Astronomers on Tuesday said that for the first time they had spotted two Earth-sized worlds orbiting a Sun-like star, in another big advance in the search for so-called exoplanets.

One of the planets is just three percent bigger than Earth and the other is 13 percent smaller, which would make it a bit tinier than Venus, they reported online in the British science journal Nature.

The planets are inferred to have a rocky composition similar to Earth's but they orbit so close to their star, Kepler-20, that the temperature is likely to be far too high to nurture life.

The larger planet, Kepler-20f, completes a "year" in 19.5 days and may have a thick water-vapour atmosphere, while the smaller one, Kepler-20e, zips around the star in just 6.1 days.

Spotting the pair is a technical feat.

They are the smallest exoplanets to be found since the first world beyond our Solar System was officially detected in 1995. Their distance, too, is enormous: Kepler-20 is about a thousand light years from Earth.

So far 709 planets have been netted in 534 star systems, according to a tally compiled by the Extrasolar Planets Encyclopaedia (http://exoplanet.eu/catalog.php).

Almost all are gas giants or are located too close or too far from their star to allow water -- the stuff of life -- to exist in liquid form.

Only three have been confirmed as being rocky and orbiting in the "Goldilocks zone," where the temperature is balmy.

Two of the three are Gliese 581d and HD 85512b, orbiting stars that are cooler and smaller than the Sun. The third is Kepler-22b, unveiled on December 5, which is 2.4 times the size of the Earth, orbiting a Sun-like star every 290 days.

The two new discoveries were found by a team led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics, using NASA's Kepler orbiting space telescope.

A 600-million-dollar mission launched in March 2009, Kepler monitors more than 150,000 stars for tiny wobbles in light.

This could signal a planet which is passing in front of the star and is thus dimming the light reaching the telescope.

So far, Kepler has notched up 2,326 "planet candidates" -- sightings that could turn out to be exoplanets if they are confirmed by further observations.

NASA astronomers assign the name of Kepler and a number to a star where the telescope has found exoplanets. The exoplanets are then identified by a lower-case letter in order of discovery -- for instance Kepler-12b is the second exoplanet to have been found orbiting the star Kepler-12.