PLATO (PLAnetary Transits and Oscillations of stars), a candidate mission for the M3 slot in ESA' s Cosmic Vision programme, will characterise exoplanetary systems by detecting planetary transits and conducting asteroseismology of their parent stars.

Instrument concepts

Since the detection of exoplanets is of a statistical nature, due to the fact that the proportion of stars having exoplanets is unknown, PLATO will need to observe as many stars of the necessary type as possible, as accurately as possible. This can be translated into three main mission requirements:

• large field-of-view– more stars can be seen when the observed sky field is larger;
• large collecting area – a large collecting area results in observing more faint stars with the required accuracy;
• long mission lifetime – a long mission lifetime allows for several sky fields to be observed over periods of several years.

The first two mission design drivers are difficult to combine. The best observation strategy has been determined to be the 'staring' concept, as opposed to a Gaia-like spinning concept. Using the staring concept, the observations are conducted using multiple individual telescopes, in order to comply with the demanding requirements on large field-of-view and large collecting area. Using a number of sub-apertures gives several advantages compared to using one large aperture; it allows the stringent requirements in terms of signal-to-noise to be met and the large sky field size required for observing sufficient stellar targets. It also achieves a higher level of redundancy, since the loss of a telescope will reduce the signal-to-noise ratio, but not cause the mission to fail.