Debanjan Sengupta^1,2, Sarah E. Dodson-Robinson^1,3, Yasuhiro Hasegawa², and Neal J. Turner²
Astrophysical Journal 874, 26 Link to Article [DOI: 10.3847/1538-4357/aafc36 ]
¹Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA

Despite making a small contribution to total protoplanetary disk mass, dust affects the disk temperature by controlling the absorption of starlight. As grains grow from their initial interstellar-medium-like size distribution, settling depletes the disk’s upper layers of dust and decreases the optical depth, cooling the interior. Here we investigate the effect of collisional growth of dust grains and their dynamics on the thermal and optical profile of the disk, and explore the possibility that cooling induced by grain growth and settling could lead to gravitational instability. We develop a Monte Carlo dust collision model with a weighting technique and allow particles to collisionally evolve through sticking and fragmentation, along with vertical settling and turbulent mixing. We explore three disk models and perform simulations for both constant and spatially variable turbulence profile. We then calculate mean wavelength-dependent opacities for the evolving disks and perform radiative transfer to calculate the temperature profile. Finally, we calculate the Toomre Q parameter, a measure of the disk’s stability against self-gravity, after it reaches a steady-state dust-size distribution. We find that even weak turbulence can keep submicrometer-sized particles stirred in the disk’s upper layer, affecting its optical and thermal profiles, and the growth of large particles in the midplane can make a massive disk optically thick at millimeter wavelengths, making it difficult to calculate the surface density of dust available for planet formation in the inner disk. Also, for all our initially marginally stable annuli, we find a small but noticeable reduction in Q.

Yanqin Wu
Astrophysical Journal 874, 91 Link to Article [DOI: 10.3847/1538-4357/ab06f8 ]
Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada

The majority of the transiting planets discovered by the Kepler mission (called super-Earths here, includes the so-called “sub-Neptunes”) orbit close to their stars. As such, photoevaporation of their hydrogen envelopes etches sharp features in an otherwise bland space spanned by planet radius and orbital period. This, in turn, can be exploited to reveal the mass of these planets, in addition to techniques such as radial velocity and transit-timing-variation. Here, using updated radii for Keplerplanet hosts from Gaia DR2, I show that the photoevaporation features shift systematically to larger radii for planets around more massive stars (ranging from M-dwarfs to F-dwarfs), corresponding to a nearly linear scaling between planet mass and its host mass. By modeling planet evolution under photoevaporation, one further deduces that the masses of super-Earths peak narrowly around 8 M_⊕(M _*/M _⊙). When such a stellar mass dependence is scaled out, Kepler planets appear to be a homogeneous population surprisingly uniform in mass, in core composition (likely terrestrial), and in initial mass fraction of their H/He envelope (a couple percent). The masses of these planets do not appear to depend on the metallicity values of their host stars, while they may weakly depend on the orbital separation. Taken together, the simplest interpretation of our results is that super-Earths are at the so-called “thermal mass”, where the planet’s Hill radius is equal to the vertical scale height of the gas disk.

¹C.J.Manser et al. (>10)
Science 364, 66-69 Link to Article [DOI: 10.1126/science.aat5330]
¹Department of Physics, University of Warwick, Coventry CV4 7AL, UK. Reprinted with permission from AAAS

Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca ii emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength.

¹D.S.Lauretta et al. (>10)
Nature 568, 55-60 Link to Article [https://doi.org/10.1038/s41586-019-1033-6]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

^1,2,3Xiaojia Zeng,¹Yingli Shang,^1,4Shijie Li,^1,4,2Xiongyao Li,⁵Shijie Wang,^1,4,2Yang Li
Earth, Planets and Space 71,38 Link to Article [https://doi.org/10.1186/s40623-019-1015-9]
¹Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China
²Key Laboratory of Space Manufacturing Technology, Chinese Academy of Sciences, Beijing, 100094, China
³University of Chinese Academy of Sciences, Beijing, 100049, China
⁴CAS Center for Excellence in Comparative Planetology, Hefei, China
⁵State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Grzegorz Racki,^2,3Christian Koeberl
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13280]
¹Faculty of Earth Sciences, University of Silesia, ul. Będzińska 60, 41‐200 Sosnowiec, Poland
²Natural History Museum Vienna, Burgring 7, 1010 Vienna, Austria
³Department of Lithospheric Research, University of Vienna, Althanstrasse, 1090 Vienna, Austria
Published by arrangement with John Wiley & Sons

Franz von Paula Gruithuisen (1774–1852), the Bavarian medic, physician, and astronomer, enfant terrible of German science, is known for his insightful observations and many extravagant conceptions. However, since the seminal monograph of Baldwin (1949), he is also referenced for early contributions to the meteoritic origin concept of lunar craters. His most commonly cited paper of 1828 is analyzed here for the first time in some detail. For Gruithuisen, impact phenomena were only an outcome of a more general cosmogenic theory, which assumed planet and satellite growth by concentric shell‐like coalescence of the cosmic bodies. The aggregation theory thus defined was initiated in 1794 by Chladni, developed by the Bierberstein brothers and Anton Zach. Gruithuisen was notably the first person to formulate a nascent concept of lunar crater mechanics. This cratering process, as he thought, is based on an uneven gravitational subsidence of concentrically layered spherical impactors (=the solid core of comet) into the plastic sediments. Only the more resistant and heavy central portion of the body was submerging deeper, and therefore, the circular terrace‐like rim of the ring mountains was formed. Gruithuisen tried also to recognize terrestrial equivalents of large‐scale crater‐like mountains on the Moon, and speculated on other impact consequences, such as a catastrophic influence on the history of the biosphere and a cometary source of the terrestrial hydrosphere. These ideas found several conceptual followers in the vital German science of the last decades of 19th century. Thus, despite principal errors in the gravitationally penetrative cratering model, we confirm the claim of recognition of Gruithuisen as one of the founders of the impact hypothesis.

¹Steven B. Simon,^2,3Alexander N. Krot,²Kazuhide Nagashima
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13282]
¹Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, 87131 USA
²Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
³Geoscience Institute/Mineralogy, Goethe University Frankfurt, Altenhoeferallee 1, 60438 Frankfurt am Main, Germany
Published by arrangement with John Wiley & Sons

The distribution of the short‐lived radionuclide ²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions in CO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearing CAIs in the Dominion Range (DOM) 08006 (CO3.0) and DOM 08004 (CO3.1) chondrites. All minerals in DOM 08006 CAIs as well as hibonite, spinel, and pyroxene in DOM 08004 are uniformly ¹⁶O‐rich (Δ¹⁷O = −25 to −20‰) but grossite and melilite in DOM 08004 CAIs are not; Δ¹⁷O of grossite and melilite range from ~ −11 to ~0‰ and from ~ −23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial ²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀ is seen, with four having (²⁶Al/²⁷Al)₀ ≤1.1 × 10⁻⁵ and six having (²⁶Al/²⁷Al)₀ ≥3.7 × 10⁻⁵. Five of the ²⁶Al‐rich CAIs have (²⁶Al/²⁷Al)₀ within error of 4.5 × 10⁻⁵; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10⁻⁵ given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the ²⁶Al‐poor CAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in the DOM 08006 CAIs, as well as spinel, hibonite, and Al‐diopside in the DOM 08004 CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where the CO grossite‐bearing CAIs originated. Oxygen isotopic heterogeneity in CAIs from DOM 08004 resulted from exchange between the initially ¹⁶O‐rich (Δ¹⁷O ~−24‰) melilite and grossite and ¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on the CO chondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic ²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on the CO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected most CAIs in CO ≥3.1 chondrites.

¹Ingo Leya, ¹Peter C. Stephenson
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13288]
¹Space Research and Planetology, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

We report newly measured noble gas isotopic concentrations of He, Ne, and Ar for 21 samples from the 10 ureilites, DaG 084, DaG 319, DaG 340, Dho 132, HaH 126, JaH 422, JaH 424, Kenna, NWA 5928, and RaS 247, including the results of both single and stepwise heating extractions. Cosmic ray exposure (CRE) ages calculated using model calculations that fully account for all shielding depths and a wide range of preatmospheric radii, and are tailored to ureilite chemistry, range from 3.7 Ma for Dho 132 to 36.3 Ma for one of several measured Kenna samples. In a Ne‐three‐isotope plot, the data for DaG 340 and JaH 422 plot below the Necos/Neureilite mixing envelope, possibly indicating the presence of Ne produced from solar cosmic rays. In combination with literature data and correcting for pairing, we established a fully consistent database containing 100 samples from 40 different ureilites. The CRE age histogram shows a trend of decreasing meteorite number with increasing CRE age. We speculate that the parent body of the known ureilites is moving closer to a resonance and/or that there is a loss mechanism that acts on ureilites independent of their size. In addition, there is a slight indication for a peak in the range 30 Ma, which might indicate a larger impact on the ureilite daughter body. Finally, we confirm earlier results that the majority of the studied ureilites have relatively small preatmospheric radii less or equal ~20 cm.

¹Robert M. Hazen
American Mineralogist 104, 468-470 Link to Article [https://doi.org/10.2138/am-2019-6745]
¹Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A. Orcid 0000-0003-4163-8644
Copyright: The Mineralogical Society of America

The Earth in Five Reactions Workshop posed two significant challenges: (1) the formulation of a conceptual definition of “reaction” and (2) the identification and ranking of the “most important reactions” in the context of planetary evolution. Attempted answers to those challenges, collated in this collection of articles, reflect both the opportunities and hurdles when scientists deal with questions of meaning and value.

¹Kevin J.Walsh,¹Harold F.Levison
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.03.031]
¹Southwest Research Institute, 1050 Walnut St. Suite 300, Boulder, CO 80302, USA
Copyright Elsevier

We present numerical simulations of terrestrial planet formation that examine the growth continuously from planetesimals to planets in the inner Solar System. Previous studies show that the growth will be inside-out, but it is still common practice to assume that the entire inner disk will eventually reach a bi-modal distribution of embryos and planetesimals. For the combinations of disk mass, initial planetesimal radius and gas disk lifetime explored in this work the entire disk never reaches a simple bi-modal mass distribution.
We find that the inside-out growth is amplified by the combined effects of collisional evolution of solid bodies and interactions with a dissipating gas disk. This leads to oligarchic growth never being achieved in different places of the disk at the same time, where in some cases the disk can simultaneously support chaotic growth and giant impacts inside 1 au and runaway growth beyond 2 au. The planetesimal population is efficiently depleted in the inner disk where embryo growth primarily advances in the presence of a significant gas disk. Further out in the disk growth is slower relative to the gas disk dissipation, resulting in more excited planetesimals at the same stage of growth and less efficient accretion. This same effect drives mass loss due to collisional grinding strongly altering the surface density of the accreted planets relative to the initial mass distribution. This effect decreases the Mars-to-Earth mass ratios compared to previous works with no collisional grinding. Similar to some previous findings utilizing vastly different growth scenarios these simulations produce a first generation of planetary embryos that are stable for 10–20 Myr, or 5–10 e-folding times of the gas dissipation timescale, before having an instability and entering the chaotic growth stage.