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In 1929, the observation of the relative motion among galaxies by Hubble was interpreted as evidence of the cosmological principle, according to which no position (galaxy) in the universe is privileged. The discovery in 1964 of the electromagnetic fossil radiation at close to 3 K provided the missing experimental link between the unique thermodynamic origin of the universe and the present observable stellar era. These are the foundations of the standard cosmological model. The thermodynamic origin and the various stages of evolution provide the framework to understand the process of nucleosynthesis, and we expect that the relative abundances of the chemical elements in the universe are uniform on the large scale.
At this point, we might ask whether it makes sense to talk about the formulation of a “bio-cosmological principle,” stating that the probability of finding life in the universe is uniform, with no privilege for our galaxy. But right away we find a difficulty: in the standard cosmological model the observables are well defined by physics. Instead the assumed probability of finding life in the universe refers to something – life – that is not defined rigorously. We do not have a definition of life usable everywhere; we are in a pre-Galilean stage.
The science of extra-solar planets is one of the most rapidly changing areas of astrophysics and since 1995 the number of planets known has increased by almost two orders of magnitude. A combination of ground-based surveys and dedicated space missions has resulted in 560-plus planets being detected, and over 1200 that await confirmation. NASA's Kepler mission has opened up the possibility of discovering Earth-like planets in the habitable zone around some of the 100,000 stars it is surveying during its 3 to 4-year lifetime. The new ESA's Gaia mission is expected to discover thousands of new planets around stars within 200 parsecs of the Sun. The key challenge now is moving on from discovery, important though that remains, to characterisation: what are these planets actually like, and why are they as they are?
In the past ten years, we have learned how to obtain the first spectra of exoplanets using transit transmission and emission spectroscopy. With the high stability of Spitzer, Hubble, and large ground-based telescopes the spectra of bright close-in massive planets can be obtained and species like water vapour, methane, carbon monoxide and dioxide have been detected. With transit science came the first tangible remote sensing of these planetary bodies and so one can start to extrapolate from what has been learnt from Solar System probes to what one might plan to learn about their faraway siblings. As we learn more about the atmospheres, surfaces and near-surfaces of these remote bodies, we will begin to build up a clearer picture of their construction, history and suitability for life.
The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. By characterising spectroscopically more bodies in different environments we will take detailed planetology out of the Solar System and into the Galaxy as a whole.
EChO has now been selected by the European Space Agency to be assessed as one of four M3 mission candidates.
The SEE COAST concept is designed with the objective to characterize extrasolar planets and possibly Super Earths via spectro-polarimetric imaging in reflected light. A space mission complementary to ground-based near IR planet finders is a first secure step towards the characterization of planets with mass and atmosphere comparable to that of the Earth. The accessibility to the Visible spectrum is unique and with important scientific returns.
We have now entered a phase of extrasolar planets characterization: probing their atmospheres for molecules, constraining their horizontal and vertical temperature profiles and estimating the contribution of clouds and hazes. We review here the current situation with ground-based and space-based observations, and present the transmission spectra of HD189733b in the spectral range 0.5-24 microns.
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