Kepler

The Kepler mission, led by NASA Ames, is a spaceborne telescope designed to search for planets about other stars by observing the small dip in light received from the star caused by a planet's transit in front of it. The mission is named after the German astronomer Johannes Kepler (1571-1630), who deduced his three famous orbital laws in 1609 (First and Second Law) and 1619 (Third Law). Kepler will continuously and simultaneously monitor the brightnesses of approximately 100,000 A-K dwarf

Figure 8.9. A schematic of the Kepler telescope, due for launch in 2009 to search for extrasolar planetary transits.

Figure 8.9. A schematic of the Kepler telescope, due

Photometer Electronics

Spacecraft Electronics

Photometer Electronics

Courtesy of NASA.

Spacecraft Electronics

(main-sequence) stars brighter than magnitude 14 in the region of the Cygnus constellation along the Milky Way. The star field in this region is very dense and far enough from the ecliptic plane so as not to be obscured by the Sun at any time of the year and also to avoid confusion resulting from possible occultations by asteroids and Kuiper Belt Objects.

The Kepler spacecraft is basically a Schmidt telescope with a 1.4 m diameter primary mirror and a 0.95m aperture (Figures 8.9, 8.10). The current planned spacecraft mass is ^900 kg, and the spacecraft will be placed in an Earth-trailing, heliocentric orbit where it will continuously monitor the Cygnus star field for a period of 4 years. It is currently planned to launch Kepler in 2009 using a Delta II launcher. The Kepler telescope will operate as a differential photometer utilizing forty-two 50 x 25 mm CCD arrays, each containing 2,200 x 1,024 pixels, and the FOV of the combined instrument will be roughly 12° in diameter. The CCDs will not record images as such, but instead will monitor and return data from those CCD pixels where there are stars with a visible magnitude greater than 14. The image system is intentionally defocused to an angular resolution of 10" to improve photometric precision and the signals from the CCD arrays will be integrated for 15 minutes. The orbital position of the telescope chosen is required to ensure low background loading and to allow unbroken observations which are essential for the mission.

Although the probability of a planet passing in front of an individual star during Kepler's mission is small, it is certainly not insignificant and Kepler will monitor so many stars that current formation and accretion models predict the following results:

1. Observation of about 50 transits of Earth-mass planets at a distance of AU.

2. Observation of thousands of transits of Earth-mass planets for orbits significantly less than 1 AU.

42 CCDp vipwinn >100 sq I

Interface electronics with15 minute integrations

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1 4 m uia. primary mirror

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Sunshade hr:■;;;! fi!;:^; s^somtiV wiiii Figure 8.10. Schematic of 2' i:ol:3- nr-vioni lo'is?s a'id Kepler's optical system.

Courtesy of NASA.

0.95 m dia, Schmidt corrector

Sunshade

3. Observation of reflected light from ^900 giant planets with periods <1 week.

4. Observation of about 130 transits of inner-orbit giants, with albedo determination for ~100 of these.

5. Determination of the density of ~35 inner-orbit giant planets.

6. Observation of the transit of ~30 outer-orbit giants.

While the Kepler mission is designed primarily to search for terrestrial planets in the "habitable zone'' about other stars, the detection method is less biased than the Doppler shift method, and so Kepler should be able to build up a much more representative statistical picture of the distribution of planet sizes and orbital distances in our galaxy. Such data will provide a strong test of current formation theories, and indeed on the representativeness of our own solar system.

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