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Planets with sizes between that of Earth (with radius [R.sub.⊕] and (Neptune (about 4[R.sub.⊕) are now known to be common around Sun-like stars (1-3). Most such planets have been discovered through the transit technique, by which the planet's size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet's mass--and hence its density, which is a clue to its composition--is more difficult. Planets of size 2-4[R.sub.⊕ have proved to have a wide range of densities, implying a diversity of compositions (4,5), but these measurements did not extend to planets as small as Earth. Here we report Doppler spectroscopic measurements of the mass of the Earth-sized planet Kepler-78b, which orbits its host star every 8.5 hours (ref. 6). Given a radius of 1.20 ± 0.09[R.sub.⊕] and a mass of 1.69 ± 0.41[M.sub.[direct sum]], the planet's mean density of 5.3 ± 1.8 g [cm.sup.-3] is similar to Earth's, suggesting a composition of rock and iron., Kepler-78 is one of approximately 150,000 stars whose brightness was precisely measured at 30-min intervals for four years by NASA's Kepler spacecraft (7). This star is somewhat smaller, less massive [...]

Most stars and their planets form in open clusters. Over 95 per cent of such clusters have stellar densities too low (less than a hundred stars per cubic parsec) to withstand internal and external dynamical stresses and fall apart within a few hundred million years (1). Older open clusters have survived by virtue of being richer and denser in stars (1,000 to 10,000 per cubic parsec) when they formed. Such clusters represent a stellar environment very different from the birthplace of the Sun and other planet-hosting field stars. So far more than 800 planets have been found around Sun-like stars in the field (2). The field planets are usually the size of Neptune or smaller (3-5). In contrast, only four planets have been found orbiting stars in open clusters (6-8), all with masses similar to or greater than that of Jupiter. Here we report observations of the transits of two Sun-like stars by planets smaller than Neptune in the billion-year-old open cluster NGC6811. This demonstrates that small planets can form and survive in a dense cluster environment, and implies that the frequency and properties of planets in open clusters are consistent with those of planets around field stars in the Galaxy., Previous planet surveys in clusters have suffered from insufficient sensitivity to detect small planets, and from sample sizes barely large enough to find the less common larger planets (9). The [...]

How to Find a Habitable Planet by James Kasting Princeton University press: 2010. 360 pp. $29.95, 20.95 £ In the 15 years since the discovery of the first planet around [...]