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 $11\pm4$ 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 $\sim200$ d. Extrapolating, one finds $5.7^{+1.7}_{-2.2}$ 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

Astronomical Anomalies: Their Role in the Quest for Extraterrestrial Life

Astronomers occasionally detect an object having unexpected shape, unexplainable photometry, or unprecedented spectra that are inconsistent with our contemporary knowledge of the universe. Upon careful assessment, many of these anomalies are discarded as mere noise, contamination, or faulty analysis. But some anomalies survive scrutiny to yield new astronomical objects and physical processes. Examples of validated anomalies include quasars, pulsars, and periodic Doppler shifts of Sun-like stars caused by the gravitational pull of orbiting planets. Other anomalies persist as mysteries, including Fast Radio Bursts, dark energy, 'Oumuamua as an alien spaceship, and simultaneously vanishing stars. Advanced technological life may present astronomers with anomalies that require carefully designed observations from multiple vantage points simultaneously and with real-time spectroscopy.Comment: 9 pages. This paper is for the general publi

Empirical Limits on Radial Velocity Planet Detection for Young Stars

We report initial results from our long term search using precision radial velocities for planetary-mass companions located within a few AU of stars younger than the Sun. Based on a sample of >150 stars, we define a floor in the radial velocity scatter, sigma_RV, as a function of the chromospheric activity level R'_{HK}. This lower bound to the jitter, which increases with increasing stellar activity, sets the minimum planet mass that could be detected. Adopting a median activity-age relationship reveals the astrophysical limits to planet masses discernable via radial velocity monitoring, as a function of stellar age. Considering solar-mass primaries having the mean jitter-activity level, when they are younger than 100 / 300 / 1000 Myr, the stochastic jitter component in radial velocity measurements restricts detectable companion masses to > 0.3 / 0.2 / 0.1 M_Jupiter. These numbers require a large number -- several tens -- of radial velocity observations taken over a time frame longer than the orbital period. Lower companion mass limits can be achieved for stars with less than the mean jitter and/or with an increased number of observations.Comment: 5 pages, to appear the 18th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun proceedings edited by G. van Belle & H. Harri

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

Optical SETI: A Spectroscopic Search for Laser Emission from Nearby Stars

We have searched for nonastrophysical emission lines in the optical spectra of 577 nearby F, G, K, and M main-sequence stars. Emission lines of astrophysical origin would also have been detected, such as from a time--variable chromosphere or infalling comets. We examined ~20 spectra per star obtained during four years with the Keck/HIRES spectrometer at a resolution of 5 km/s, with a detection threshold 3% of the continuum flux level. We searched each spectrum from 4000-5000 angstroms for emission lines having widths too narrow to be natural from the host star, as well as for lines broadened by astrophysical mechanisms. We would have detected lasers that emit a power, P>60 kW, for a typical beam width of ~0.01 arcsec (diffraction-limit from a 10-m aperture) if directed toward Earth from the star. No lines consisstent with laser emission were found.Comment: 27 pages, 11 figures, uses aastex.st