<P> In 1987, a team of scientists examined some meteorites and found grains of diamond about 2 nanometers in diameter (nanodiamonds). Trapped in them were noble gases whose isotopic signature indicated they came from outside the Solar System . Analyses of more meteorites found nanodiamonds from many different stars . The record of their origins was preserved despite a long and violent history that started when they were ejected from a star into the interstellar medium, went through the formation of the Solar System, were incorporated into a planetary body that was later broken up into meteorites, and finally crashed on the Earth's surface . </P> <P> In meteorites, nanodiamonds make up about 3 percent of the carbon and 400 parts per million of the mass . Grains of silicon carbide and graphite also have anomalous isotopic patterns . Collectively they are known as presolar grains or stardust and their properties constrain models of nucleosynthesis in giant stars and supernovae . </P> <P> It is unclear how many nanodiamonds in meteorites are really from outside the Solar System . Only a very small fraction of them contain noble gases and until recently it was not possible to study them individually . On average, the ratio of carbon - 12 to carbon - 13 matches that of the Earth's atmosphere while that of nitrogen - 14 to nitrogen - 15 matches the Sun . Techniques such as atom probe tomography will make it possible to examine individual grains, but due to the limited number of atoms, the isotopic resolution is limited . </P> <P> If most nanodiamonds did form in the Solar System, that raises the question of how this is possible . On the surface of Earth, graphite is the stable carbon mineral while larger diamonds can only be formed in the kind of temperatures and pressures that are found deep in the mantle . However, nanodiamonds are close to molecular size: one with a diameter of 2.8 nm, the median size, contains about 1800 carbon atoms . In very small minerals, surface energy is important and diamonds are more stable than graphite because the diamond structure is more compact . The crossover in stability is between 1 and 5 nm . At even smaller sizes, a variety of other forms of carbon such as fullerenes can be found as well as diamond cores wrapped in fullerenes . </P>

Which planet in our solar system rains diamonds