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1. A Gap in the Densities of Small Planets Orbiting M Dwarfs: Rigorous Statistical Confirmation Using the Open-source Code RhoPop

   https://doi.org/10.3847/PSJ/ad26f5  

   Schulze, J. G. ; Wang 王, Ji 吉. ; Johnson, J. A. ; Gaudi, B. S. ; Rodriguez Martinez, R. ; Unterborn, C. T. ; Panero, W. R. ( March 2024 , The Planetary Science Journal)

   Using mass–radius composition models, small planets (R≲ 2R_⊕) are typically classified into three types: iron-rich, nominally Earth-like, and those with solid/liquid water and/or atmosphere. These classes are generally expected to be variations within a compositional continuum. Recently, however, Luque & Pallé observed that potentially Earth-like planets around M dwarfs are separated from a lower-density population by a density gap. Meanwhile, the results of Adibekyan et al. hint that iron-rich planets around FGK stars are also a distinct population. It therefore remains unclear whether small planets represent a continuum or multiple distinct populations. Differentiating the nature of these populations will help constrain potential formation mechanisms. We present theRhoPopsoftware for identifying small-planet populations.RhoPopemploys mixture models in a hierarchical framework and a nested sampler for parameter and evidence estimates. UsingRhoPop, we confirm the two populations of Luque & Pallé with >4σsignificance. The intrinsic scatter in the Earth-like subpopulation is roughly half that expected based on stellar abundance variations in local FGK stars, perhaps implying M dwarfs have a smaller spread in the major rock-building elements (Fe, Mg, Si) than FGK stars. We applyRhoPopto the Adibekyan et al. sample and find no evidence of more than one population. We estimate the sample size required to resolve a population of planets with Mercury-like compositions from those with Earth-like compositions for various mass–radius precisions. Only 16 planets are needed when${\sigma }_{M_p}=5\%$and${\sigma }_{R_p}=1\%$. At${\sigma }_{M_p}=10\%$and${\sigma }_{R_p}=2.5\%$, however, over 154 planets are needed, an order of magnitude increase.

    

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2. Constraining the Volume of Earth's Early Oceans With a Temperature‐Dependent Mantle Water Storage Capacity Model

   https://doi.org/10.1029/2020AV000323  

   Dong, Junjie ; Fischer, Rebecca A. ; Stixrude, Lars P. ; Lithgow‐Bertelloni, Carolina R. ( March 2021 , AGU Advances)

   The water content in Earth's mantle today remains poorly constrained, but the bulk water storage capacity in the solid mantle can be quantified based on experimental data and may amount to a few times the modern surface ocean mass (OM). An appreciation of the mantle water storage capacity is indispensable to our understanding of how water may have cycled between the surface and mantle reservoirs and changed the volume of the oceans through time. In this study, we parameterized high pressure‐temperature experimental data on water storage capacities in major rock‐forming minerals to track the bulk water storage capacity in Earth's solid mantle as a function of temperature. We find that the mantle water storage capacity decreases as mantle potential temperature (T_p) increases, and its estimated value depends on the water storage capacity of bridgmanite in the lower mantle: 1.86–4.41 OM with a median of 2.29 OM for today (T_p = 1600 K), and 0.52–1.69 OM with a median of 0.72 OM for the early Earth's solid mantle (for aT_pthat was 300 K higher). An increase inT_pby 200–300 K results in a decrease in the mantle water storage capacity by – OM. We explored how the volume of early oceans may have controlled sea level during the early Archean (4–3.2 Ga) with some additional assumptions about early continents. We found that more voluminous surface oceans might have existed if the actual mantle water content today is > 0.3–0.8 OM and the early ArcheanT_pwas ≥1900 K.

    

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3. Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides

   https://doi.org/10.3847/PSJ/abd022  

   Agol, Eric ; Dorn, Caroline ; Grimm, Simon L. ; Turbet, Martin ; Ducrot, Elsa ; Delrez, Laetitia ; Gillon, Michaël ; Demory, Brice-Olivier ; Burdanov, Artem ; Barkaoui, Khalid ; et al ( January 2021 , The Planetary Science Journal)

   We have collected transit times for the TRAPPIST-1 system with the Spitzer Space Telescope over four years. We add to these ground-based, HST, and K2 transit-time measurements, and revisit anN-body dynamical analysis of the seven-planet system using our complete set of times from which we refine the mass ratios of the planets to the star. We next carry out a photodynamical analysis of the Spitzer light curves to derive the density of the host star and the planet densities. We find that all seven planets’ densities may be described with a single rocky mass–radius relation which is depleted in iron relative to Earth, with Fe 21 wt% versus 32 wt% for Earth, and otherwise Earth-like in composition. Alternatively, the planets may have an Earth-like composition but enhanced in light elements, such as a surface water layer or a core-free structure with oxidized iron in the mantle. We measure planet masses to a precision of 3%–5%, equivalent to a radial-velocity (RV) precision of 2.5 cm s⁻¹, or two orders of magnitude more precise than current RV capabilities. We find the eccentricities of the planets are very small, the orbits are extremely coplanar, and the system is stable on 10 Myr timescales. We find evidence of infrequent timing outliers, which we cannot explain with an eighth planet; we instead account for the outliers using a robust likelihood function. We forecast JWST timing observations and speculate on possible implications of the planet densities for the formation, migration, and evolution of the planet system.

    

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4. Potential Melting of Extrasolar Planets by Tidal Dissipation

   https://doi.org/10.3847/1538-4357/ad0b82  

   Seligman, Darryl Z. ; Feinstein, Adina D. ; Lai, Dong ; Welbanks, Luis ; Taylor, Aster G. ; Becker, Juliette ; Adams, Fred C. ; Morgan, Marvin ; Bergner, Jennifer B. ( January 2024 , The Astrophysical Journal)

   Tidal heating on Io due to its finite eccentricity was predicted to drive surface volcanic activity, which was subsequently confirmed by the Voyager spacecraft. Although the volcanic activity in Io is more complex, in theory volcanism can be driven by runaway melting in which the tidal heating increases as the mantle thickness decreases. We show that this runaway melting mechanism is generic for a composite planetary body with liquid core and solid mantle, provided that (i) the mantle rigidity,μ, is comparable to the central pressure, i.e.,μ/(ρgR_P) ≳ 0.1 for a body with densityρ, surface gravitational accelerationg, and radiusR_P; (ii) the surface is not molten; (iii) tides deposit sufficient energy; and (iv) the planet has nonzero eccentricity. We calculate the approximate liquid core radius as a function ofμ/(ρgR_P), and find that more than 90% of the core will melt due to this runaway forμ/(ρgR_P) ≳ 1. From all currently confirmed exoplanets, we find that the terrestrial planets in the L 98-59 system are the most promising candidates for sustaining active volcanism. However, uncertainties regarding the quality factors and the details of tidal heating and cooling mechanisms prohibit definitive claims of volcanism on any of these planets. We generate synthetic transmission spectra of these planets assuming Venus-like atmospheric compositions with an additional 5%, 50%, and 98% SO₂component, which is a tracer of volcanic activity. We find a ≳3σpreference for a model with SO₂with 5–10 transits with JWST for L 98-59bcd.

    

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5. The composition of Mars

   https://doi.org/doi:10.1016/j.gca.2020.01.011  

   Yoshizaki, Takashi ; McDonough, William F. ( March 2020 , Geochimica et cosmochimica acta)

   Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulkMars and its core. The Martian volatility trend is consistent with <7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core. 

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