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The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R⊕) still remains. They may either have a rocky core enveloped in a H₂–He gaseous envelope (gas dwarfs) or contain a significant amount of multicomponent, H₂O-dominated ices/fluids (water worlds). Planets in the mass range of 10–15 M⊕, if half-ice and half-rock by mass, have radii of 2.5 R⊕, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2- to 4-R⊕ range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are “water worlds.”

Late accretion, early mantle differentiation, and core-mantle interaction are processes that could have created subtle ¹⁸²W isotopic heterogeneities within Earth's mantle. Tungsten isotopic data for Kostomuksha komatiites dated at 2.8 billion years ago show a well-resolved ¹⁸²W excess relative to modern terrestrial samples, whereas data for Komati komatiites dated at 3.5 billion years ago show no such excess. Combined ¹⁸²W, ¹ɸ⁶,¹ɸ⁷Os, and ¹⁴²,¹⁴³Nd isotopic data indicate that the mantle source of the Kostomuksha komatiites included material from a primordial reservoir that represents either a deep mantle region that underwent metal-silicate equilibration or a product of large-scale magmatic differentiation of the mantle. The preservation, until at least 2.8 billion years ago, of this reservoir—which likely formed within the first 30 million years of solar system history—indicates that the mantle may have never been well mixed.

The measurement of magnesium isotope ratios at improved accuracy suggests that planetary compositions result from fractionation between liquid and vapour, followed by vapour escape during accretionary growth. Earth's volatile origins In comparison to primitive, chondritic meteorites, which are widely thought to be the building blocks of Earth, Earth and other differentiated planetary bodies are chemically fractionated, with Earth's crust and mantle—the 'silicate Earth'—being strongly depleted in moderately volatile elements (such as lead, zinc, indium and the alkali metals). Two papers in this week's issue suggest that this difference in composition between chondritic meteorites and Earth could be a natural consequence of vapour loss from magma on the surface of growing planetesimals. Ashley Norris and Bernard Wood examined the melting processes that would have occurred during accretion on Earth and its precursor bodies and performed vaporization experiments under conditions of fixed temperature and oxygen partial pressure. They found that the pattern of volatile-element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. Remco Hin and co-authors show that differentiated planetary bodies have isotopically heavier magnesium compositions compared to chondritic meteorites, and conclude that this could be due to the isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals. It has long been recognized that Earth and other differentiated planetary bodies are chemically fractionated compared to primitive, chondritic meteorites and, by inference, the primordial disk from which they formed. However, it is not known whether the notable volatile depletions of planetary bodies are a consequence of accretion 1 or inherited from prior nebular fractionation 2 . The isotopic compositions of the main constituents of planetary bodies can contribute to this debate 3 , 4 , 5 , 6 . Here we develop an analytical approach that corrects a major cause of measurement inaccuracy inherent in conventional methods, and show that all differentiated bodies have isotopically heavier magnesium compositions than chondritic meteorites. We argue that possible magnesium isotope fractionation during condensation of the solar nebula, core formation and silicate differentiation cannot explain these observations. However, isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals, generates appropriate residual compositions. Our modelling implies that the isotopic compositions of magnesium, silicon and iron, and the relative abundances of the major elements of Earth and other planetary bodies, are a natural consequence of substantial (about 40 per cent by mass) vapour loss from growing planetesimals by this mechanism.

The detection 1 of a dust disk around the white dwarf star G29-38 and transits from debris orbiting the white dwarf WD 1145+017 (ref. 2 ) confirmed that the photospheric trace metals found in many white dwarfs 3 arise from the accretion of tidally disrupted planetesimals 4 . The composition of these planetesimals is similar to that of rocky bodies in the inner Solar System 5 . Gravitational scattering of planetesimals towards the white dwarf requires the presence of more massive bodies 6 , yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a hot (about 27,750 kelvin) white dwarf, WD J091405.30+191412.25, that is accreting from a circumstellar gaseous disk composed of hydrogen, oxygen and sulfur at a rate of about 3.3 × 10 9 grams per second. The composition of this disk is unlike all other known planetary debris around white dwarfs 7 , but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H 2 O and H 2 S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of around 15 solar radii will undergo substantial evaporation with expected mass loss rates comparable to the accretion rate that we observe onto the white dwarf. The orbit of the planet is most probably the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of approximately 1 in 10,000 for spectroscopically detectable giant planets in close orbits around white dwarfs. Observations of an accretion disk around a hot white dwarf star reveal that the chemical abundances in its disk are similar to those thought to exist deep in icy giant planets, so the white dwarf must be accreting a giant planet.

In the solar system, the planets' compositions vary with orbital distance, with rocky planets in close orbits and lower-density gas giants in wider orbits. The detection of close-in giant planets around other stars was the first clue that this pattern is not universal and that planets' orbits can change substantially after their formation. Here, we report another violation of the orbit-composition pattern: two planets orbiting the same star with orbital distances differing by only 10% and densities differing by a factor of 8. One planet is likely a rocky \"super-Earth,\" whereas the other is more akin to Neptune. These planets are 20 times more closely spaced and have a larger density contrast than any adjacent pair of planets in the solar system.

The surface elemental composition of dwarf planet Ceres constrains its regolith ice content, aqueous alteration processes, and interior evolution. Using nuclear spectroscopy data acquired by NASA’s Dawn mission, we determined the concentrations of elemental hydrogen, iron, and potassium on Ceres. The data show that surface materials were processed by the action of water within the interior. The non-icy portion of Ceres’ carbon-bearing regolith contains similar amounts of hydrogen to those present in aqueously altered carbonaceous chondrites; however, the concentration of iron on Ceres is lower than in the aforementioned chondrites. This allows for the possibility that Ceres experienced modest ice-rock fractionation, resulting in differences between surface and bulk composition. At mid-to-high latitudes, the regolith contains high concentrations of hydrogen, consistent with broad expanses of water ice, confirming theoretical predictions that ice can survive for billions of years just beneath the surface.

The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1σ) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

Theoretical studies of giant planet formation suggest that substantial quantities of metals—elements heavier than hydrogen and helium—can be delivered by solid accretion during the envelope-assembly phase. This process of metal enhancement of the envelope is believed to diminish as a function of planet mass, leading to predictions for a mass-metallicity relationship. Supporting evidence for this picture is provided by the abundance of CH 4 in solar system giant planets, where CH 4 abundance, unlike H 2 O, is unaffected by condensate cloud formation. However, all of the solar system giants exhibit some evidence for stratification of metals outside of their cores. In this context, two fundamental questions are whether metallicity of giant planets inferred from observations of the outer envelope layers represents the bulk metallicity of these planets, and if not, how are metals distributed within giant planets. Comparing the mass-metallicity relationship for solar system giant planets, inferred from the observed CH 4 abundance, with various tracers of metallicity in the exoplanet population, has yielded a range of results. There is evidence of a solar-system-like mass-metallicity trend using bulk density estimates of exoplanet metallicity. However, transit-spectroscopy-based tracers of exoplanet metallicity, which probe only the outer layers of the envelope, are less clear about a mass-metallicity trend and raise the question of whether radial composition gradients exist in some giant exoplanets. The large number of known exoplanets enables statistical characterization of planet properties. We develop a formalism for comparing both the metallicity inferred for the outer envelope and the metallicity inferred using the bulk density and show this combination may offer insights into the broader question of metal stratification within planetary envelopes. Our analysis suggests that future exoplanet observations with JWST and Ariel will be able to shed light on the conditions governing radial composition gradients in exoplanets and, perhaps, provide information about the factors controlling stratification and convection in our solar system gas giants.

It remains contentious whether the meteoritic material delivered to the terrestrial planets after the end of core formation was rich or poor in water and other volatiles. As Venus’s atmosphere has probably experienced less volatile recycling over its history than Earth’s, it may be possible to constrain the volatile delivery to the primitive Venusian atmosphere from the planet’s present-day atmospheric composition. Here we investigate the long-term evolution of Venus using self-consistent numerical simulations of global thermochemical mantle convection coupled with both an atmospheric evolution model and a late accretion N-body delivery model. We found that atmospheric escape is only able to remove a limited amount of water over the history of the planet, and that the late accretion of wet material exceeds this sink and would result in a present-day atmosphere that is too rich in volatiles. A preferentially dry composition of the late accretion impactors is most consistent with measurements of atmospheric H2O, CO2 and N2. Hence, we suggest that the late accreted material delivered to Venus was mostly dry enstatite chondrite, consistent with isotopic data for Earth, with less than 2.5% (by mass) wet carbonaceous chondrites. In this scenario, the majority of Venus’s and Earth’s water would have been delivered during the main accretion phase.Venus’s atmospheric composition suggests limited water delivery to the terrestrial planets by late accretion, according to numerical simulations of the interior and atmospheric evolution of Venus under various late accretion scenarios.

X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets. Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust. The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance.