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stardust2asteroids Report Summary

Project ID: 616027
Funded under: FP7-IDEAS-ERC
Country: Denmark

Mid-Term Report Summary - STARDUST2ASTEROIDS (Stardust to asteroids: Unravelling the formation and earliest evolution of a habitable solar system)

The main objective of the STARDUST2ASTEROIDS project is to investigate the timescales and processes -- including the role of supernovas -- leading to the formation of the solar system by measurement of isotopic variations in meteorites. To achieve our objectives, we integrate long-lived and short-lived radioisotope chronometers with the presence/absence of nucleosynthetic anomalies in various meteorites and meteoritic components. This allows us to provide insights into the formation and earliest evolution of the solar system and, hence, the initial conditions required for the formation of habitable words like Earth. During the first half of the grant period, we have made significant progress toward our main objectives, including a total of 20 publications published in top tier peer-reviewed journals. We briefly highlight below some of the major discoveries.

Our research programme requires development of novel analytical techniques allowing isotope ratios to be measured to unprecedented precision and accuracy. This reflects the need for analyzing minute quantities of rare materials and the higher level of precision and accuracy needed to tackle a significant part of our research program. We have developed new protocols for the high-precision measurements of Cr, Pt and Nd isotopes, which were reported in a number of articles published in technique-oriented journals in 2014 and 2016. Importantly, these new technical developments form the basis of a number high-impact results published during the first half of the grant period.

Comets are pristine, volatile-rich objects formed beyond the orbits of the gas giants and, thus, thought to preserve a record of the primordial molecular cloud material parental to our Solar System. Using high-precision magnesium and chromium isotopes, we have showed that a that a class of pristine chondrites, the metal-rich carbonaceous chondrites, has a signature distinct from most inner solar system planets and asteroids. This signature is consistent with that predicted for unprocessed primordial molecular cloud material, suggesting that - similar to comets - metal-rich carbonaceous chondrites are samples of asteroids that accreted in the outer solar system. Therefore, these objects may provide a direct window into the formation history of the outer solar system. These results were reported in the Proceedings of the National Academy of Sciences in 2016.

We have also made significant progress towards our understanding of the formation history of chondrules, the most abundant early Solar System solids, and their role in promoting the accretion of asteroidal bodies and planetary embryos. Building on an earlier report defining the first absolute assumption free chronology of chondrules, we have now investigated the absolute ages of 25 chondrules from various chondrites indicating that these once molten millimetre spheres formed over a period of approximately 3 Myr, with approximately 50% of the chondrules recording ages that are within 1 Myr of solar system formation. These results establish that chondrules were present in the protoplanetary disk for its entire lifetime and, therefore, may promote the growth of asteroids and planetary embryos. To test this hypothesis, we have used supercomputer simulations to investigate the accretion of chondrules onto planetesimal and the continuous growth into planetary embryos. We show that this process is efficient and occurs over the lifetime of the disk, implying that chondrules represent a key ingredient in planet formation. These results were reported in the journal Science Advances in 2015.

Finally, we have collaborated with scientists in the field exoplanet research, to better understand the role of the host-star metallicity, a proxy for the initial amount of solids present in protoplanetary disks, in regulating planet formation processes. In detail, we obtained precise stellar parameters, including metallicities, for 403 stars orbited by 596 exoplanet candidates. This allowed us for the first time to test whether subtle but fundamental differences exist in the host-star metallicity of small exoplanets, which may be linked to distinct physical properties of the underlying planet populations. We find that the exoplanet sample can be categorized into three populations defined by statistically distinct metallicities, with transitions at 1.7 and 3.4 Earth radii. We interpret these three populations as reflecting the formation regimes of terrestrial-like exoplanets, rocky cores with modest H/He envelopes (which we denote gas-dwarfs planets) and gas-giant planets. These results were published in the journal Nature in 2014.

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