Gerrit Budde, Christoph Burkhardt & Thorsten Kleine
Nature Astronomy Link to Article [https://www.nature.com/articles/s41550-019-0779-y]
Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany

Earth grew through collisions with Moon-sized to Mars-sized planetary embryos from the inner Solar System, but it also accreted material from greater heliocentric distances^1,2, including carbonaceous chondrite-like bodies, the likely source of Earth’s water and highly volatile species^3,4. Understanding when and how this material was added to Earth is critical for constraining the dynamics of terrestrial planet formation and the fundamental processes by which Earth became habitable. However, earlier studies inferred very different timescales for the delivery of carbonaceous chondrite-like bodies, depending on assumptions about the nature of Earth’s building materials^{5,6,7,8,9,10,11}. Here we show that the Mo isotopic composition of Earth’s primitive mantle falls between those of the non-carbonaceous and carbonaceous reservoirs^12,13,14,15, and that this observation allows us to quantify the accretion of carbonaceous chondrite-like material to Earth independently of assumptions about its building blocks. As most of the Mo in the primitive mantle was delivered by late-stage impactors¹⁰, our data demonstrate that Earth accreted carbonaceous bodies late in its growth history, probably through the Moon-forming impact. This late delivery of carbonaceous material probably resulted from an orbital instability of the gas giant planets, and it demonstrates that Earth’s habitability is strongly tied to the very late stages of its growth.

Francesco C. Pignatale^1,2, Sébastien Charnoz¹, Marc Chaussidon¹, and Emmanuel Jacquet²
Astrophysical Journal Letters 867, L23 Link to Article [DOI: 10.3847/2041-8213/aaeb22]
¹Institut de Physique du Globe de Paris (IPGP) 1 rue Jussieu, F-75005, Paris, France
²Muséum national d’Histoire naturelle, UMR 7590, CP52 57 rue Cuvier, F-75005, Paris, France

Chondritic meteorites, the building blocks of terrestrial planets, are made of an out-of-equilibrium assemblage of solids formed at high and low temperatures, either in our Solar system or previous generations of stars. For decades this was considered to result from large-scale transport processes in the Sun’s isolated accretion disk. However, mounting evidence suggests that refractory inclusions in chondrites formed contemporaneously with the disk building. Here we numerically investigate, using a 1D model and several physical and chemical processes, the formation and transport of rocky materials during the collapse of the Sun’s parent cloud and the consequent assembling of the Solar Nebula. The interplay between the cloud collapse, the dynamics of gas and dust, vaporization, recondensation, and thermal processing of different species in the disk results in a local mixing of solids with different thermal histories. Moreover, our results also explain the overabundance of refractory materials far from the Sun and their short-formation timescales, during the first tens of kyr of the Sun, corresponding to class 0-I, opening new windows into the origin of the compositional diversity of chondrites.

Judit Szulágyi^1,2, Marco Cilibrasi¹, and Lucio Mayer¹
Astrophysical Journal Letters 868, L13 Link to Article [DOI: 10.3847/2041-8213/aaeed6]
¹Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
²Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093, Zurich, Switzerland

Satellites of giant planets have been thought to form in gaseous circumplanetary disks (CPDs) during the late planet-formation phase, but it was unknown whether or not smaller-mass planets such as the ice giants could form such disks, and thus moons, there. We combined radiative hydrodynamical simulations with satellite population synthesis to investigate the question in the case of Uranus and Neptune. For both ice giants we found that a gaseous CPD is created at the end of their formation. The population synthesis confirmed that Uranian-like, icy, prograde satellite system could form in these CPDs within a couple of 10⁵ yr. This means that Neptune could have a Uranian-like moon system originally that was wiped away by the capture of Triton. Furthermore, the current moons of Uranus can be reproduced by our model without the need for planet–planet impact to create a debris disk for the moons to grow. These results highlight that even ice giants—among the most common mass category of exoplanets—can also form satellites, opening a way to a potentially much larger population of exomoons than previously thought.

Nienke van der Marel¹, Jonathan P. Williams², and Simon Bruderer³
Astrophysical Journal Letters 867, L14 Link to Article [DOI: 10.3847/2041-8213/aae88e]
¹Herzberg Astronomy & Astrophysics Programs, National Research Council of Canada, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada
²Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, 96822 Honolulu, HI, USA
³Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 2, D-85741 Garching bei München, Germany

High-resolution Atacama Large Millimeter/submillimeter Array observations of protoplanetary disks have revealed that many, if not all, primordial disks consist of ring-like dust structures. The origin of these dust rings remains unclear, but a common explanation is the presence of planetary companions that have cleared gaps along their orbit and trapped the dust at the gap edge. A signature of this scenario is a decrease of gas density inside these gaps. In a recent work, Isella et al. derived drops in gas density that are consistent with Saturn-mass planets inside the gaps in the HD 163296 disk through spatially resolved CO isotopologue observations. However, as CO abundance and temperature depends on a large range of factors, the interpretation of CO emission is non-trivial. We use the physical–chemical code DALI to show that the gas temperature increases inside dust density gaps, implying that any gaps in the gas, if present, would have to be much deeper, consistent with planet masses >M _Jup. Furthermore, we show that a model with increased grain growth at certain radii, as expected at a snowline, can reproduce the dust rings in HD 163296 equally well without the need for companions. This scenario can explain both younger and older disks with observed gaps, as gaps have been seen in systems as young <1 Myr. While the origin of the rings in HD 163296 remains unclear, these modeling results demonstrate that care has to be taken when interpreting CO emission in protoplanetary disk observations.