The search for extrasolar planets, those beyond our solar system, has been transformed in recent years by exploiting the transit method for their discovery. Looking for the minuscule dip in a star's light when a planet crosses in front of it promises to reveal small rocky worlds and even ones within the star's habitable zone where liquid water and possibly life might be found.

Foremost in the quest has been Nasa's Kepler spacecraft which stares at a region of sky between Deneb and Vega and monitors continuously the brightness of some 150,000 stars. It is now three years into a mission that was initially due end later this year but has just been prolonged until 2016.

The extension is good news for the army of citizen scientists who scan Kepler's data for the tell-tale dips in stellar brightness that might indicate or confirm a new planet – ones that might have been missed by Kepler's automated algorithms. Anyone can get involved in this venture by signing up at planethunters.org as did many thousands of others as a result of the BBC's Stargazing Live programmes in January.

Our artist's impression shows an imaginary view from the vicinity of Kepler-16b, the first planet known to orbit two stars. Announced last September, it has the unofficial name Tatooine after the double-sun planet in the Star Wars series. Though it orbits near the outer edge of the system's habitable zone, it is Saturn-sized, largely gaseous and may be a poor candidate for life.

Other Kepler finds are more promising, including Kepler-22b which is a super-Earth (2.4 times Earth's diameter) located comfortably within the habitable zone. Some have speculated that it has planet-wide oceans, but confirmation may be difficult even using the James Webb Space Telescope or the next generation of gigantic ground-based telescopes.

Kepler's extended mission greatly improves the chances of finding Earth-like planets. To verify that any planet exists, Kepler needs to detect at least three regularly-spaced dips in its star's brightness. In the case of a planet like the Earth, in a one-year orbit, that would take a minimum of two or three years.

The tally of confirmed extrasolar planets stood at 763 a week ago, of which 61 are credited to Kepler. However, Kepler has more than 2,300 candidate planets awaiting confirmation of which perhaps 9% may be similar in size to the Earth, with the rest larger. Estimates based on Kepler's findings suggest that our galaxy alone contains at least 50 billion planets, with at least 500 million within the habitable zone of their star.