 wobbling slightly under the gravitational pull of the planet going round it.
Professor Natalie Batalha of San Jose State University, who helps lead the Kepler science mission for NASA, stressed the significance of the observation to the BBC.
“We’re always pushing down toward smaller and less massive, so it’s natural that we’re arriving there,” she said. “But perhaps what’s not so natural is that we’ve pinned down the properties of this planet with such fantastic accuracy that we’re able to say without a doubt that this is a rocky world, something that you could actually stand on.”
At the end of last year the official NASA count of confirmed exoplanets observed reached 500, compared to only 50 a decade ago . More than 100 of these were discovered in 2010. The Paris-based Extra Solar Planets Encyclopaedia  now has clocked a total of 518 planets.
Kepler is continually monitoring the brightness of more than 145,000 stars, and it is expected that many more exoplanets will be discovered over the next year or so. As the data already downloaded is analysed for exoplanets with longer orbits than that of Kepler 10-b, the likelihood of finding planets that are similar to the earth increases.
Before the discovery of Kepler-10b the smallest exoplanet known was Corot-7b, found at the beginning of 2009, with a diameter 1.7 times the earth’s and a mass five times bigger. It was found by the French-led Corot spacecraft and is even closer to its parent star than Kepler-10b, with a faster orbit. Corot-7b is also thought to be rocky and has a density somewhat less than if it were completely made of iron.
Last October, a team at NASA and the University of California, Berkeley, announced the result of a study of 166 stars over a five-year period at the Keck telescope in Hawaii , with exoplanets ranging from three to 1,000 times the mass of the earth. The exoplanets were all in orbits close to their stars, at a distance less than a quarter of the distance of the earth to the sun. Remarkably, it emerged that there was a distinctive trend in the distribution of exoplanet masses, with smaller planets outnumbering larger ones. Extrapolating from their data, the research team estimated that 23 percent of stars in our galaxy, or 46 billion stars, would have earth-sized planets orbiting the hot zone close to the star.
This result contradicts current theories on planet formation, which suggest that smaller planets would tend not to form in closer orbits. Nevertheless it gives support to the existence of large numbers of earth-size planets in more distant orbits and so possibly habitable. Andrew Howard of the University of California, Berkeley, lead author of the study explained, “During planet formation, small bodies similar to asteroids and comets stick together, eventually growing to Earth-size and beyond. Not all of the planets grow large enough to become giant planets like Saturn and Jupiter.” Howard added, “It’s natural for lots of these building blocks, the small planets, to be left over in this process.”
A series of discoveries of exoplanets with fascinating properties have been made of recent months. In September researchers from the University of California, Santa Cruz, and the Carnegie Institution of Washington, announced they had found two exoplanets orbiting around the nearby red dwarf star Gliese 581. Their findings are based on 11 years of observations at the Keck Observatory. One of these planets, Gliese 581g, has a mass three to four times that of the Earth and an orbital period of fewer than 37 days. It is thought to be the first exoplanet discovered that lies in the habitable zone of the star, with temperatures in the range required for liquid water to exist on the planet’s surface and to sustain an atmosphere.
In November, the first exoplanet from outside of our Milky Way galaxy was found. This Jupiter-sized planet orbits a star known as HIP 13044 that is part of the so-called Helmi stream. These are stars that originally belonged to a dwarf galaxy that was “eaten up” by our Milky Way some six to nine billion years ago.
Last month 