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Galactic gravitational microlensing is a powerful technique to detect extrasolar planets at large orbital distances from their stars, from giant down to Earth-mass planets. We report a statistical analysis (Cassan et al. 2012) that combines six years of microlensing observations gathered between 2002 to 2007 by the PLANET and OGLE collaborations. From these data, we estimate the frequency of cool extrasolar planets, with masses ranging from 5 Earths to 10 Jupiters and orbits between 0.5 to 10 Astronomical Units. We find that in average, one in six stars has a Jupiter-like gas giant as companion planet, that about half the stars are orbited by a Neptune-like giant, and two-thirds are associated to super-Earths. Our study also suggests that planets should be ubiquitous throughout the Galaxy. Current deployment of wide-field imagers and possible space-based observations onboard ESA spacecraft EUCLID will soon allow a large increase of the number of monitored microlensing events. These new observatories should provide in a near future a more detailed view on planet abundance as a function of mass.
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.
Microlensing has proven to be a valuable tool to search for extrasolar planets of Jovian- to Super-Earth-mass planets at orbits of a few AU. Since planetary signals are of very short duration, an intense and continuous monitoring is required. This is achieved by ground-based networks of telescopes (PLANET/RoboNET, μFUN) following up targets, which are identified as microlensing events by single dedicated telescopes (OGLE, MOA). Microlensing has led to four already published detections of extrasolar planets, one of them being OGLE 2005-BLG-390Lb, a planet of only ∼5.5 M⊕ orbiting its M-dwarf host star at ∼2.6 AU. Very recent observations (May–September 2007) provided four more planetary candidates, still under study, that will double the number of detections. For non-planetary microlensing events observed from 1995 to 2006 we compute detection efficiency diagrams, which can then be used to derive an estimate of the Galactic abundance of cool planets in the mass regime from Jupiters to Sub-Neptunes.
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