PLATO is a post-CoRoT and post-Kepler mission among the call for proposals for ESA's 2015-2025 (Cosmic Vision) programme. The main idea is that the formation and evolution of stars, their magnetic fields and their planets are intrinsically linked and should be studied simultaneously. PLATO therefore proposes:

• to select a large sample of stars of all masses and ages (the sample including more than 100000 objects);

• to search for planets and define their characteristics by the transit method;

• to study the rotation and internal structure of the same stars by asteroseismology;

• detect, describe and map the magnetic fields of the same stars by UV monitoring and by tomography techniques.

Unlike CoRoT or Kepler, the idea for PLATO is to try to observe over the widest possible field, so as to be able to select not just the brightest objects. PLATO's field may therefore be more than 30° x 30° (i.e., nearly 1000 square degrees!).

The basic features of PLATO are currently being defined, but observing such a vast field may be envisaged as consisting of a mosaic of small telescopes (10 cm in diameter), together covering the whole field, with a photometric performance comparable with that of a telescope 1 m in diameter.

Other Space-Borne Observatories

Several space-borne observatories were used to study transiting planets.

The MOST mission (Microvariability and Oscillations of STars) is the first Canadian space observatory. It is a micro-satellite with a 15-cm telescope allowing high-accuracy photometry designed to detect stellar oscillations and the reflected light from transiting planets. The observation is performed thanks to an array of Fabry lenses which project a large, stable image of the telescope pupil. The photometry is obtained by measuring the stellar flux within the telescope's pupil. Because of stray light problems, the accuracy of MOST did not permit a definitive conclusion about the reflected light from HD 209458 b.

Even though the Hubble Space Telescope (HST) was not particularly designed to the search for planets by the measurement of transit dimming, it was the first space facility to give the lightcurve of a transiting system (HD 209458 b, Brown et al. 2001; see references for Chap. 2). HST instruments have been used for many observations, particularly the Wide Field Planetary Camera, the STIS and the NIC-MOS instruments. HST was used to perform high-accuracy photometry of a globular cluster (47 Tucanae) but did not detect any transits there. HST was also used in the detection of sodium in the atmosphere of HD 209458 b (Charbonneau et al. 2002) and in obtaining an accurate determination of the chromatic diameter of this object, enhancing the role of the atmosphere.

The Spitzer observatory (see Sect. and particularly the IRS spectrograph, is currently used to observe secondary transits of transiting stars. Comparison with models of hot planets allowed the announcement of the detection of several gaseous species, such as H20 and CH4, in the atmospheres of HD 209458 b and / or HD 189733 b. It is currently one of the productive tools in determining the characteristics of the atmospheres of hot planets, because the observation of a secondary transit allows differentiation between the stellar and the planetary flux (this is thus a direct detection of the atmosphere), allowing direct detection of molecular species. These kinds of observation should be performed at a higher spectral resolution using the JWST (see Sect.

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