William J. Borucki

The Vulcan Photometer: A Dedicated Photometer for Extrasolar Planet Searches

A small CCD photometer dedicated to the detection of extrasolar planets has been developed and put into operation at Mount Hamilton, California. It simultaneously monitors 6000 stars brighter than 13th magnitude in its 49 deg 2 field of view. Observations are conducted all night every clear night of the year. A single field is monitored at a cadence of eight images per hour for a period of about 3 months. When the data are folded for the purpose of discovering low-amplitude transits, transit amplitudes of 1% are readily detected. This precision is sufficient to find Jovian-size planets orbiting solar-like stars, which have signal amplitudes from 1% to 2% depending on the inflation of the planets atmosphere and the size of the star. An investigation of possible noise sources indicates that neither star field crowding, scintillation noise, nor photon shot noise are the major noise sources for stars brighter than visual magnitude 11.6. Over one hundred variable stars have been found in each star field. About 50 of these stars are eclipsing binary stars, several with transit amplitudes of only a few percent. Three stars that showed only primary transits were examined with high-precision spectroscopy. Two were found to be nearly identical stars in binary pairs orbiting at double the photometric period. Spectroscopic observations showed the third star to be a high mass ratio single-lined binary. On 1999 November 22 the transit of a planet orbiting HD 209458 was observed and the predicted amplitude and immersion times were confirmed. These observations show that the photometer and the data reduction and analysis algorithms have the necessary precision to find companions with the expected area ratio for Jovian-size planets orbiting solar-like stars.

Test of CCD Precision Limits for Differential Photometry

Results of tests to demonstrate the very high differential photometric stability of CCD light sensors are presented. The measurements reported here demonstrate that in a controlled laboratory environment, a front-illuminated CCD can provide differential photometric measurements with CCDs and implications for spaceborne applications are discussed.

A decade of extrasolar planets around normal stars: The Kepler Mission: Design, expected science results, opportunities to participate

Kepler is a Discovery-class mission designed to determine the frequency of Earth-size and smaller planets in and near the habitable zone (HZ) of spectral type F through M dwarf stars. The instrument consists of a 0.95 m aperture photometer to do high precision photometry of 100,000 solar-like stars to search for patterns of transits. The depth and repetition time of transits provide the size of the planet relative to the star and its orbital period. Multi-band ground-based observation of these stars is currently underway to estimate the stellar parameters and to choose appropriate targets. With these parameters, the true planet radius and orbit scale, hence the relation to the HZ can be determined. These spectra are also used to discover the relationships between the characteristics of planets and the stars they orbit. In particular, the association of planet size and occurrence frequency with stellar mass and metallicity will be investigated. At the end of the four year mission, several hundred terrestrial planets should be discovered with periods between 1 day and 400 days if such planets are common. A null result would imply that terrestrial planets are rare. Based on the results of the recent Doppler-velocity discoveries, over a thousand giant planets will also be found. Information on the albedos and densities of those giants showing transits will be obtained. The mission is now in Phase C/D development and is scheduled for launch in 2008 into a 372-day heliocentric orbit.

Kepler system numerical model for the detection of extrasolar terrestrial planets

The objective of the NASA Ames Kepler mission is the detection of extrasolar terrestrial-size planets through transit photometry. In an effort to optimize the Kepler system design, Ball Aerospace has developed a numerical photometer model to simulate the sensor as well as stars and hypothetical planetary transits. The model emulates the temporal behavior of the incident light from 100 stars (with various visual magnitudes) on one CCD of the Kepler focal plane array. Simulated transits are inserted into the light curves of the stars for transit detection signal-to-noise ratio analyses. The Kepler photometer model simulates all significant CCD characteristics such as dark current, shot noise, read out noise, residual non-uniformity, intrapixel gain variation, charge spill over, well capacity, spectral response, charge transfer efficiency, read out smearing, and others. The noise effects resulting from background stars are also considered. The optical system is also simulated to accurately estimate system optical point spread functions and optical attenuation. In addition, spacecraft pointing and jitter are incorporated. The model includes on-board processing effects such as analog-to-digital conversion, photometric aperture extraction, and 15-minute frame co-addition. Results from the model exhibit good agreement with NASA Ames lab data and are used in subsequent signal-to-noise ratio analyses to assess the transit detection capability. The reported simulations are run using system requirements rather than predicted performance to guarantee that mission science objectives can be attained. The Kepler Photometer Model has given substantial insight into the Kepler system design by offering a straightforward means of assessing system design impacts on the ability to detect planetary transits. It is used as one of the various tools for the establishment of system requirements to ensure mission success.

Stellar Variability Observed with Kepler

The Kepler photometer was launched in March 2009 initiating NASA’s search for Earth-size planets orbiting in the habitable zone of their star. After three years of science operations, Kepler has proven to be a veritable cornucopia of science results, both for exoplanets and for astrophysics. The phenomenal photometric precision and continuous observations required in order to identify small, rocky transiting planets enables the study of a large range of phenomena contributing to stellar variability for many thousands of solar-like stars in Kepler’s field of view in exquisite detail. These effects range from <1 ppm acoustic oscillations on timescales from a few minutes and longward, to flares on timescales of hours, to spot-induced modulation on timescales of days to weeks to activity cycles on timescales of months to years. Recent improvements to the science pipeline have greatly enhanced Kepler’s ability to reject instrumental signatures while better preserving intrinsic stellar variability, opening up the timescales for study well beyond 10 days. We give an overview of the stellar variability we see across the full range of spectral types observed by Kepler, from the cool, small red M stars to the hot, large late A stars, both in terms of amplitude as well as timescale. We also present a picture of what the extended mission will likely bring to the field of stellar variability as we progress from a 3.5 year mission to a 7.5+ year mission.

FRESIP - Searching for Earth sized planets

This paper describes an instrument concept for the FRequency of Earth Sized Inner Planets (FRESIP) program. An orbiting photometer with a one-meter class telescope will be used to continuously monitor the brightness of over 5,000 target stars. A drop in light intensity will be detected when a planetary transit occurs within the telescope line of sight. Subsequent detections and confirmed predictions would constitute a planetary detection.