On the 18th of Febraury 2010, Solar Orbiter has been selected by the ESA's Science Programme Committee as one of the M-class missions that enter the so-called ``definition phase'', the last step before the final decision that will determine which mission has to be implemented.

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Two other missions were chosen to enter this pre-final stage: Euclid, that will address the fundamental questions concerning the nature of dark energy and dark matter; and PLATO, whose main objective consists in detecting extra-solar planets and hence to put constraints on the frequency of planets around stars.
Among these three missions only two of them will be selected for the M-class launch slots. The final decision will be taken around the mid-2011, when the definition activities are expected to be completed. The Science Programme Commettee will then be able to verifsy which mission fits within the available budget, this appearing to be a crucial criterium in the evaluation process.
More information can be found here in the ESA Cosmic Vision programme.