
We carry out an independent search of Kepler photometry for small transiting planets with sizes 0.5-8.0 times that of Earth and orbital periods between 5 and 50 days, with the goal of measuring the fraction of stars harboring such planets. We use a new transit search algorithm, TERRA, optimized to detect small planets around photometrically quiet stars. We restrict our stellar sample to include the 12,000 stars having the lowest photometric noise in the Kepler survey, thereby maximizing the detectability of Earth-size planets. We report 129 planet candidates having radii less than 6 R_E found in three years of Kepler photometry (quarters 1-12). Forty-seven of these candidates are not in Batalha et al., which only analyzed photometry from quarters 1-6. We gather Keck HIRES spectra for the majority of these targets leading to precise stellar radii and hence precise planet radii. We make a detailed measurement of the completeness of our planet search. We inject synthetic dimmings from mock transiting planets into the actual Kepler photometry. We then analyze that injected photometry with our TERRA pipeline to assess our detection completeness for planets of different sizes and orbital periods. We compute the occurrence of planets as a function of planet radius and period, correcting for the detection completeness as well as the geometric probability of transit, R⋆/a. The resulting distribution of planet sizes exhibits a power law rise in occurrence from 5.7 R_E down to 2 R_E, as found in Howard et al. That rise clearly ends at 2 R_E . The occurrence of planets is consistent with constant from 2 R_E toward 1 R_E . This unexpected plateau in planet occurrence at 2 R_E suggests distinct planet formation processes for planets above and below 2 R_E . We find that 15.1^(+1.8)_(-2.7)% of solar type stars—roughly one in six—has a 1-2 R_E planet with P = 5-50 days
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