The search for extrasolar planets is one of the areas of astrophysics that has received a great boost in recent years from the use of ever more sophisticated observational techniques, data reduction and analysis. Given the obvious scientific and cultural interest in the subject, with its consequent public impact, the field is in continuous development and ever more sophisticated instruments and more ambitious space missions are being developed. By its nature, the search for extrasolar planets requires large telescopes, and in some cases, dedicated instruments. Apart from surveys from the ground with high precision spectrographs like HARPS and SOPHIE (and in the future ESPRESSO and CODEX), the photometric stability guaranteed by space missions allows for the detection of transits by small planets. The two missions of this kind currently in orbit are CoRoT and Kepler.
So far, over 500 extrasolar planets have been found, and new discoveries are constantly being made. Most planets found so far are Jupiter type planets located very close to their host star. This is simply a consequence of the greater ease with which such planets can be detected with the spectroscopic, radial velocity technique (Doppler effect). Nonetheless, recently it has been possible to detect a certain number of planets with masses well below that of Jupiter. Among these are the so-called super-Earths, that is, rocky planets with a mass between 2 and 10 times the mass of Earth. The current challenge is to find them also around stars of low mass with orbital radii that would permit the presence of liquid water on the surface of the planet. It is also expected that in this case the combination of the light curve and the radial velocity will allow the determination of the absolute dimensions of the star and the planet with great precision. In the case of planets passing in front of their stars, spectroscopy also allows the study of the chemical composition of the atmosphere. The few planets with long periods located at large distances from their stars have been discovered using direct imaging, timing measurements, in the case of variable stars, or via microlensing, but in none of these cases is it possible to determine the physical properties.
The large number of extrasolar planets discovered allows the study of planetary formation mechanisms to begin, in a context far wider than that permitted by our own solar system. In particular, the discovery of "Hot Jupiters" has called into question the accepted mechanisms for the formation and evolution of planets, demonstrating that planetary systems very different from our own can exist, even in complex, and seeming inappropriate, environments. The presence of these planets, that could not have formed where they are currently observed, has led to substantial modifications to models of planetary system formation. The application of the techniques of asteroseismology should guarantee a significant improvement in this field thanks to the determination of the age of the star.
Observations at different wavelengths have permitted the derivation of spectral information such as the identification of water molecules even in hot planets. These observations give the possibility of studying the composition of planetary atmospheres, and to compare the results with theoretical predictions. For this reason, space missions are being conceived, both in Europe and the USA, with the ultimate objective of identifying molecules that would indicate the presence of life on terrestrial planets. The path to this goal is long and extremely ambitious and will require a series of intermediate steps that include the complete characterisation of planets in terms of mass, radius, composition, internal structure, habitability and magnetic properties, as well as the properties of their atmospheres in terms of climatic conditions, temperature distribution, density and chemical composition. It should be pointed out that the study of the interaction between stars and planets in close orbits is one of the fields of research active in Italy thanks to the considerable experience gained in the study of stellar activity.
Italy is participating in the development of the SPHERE and EPICS instruments for the detection of planets via direct imaging. SPHERE is designed to search for Jupiter type planets, while EPICS is for smaller planets. Finally, the Italian community is strongly involved in the development of ESA's space mission, PLATO, designed for the detection of transits of planets of terrestrial dimensions located in the habitable zone and the simultaneous characterisation of the asteroseismology of the central star. The scientific motivations for PLATO are the natural continuation of those that inspired the CoRoT and Kepler missions.