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1. Formation of Super-Earths and Mini-Neptunes from Rings of Planetesimals

   https://doi.org/10.3847/2041-8213/ada3d1  

   Shibata, Sho ; Izidoro, Andre ( January 2025 , The Astrophysical Journal Letters)

   The solar system's planetary architecture has been proposed to be consistent with the terrestrial and giant planets forming from material rings at ∼1 au and ∼5 au, respectively. Here, we show that super-Earths and mini-Neptunes may share a similar formation pathway. In our simulations conducted with a diskα-viscosity of 4 × 10⁻³, super-Earths accrete from rings of rocky material in the inner disk, growing predominantly via planetesimal accretion. Mini-Neptunes primarily originate from rings located beyond the water snowline, forming via pebble accretion. Our simulations broadly match the period-ratio distribution, the intrasystem size uniformity, and the planet multiplicity distribution of exoplanets. The radius valley constrains the typical total mass available for rocky planet formation to be less than 3–6M_⊕. Our results predict that planets at ∼1 au in systems with close-in super-Earths and mini-Neptunes are predominantly water-rich. Though relatively uncommon, at ∼1% level, such systems might also host rocky Earth-sized planets in the habitable zone that underwent late giant impacts, akin to the Moon-forming event.

    

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2. Neptune Odyssey: A Flagship Concept for the Exploration of the Neptune–Triton System

   https://doi.org/10.3847/PSJ/abf654  

   Rymer, Abigail M. ; Runyon, Kirby D. ; Clyde, Brenda ; Núñez, Jorge I. ; Nikoukar, Romina ; Soderlund, Krista M. ; Sayanagi, Kunio ; Hofstadter, Mark ; Quick, Lynnae C. ; Stern, S. Alan ; et al ( September 2021 , The Planetary Science Journal)

   The Neptune Odyssey mission concept is a Flagship-class orbiter and atmospheric probe to the Neptune–Triton system. This bold mission of exploration would orbit an ice-giant planet to study the planet, its rings, small satellites, space environment, and the planet-sized moon Triton. Triton is a captured dwarf planet from the Kuiper Belt, twin of Pluto, and likely ocean world. Odyssey addresses Neptune system-level science, with equal priorities placed on Neptune, its rings, moons, space environment, and Triton. Between Uranus and Neptune, the latter is unique in providing simultaneous access to both an ice giant and a Kuiper Belt dwarf planet. The spacecraft—in a class equivalent to the NASA/ESA/ASI Cassini spacecraft—would launch by 2031 on a Space Launch System or equivalent launch vehicle and utilize a Jupiter gravity assist for a 12 yr cruise to Neptune and a 4 yr prime orbital mission; alternatively a launch after 2031 would have a 16 yr direct-to-Neptune cruise phase. Our solution provides annual launch opportunities and allows for an easy upgrade to the shorter (12 yr) cruise. Odyssey would orbit Neptune retrograde (prograde with respect to Triton), using the moon's gravity to shape the orbital tour and allow coverage of Triton, Neptune, and the space environment. The atmospheric entry probe would descend in ∼37 minutes to the 10 bar pressure level in Neptune's atmosphere just before Odyssey's orbit-insertion engine burn. Odyssey's mission would end by conducting a Cassini-like “Grand Finale,” passing inside the rings and ultimately taking a final great plunge into Neptune's atmosphere.

    

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3. Toward a Self-consistent Evaluation of Gas Dwarf Scenarios for Temperate Sub-Neptunes

   https://doi.org/10.3847/1538-4357/ad6c38  

   Rigby, Frances_E ; Pica-Ciamarra, Lorenzo ; Holmberg, Måns ; Madhusudhan, Nikku ; Constantinou, Savvas ; Schaefer, Laura ; Deng, Jie ; Lee, Kanani_K_M ; Moses, Julianne_I ( October 2024 , The Astrophysical Journal)

   The recent JWST detections of carbon-bearing molecules in a habitable-zone sub-Neptune have opened a new era in the study of low-mass exoplanets. The sub-Neptune regime spans a wide diversity of planetary interiors and atmospheres not witnessed in the solar system, including mini-Neptunes, super-Earths, and water worlds. Recent works have investigated the possibility of gas dwarfs, with rocky interiors and thick H₂-rich atmospheres, to explain aspects of the sub-Neptune population, including the radius valley. Interactions between the H₂-rich envelope and a potential magma ocean may lead to observable atmospheric signatures. We report a coupled interior-atmosphere modeling framework for gas dwarfs to investigate the plausibility of magma oceans on such planets and their observable diagnostics. We find that the surface–atmosphere interactions and atmospheric composition are sensitive to a wide range of parameters, including the atmospheric and internal structure, mineral composition, volatile solubility and atmospheric chemistry. While magma oceans are typically associated with high-temperature rocky planets, we assess if such conditions may be admissible and observable for temperate sub-Neptunes. We find that a holistic modeling approach is required for this purpose and to avoid unphysical model solutions. Using our model framework, we consider the habitable-zone sub-Neptune K2-18 b as a case study and find that its observed atmospheric composition is incompatible with a magma ocean scenario. We identify key atmospheric molecular and elemental diagnostics, including the abundances of CO₂, CO, NH₃, and, potentially, S-bearing species. Our study also underscores the need for fundamental material properties for accurate modeling of such planets.

    

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4. Heat and charge transport in H2O at ice-giant conditions from ab initio molecular dynamics simulations

   https://doi.org/10.1038/s41467-020-17275-5  

   Grasselli, Federico ; Stixrude, Lars ; Baroni, Stefano ( July 2020 , Nature Communications)

   The impact of the inner structure and thermal history of planets on their observable features, such as luminosity or magnetic field, crucially depends on the poorly known heat and charge transport properties of their internal layers. The thermal and electric conductivities of different phases of water (liquid, solid, and super-ionic) occurring in the interior of ice giant planets, such as Uranus or Neptune, are evaluated from equilibrium ab initio molecular dynamics, leveraging recent progresses in the theory and data analysis of transport in extended systems. The implications of our findings on the evolution models of the ice giants are briefly discussed.

    

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5. The interiors of Uranus and Neptune: current understanding and open questions

   https://doi.org/10.1098/rsta.2019.0474  

   Helled, Ravit ; Fortney, Jonathan J. ( December 2020 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   null (Ed.)

   Uranus and Neptune form a distinct class of planets in our Solar System. Given this fact, and ubiquity of similar-mass planets in other planetary systems, it is essential to understand their interior structure and composition. However, there are more open questions regarding these planets than answers. In this review, we concentrate on the things we do not know about the interiors of Uranus and Neptune with a focus on why the planets may be different, rather than the same. We next summarize the knowledge about the planets’ internal structure and evolution. Finally, we identify the topics that should be investigated further on the theoretical front as well as required observations from space missions. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’. 

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