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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. 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. The field planets are usually the size of Neptune or smaller. In contrast, only four planets have been found orbiting stars in open clusters, 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., Comment: 18 pages, 6 figures, 1 table (main text + supplementary information)

There has been a long-running debate about the origins of the eight (at last count) metal-rich meteorites called CB chondrites, named after Bencubbin in Australia where the first one was found in 1930. Their origins are important as an indication of the conditions prevailing in the early Solar System. New lead-207/lead-206 ages of chondrules (small round inclusions) in two CB chondrites are inconsistent with an origin in shockwave heating in the solar nebula, the usual source of chondrules in other chondritic meteorites. Instead they seem to have formed 5 million years later from material generated by collisions between planet-sized objects in the early Solar System. Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves1,2) in the protoplanetary disk around the early Sun. The ages of primitive chondrules3,4,5,6 in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 ± 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars7. Here we report the 207Pb–206Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 ± 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 ± 0.9 Myr), which formed during a single-stage, highly energetic event8,9,10,11. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave2. We conclude that chondrules and metal grains in the CB chondrites formed from a vapour–melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System12.

The geochemical composition of the Earth's upper mantle1,2,3 is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re–Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt3. Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re–Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a ‘plum pudding’ distribution in the upper mantle, rather than from individual melt depletion events.