Smallest rocky planet outside our solar system discovered
28 January 2011
NASA has confirmed this month that its Kepler space observatory has now identified the smallest yet planet outside our solar system, exoplanet Kepler-10b. It is also the first time that NASA has found a planet known to be made of rock, as opposed to gas giants that are similar to Jupiter. This is because its diameter is known to be 1.4 times that of the earth and its mass is 4.6 times that of the earth, so its density is comparable to that of iron.
However, it is otherwise quite different from our planet as it orbits its star in less than one day, and is 20 times closer to it than Mercury is to the sun. Its surface will consequently be very hot and it cannot be in the habitable zone that would support life. Nevertheless, it is a considerable step towards finding earth-like planets in other solar systems. Geoffrey Marcy, of the University of California Berkeley a pioneer of exoplanet research, said that the find “will be marked as among the most profound scientific discoveries in human history.”
NASA’s Kepler mission was launched in 2009 specifically designed to look for earth-like planets orbiting stars in our Milky Way galaxy. The existence of Kepler-10b was based on data collected from May 2009 to January last year. Kepler uses a specially designed, extremely accurate photometer that measures the change in the brightness of a star when a planet passes in front of it.
Because the parent star of 10-b is one of the brighter stars targeted by Kepler, scientists can observe variations in its light very accurately, telling them much about its properties. Kepler is in orbit round the sun, not the earth, enabling it to focus continuously on selected stars without the earth blocking its field of vision.
Observations made by the ground-based Keck telescope in Hawaii of the light spectrum from the parent star confirmed the existence of a planet. These observations showed characteristic movements, known as a Doppler shift, which show that the star is 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 tre