Our Sun missed the stellar "baby boom" that erupted in our young Milky Way galaxy 10 billion years ago. During that time the Milky Way was churning out stars 30 times faster than it does today. Our galaxy was ablaze with a firestorm of star birth as its rich reservoir of hydrogen gas compressed under gravity, creating myriad stars. But our Sun was not one of them. It was a late "boomer," arising 5 billion years later, when star birth had plunged to a trickle.
A primordial origin for the compositional similarity between the Earth and the Moon
Nature 520, 7546 (2015). doi:10.1038/nature14333
Authors: Alessandra Mastrobuono-Battisti, Hagai B. Perets & Sean N. Raymond
Most of the properties of the Earth–Moon system can be explained by a collision between a planetary embryo (giant impactor) and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic compositions show them to be highly similar. In contrast, the compositions of other Solar System bodies are significantly different from those of the Earth and Moon, suggesting that different Solar System bodies have distinct compositions. This challenges the giant impact scenario, because the Moon-forming impactor must then also be thought to have a composition different from that of the proto-Earth. Here we track the feeding zones of growing planets in a suite of simulations of planetary accretion, to measure the composition of Moon-forming impactors. We find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are statistically more similar to the planets they impact. A large fraction of planet–impactor pairs have almost identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.
Saturn’s fast spin determined from its gravitational field and oblateness
Nature 520, 7546 (2015). doi:10.1038/nature14278
Authors: Ravit Helled, Eli Galanti & Yohai Kaspi
The alignment of Saturn’s magnetic pole with its rotation axis precludes the use of magnetic field measurements to determine its rotation period. The period was previously determined from radio measurements by the Voyager spacecraft to be 10 h 39 min 22.4 s (ref. 2). When the Cassini spacecraft measured a period of 10 h 47 min 6 s, which was additionally found to change between sequential measurements, it became clear that the radio period could not be used to determine the bulk planetary rotation period. Estimates based upon Saturn’s measured wind fields have increased the uncertainty even more, giving numbers smaller than the Voyager rotation period, and at present Saturn’s rotation period is thought to be between 10 h 32 min and 10 h 47 min, which is unsatisfactory for such a fundamental property. Here we report a period of 10 h 32 min 45 s ± 46 s, based upon an optimization approach using Saturn’s measured gravitational field and limits on the observed shape and possible internal density profiles. Moreover, even when solely using the constraints from its gravitational field, the rotation period can be inferred with a precision of several minutes. To validate our method, we applied the same procedure to Jupiter and correctly recovered its well-known rotation period.