For the first time in history we now know of more planets outside our solar system than in it. All of these extrasolar planets are about the size of Jupiter or larger. The Kepler Mission proposes to search for hundreds of Earth-size planets. The concept consists of monitoring 100,000 stars continuously for four years for planetary transits. An Earth-sized transit of a solar-like star produces a relative change in brightness of 8x10-3 for a duration of a few to 16 hours, depending on the orbit and inclination of the planet. A technology demonstration showed that a relative precision of better than 2x10-5 is achievable when all of the realistic noise sources are incorporated in a full-up end-to-end system. A commercially available back-illuminated charged coupled device (CCD) was used for the tests. The same device can be used in the proposed Kepler Mission.
The technology demonstration test facility incorporated the ability to control and measure the following effects on the noise performance of the end-to-end system: varying the CCD operating temperature; changing the focus; varying the photometric aperture; operating over a dynamic range of five stellar magnitudes; working in a crowded star field; reading out the CCD without a shutter; translating the image to several discrete magnitudes brighter than the brightest target stars; operating with spacecraft jitter up to ten times the anticipated amplitude; and simulating the effects of cosmic rays and stellar variability.
The testbed source incorporates all the characteristics of the real sky that are important to the measurements. It produces the same flux as real 9th to 14th magnitude stars, has the same spectral color as the Sun, has the same star density as the Cygnus region of the Milky Way down to stars as faint as 19th magnitude, has several 4th magnitude stars, and has the ability to produce Earth-size transits for selected stars. The camera simulates all the functions to be performed by the space-borne photometer, namely, fast optics, a flight-type CCD, readout without a shutter, a high-speed readout of one megapixel per second, and proto-flight data reduction and analysis software. Piezoelectric transducers are used to provide tip-tilt of the camera to reproduce the motion caused by spacecraft pointing jitter.
To fully demonstrate the concept, transits were created during the testing. Representative transits are shown in figure 1 for 9th (left), 12th (middle), and 14th (right) magnitude stars. The transit depth is given in equivalent Earth size, and the error bars are the one-sigma noise for the data.
Point of Contact: D. Koch
(650) 604-6548
dkoch@mail.arc.nasa.gov
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