Formation of Planetary Systems The search for planets and life outside the Solar System had already been going on for decades when a real revolution within this exciting field of science occurred in 1995 with the discovery of the first extra-solar planet (exoplanet) by astronomers Michel Mayor and Didier Queloz from the Geneva Observatory (Switzerland). It was followed quickly by the discovery of two more giant planets by Geoffrey Marcy and Paul Butler of Lick Observatory (California, USA). Now, six years later, about 70 such giant planets have been found. This implies that at least 3-5% of all sun-like stars possess giant planets. Since this represents only the fraction of stars having planets that can be detected with currently available instruments, we must conclude that planet formation is not an extraordinary event, but rather quite a common occurrence. How representative is our Solar System? Our Solar System forms the basis for most of our information about how planetary systems may develop. However, the degree to which it is actually representative of all planetary systems is quite unclear. In fact, it now appears to be very different from all those systems discovered thus far. Indeed, in contrast to the giant planets in our own system (Jupiter, Saturn, Uranus, Neptune), most of the newly discovered exoplanets have much smaller orbits and many of them also have rather large eccentricities (i.e. the orbits are less circular). Although there is clearly a strong observational bias against detecting distant and/or small planets with current telescopes and instruments, it is significant that none of these newly discovered exoplanets should have existed according to the conventional formation theory! Does this mean that our Solar System is unusual or maybe even unique, or have we simply not yet been able to detect the right kind of systems elsewhere? To answer this central question we must attempt to obtain the fullest possible information about the possible diversity among existing planetary systems. At the same time, we must also try to reach a much better understanding of the physical processes that underlie their formation and evolution. Current ideas about planetary formation Current theories consider planetary formation as a natural and common process that takes place in connection with star formation. When a protostar contracts from an interstellar cloud, a protoplanetary disk (proplyd) of dust and gas forms around the newborn star. Small aggregates (dust and icy grains) are formed by condensation in this disk. Many of them collide to form larger and larger objects - in this way, planet-sized objects are eventually built up. Moving around the young star at the centre, these bodies gradually "sweep up" the smaller objects they encounter in their orbits, but many smaller bodies, like those we know in our solar system as asteroids and comets, may still survive this "cleaning" action. In our own Solar System, the Earth was formed by these processes and it went through a rather turbulent period while it was still a young object. It appears that its natural satellite, the Moon, was formed by a major collision some 4.5 billion years ago between the young Earth and a (Mars-sized ?) planet.

Life in the Universe
  Formation of Planetary Systems
    Planetary Formation
    Small Bodies
    Early Earth