In 2011, it was announced with great fanfare that Nasa's planet-hunting Kepler space telescope had detected 1,235 possible extra-solar planets during its first four months of operation including 68 approximately Earth-size worlds. Five of those potential worlds are orbiting in the parent star's habitable 'Goldilocks' zone, where conditions are 'just right' for liquid water to exist in environments favourable for life.

The planetary candidates must be confirmed by additional observations and analysis, and it will take another two years to collect enough data to determine how common Earth-like worlds might be. But based on the initial results, "Kepler's blown the lid off everything we know about extra-solar planets", according to Debra Fischer, an astronomer at Yale University, in reference to planets outside the solar system.

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011 Return to Ask the Astronomer