Kepler

The Kepler space mission (Fig. 8.11) being prepared by NASA is similar in principle to CoRoT. In this context it may be considered a second-generation mission relative to CoRoT, which should be seen as a pioneer in this technique. Kepler should be launched by a Delta II rocket in early 2009.

The principal differences between CoRoT and Kepler are:

• The duration of observations of single target: CoRoT observes the same field for 150 days, Kepler for 4 years. The aim of Kepler is to detect true terrestrial analogues, i.e., planets 1 AU from solar-type stars, orbiting in one Earth year. To detect and confirm the object and its period at least three transits are required: two to identify the period, and the third to confirm the nature of the event and the periodicity. The possibility of detecting long-period terrestrial-type planets (maximum period about two years), will allow - as CoRoT will do for short-period objects - a statistical analysis of the distribution in size, distance, and of the orbital parameters of the planets. CoRoT and Kepler are therefore, from a scientific point of view, the terrestrial-planet counterpart of the radial-velocity observational programmes for giant planets.

• The size of the telescope: Kepler has a telescope 95 cm in diameter (against 27 cm for COROT). The photometric curve's optimum signal-to-noise ratio (photon noise) is attained in a few minutes by Kepler, whereas CoRoT requires an hour's observation.

• The field of the telescope: Kepler observes a complete field of 105 square degrees in the constellations of Cygnus and Lyra, as against 3.5 square degrees for CoRoT. This large field allows Kepler to observe simultaneously about 100000 targets on the Main Sequence between magnitudes 9 and 15. The same field is observed for the four years of the nominal mission. The major advantage that Kepler's large field offers is that it allows a choice of targets and avoids contamination by neighbouring objects (this is the same for CoRoT, which has to point close to the galactic plane to increase the number of objects in its significantly smaller field).

On the technical side, Kepler consists of a telescope (Fig. 8.12), whose focal plane it covered by 42 CCDs, each containing 2200 x 1024 pixels. The observational passband extends from 430 to 890 nm. Photometric performance, including overall noise, stellar variability, and photon noise, is better than 2 x 10~5 for a star of magnitude 12.

The orbit chosen for Kepler is a heliocentric orbit drifting behind the Earth. This orbit allows the same field to be observed throughout the year, but still avoiding the relative positions of the Sun and Moon, which are the principal sources of parasitic light. Just like CoRoT, Kepler has a baffle that allows very little scattered light to pass.

Table 8.4 summarizes the results expected from Kepler as a function of the size of the objects, assuming that:

Solar Arrays

RCS Thruster Module (1 of 4)

Battery -

Star Tracker

Solar Arrays

RCS Thruster Module (1 of 4)

Battery -

Star Tracker

Photometer Electronics

Spacecraft Electronics

Deployable Cover

Fig. 8.12 Functional diagram of Kepler (NASA)

Photometer Electronics

Spacecraft Electronics

Deployable Cover

Photometer FPA Radiator

Fig. 8.12 Functional diagram of Kepler (NASA)

Table 8.4 Number of objects expected to be discovered by Kepler as a function of size

Average size of planets Number of objects detectable

1.3 Earth radii - 185

2.2 Earth radii - 640

• large numbers of the objects concerned exist around the stars being observed (the probability of occurrence being several tens of per cent)

• the majority of objects are of the size specified (medium size)

• the orbital period of the objects is close to one year

• observation takes place over 4 years, with 4 transits detected

• theories about stellar variability and detection criteria are conservative.

The problems facing identification of transits in photometric curves obtained by Kepler are identical to those encountered by CoRoT. Kepler needs the same algorithms to correct for instrumental bias, to search for transits, and similarly needs a comprehensive, ground-based photometric and radial-velocity programme to distinguish true planetary transits among the candidates from events that may have the same photometric signature (binaries with total eclipses, for example).

CoRoT and Kepler, combined with all the observations already made of radial velocities should enable us, in no more than 10 years' time, to have an extremely accurate picture of the statistical distribution of exoplanets, their sizes and their periods, around nearby stars. These missions should also provide statistical information concerning the multiplicity of such systems.

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