In 1995 the Geneva observatory was the first in the world to announce the discovery of a planet outside the solar system, a so-called "exoplanet". It was attached to a star in the constellation of Pegasus, 40 light years away from earth.

The study of exoplanets has since become one of the main areas of research in astronomy. Understanding them will help to understand the origin of the solar system and of the planet Earth. This research is also an important part of the search for extra-terrestrial life.

In April 2007 Swiss astronomers with colleagues from other European countries discovered the first exoplanet potentially able to sustain life, at a distance of 20.5 light years from Earth. The discovery was made using the Swiss-designed HARPS (High Accuracy Radial Velocity for Planetary Searcher) spectrograph on the telescope of the European Southern Observatory (ESO) in Chile.

This followed a number of other important discoveries. In 2004 the Geneva team and their European colleagues discovered the smallest and most earth-like planet known up to that time. It orbits the star mu Arae in the constellation of the Altar (Ara).

In May 2006 the journal Nature announced a further milestone in the search for exoplanets. A team of European astronomers, including some from Geneva and Bern, had discovered three new ones, named Neptune's Trident, in the Constellation Puppis, about 40 light years away. Of these, one is thought to have a gaseous outer envelope and a rocky, icy core, and is at a distance from its star which means that water could be present.

Many exoplanets have been discovered since the first in 1995 and the number is constantly rising. (In spring 2007 it stood at around 220.) Some are made of hot gas, like Jupiter, others are part gas and part rock, like Neptune or Uranus. The search for planets able to sustain life is particularly challenging, since they will necessarily be small, and astronomers will depend on ever improving instruments in order to detect them.

The probable nature of these exoplanets was worked out by the team at the Space Research and Planetary Sciences research division at the University in Bern which is particularly interested in the way in which exoplanets are formed.