Of more than 200 extra-solar planets so far discovered , or ‘'exoplanets', as scientists call planets found orbiting stars other than our sun, a great many are Jupiter-like gas giants orbiting very close to hot stars. The exoplanet orbiting Gliese 581 is the smallest yet found. It completes a full orbit of its parent star in just 13 days, and it is the most promising yet in terms of harbouring life. It lies in what scientists call the "Goldilocks Zone" where temperatures are just right for life to exist.

It is 14 times closer to its star than the Earth is to our Sun. However, its star is smaller and colder than our Sun and less luminous. So the planet still lies within the "habitable zone", the region around a star where water could exist as liquid.

The Gliese 581 system is now known to have three planets: the new super-Earth, a 15 Earth-mass planet orbiting even closer to the parent star, and an eight Earth-mass planet that lies further out. This is significant in terms of the solar-system formation model outlined later in this section ( see: 'Ecosphere' and 'Ecosphere II' ).

GLIESE 581

Mass: Five times Earth's mass
Orbit: 13 days
Temperature: 0C - 40C
Distance: 20.5 light years
Constellation: Libra