THE PROTOPLANETARY DISK

Accretion from dust to gas-giant planet occurred within 3 m.y. ('Pebble Accretion and the Diversity of Planetary Systems', J. E. Chambers, The Astrophysical Journal, vol. 825 (2016). In the beginning, µm-sized dust particles are embedded in a gaseous protoplanetary disk. By 0.02 m.y., mutual collisions between dust grains result in the formation of mm- to cm-sized pebbles. By 0.15 m.y., pebbles inside the ice line (~2.5–4.5 AU) have aggregated into planetesimals with diameters of 30–300 km. Just outside the ice line, aggregation of the larger ice-rich pebbles is more efficient, and larger planetesimals with diameters of ~1,500 km are formed during runaway growth. By 0.5 m.y., some of the larger planetesimals located within ~5 AU enter an oligarchic growth stage and become planetary embryos with diameters of 2,000 km. The largest embryos located just beyond the ice line begin to grow by "pebble accretion" due to the inward drift of pebbles, reaching sizes of a few Earth masses (M_⊕). By 3 m.y., the largest of these embryos exceed a critical mass (3 × M_⊕) and undergo runaway gas accretion to form gas-giant planets (Levison et al., 2015). It was argued by Desch et al. (2022 and references therein) that Jupiter's rapid growth attained a size of ≈ 20–30 M_⊕ by about 0.4–0.9 m.y. after CAIs. A large protoplanetary disk (radius = ~100 AU) and a small turbulence strength (α = 0.0005) help promote the formation of these gas giants, which ultimately clear their orbits. Inside the ice line, the growth of terrestrial planets (0.02–1.4 M_⊕) ceases due to pebble depletion in the disk.

Matsumoto et al. (2019) employed advanced techniques including synchrotron radiation–based x-ray computed nanotomography in a study of the accretionary formation of the carbonaceous parent body of the Acfer 094 chondrite. They presented a formation scenario as follows (see also schematic illustration below):

1. During migration from the outer to the inner protoplanetary disk, the Acfer 094 parent body grew through the accretion of fluffy ice-silicate aggregates (FISA) and fluffy silicate aggregates (FSA), respectively, forming a stratified planetesimal
2. As the planetesimal passed through the H2O snow line, an additional ice-bearing ultraporous lithology (UPL), which was formed through sintering of fluffy icy dust aggregates, was incorporated into the growing mantle
3. Subsequent heating of the planetesimal caused melting of ice, especially that composing the core, which led to widespread aqueous alteration
4. The parent meteoroid of Acfer 094 was ultimately ejected from the mantle of the planetesimal in an impact event

Schematic illustration of the Acfer 094 Parent Body Formation Model

click on image for a magnified view

Schematic illustration credit: Matsumoto et al., Science Advances, vol. 5, #11 (2019, open access link)
'Discovery of fossil asteroidal ice in primitive meteorite Acfer 094'
(https://doi.org/10.1126/sciadv.aax5078)

Improvements in the early chronology of the Solar System were presented by P. Koefoed (2017) based on more precise U–Pb data for the meteorites NWA 6704, NWA 10132, NWA 7325, and A-881394. The formation of crustal material of these planetesimals can be dated to as early as ~2 m.y. after CAIs, indicating that accretion of planetesimals was occurring contemporaneously with CAI formation.

Initially, Trinquier et al. (2007) concluded that at least two distinct nebular reservoirs can be resolved based on 54Cr isotopic heterogeneity among the known parent bodies. Thereafter, based on O, Cr, Ti, and Ni stable-isotopic data, Warren (2011) recognized the existence of two distinct taxonomic superclans: those which accreted inside the orbit of Jupiter where thermal processing occurred under reducing conditions, termed 'non-carbonaceous' (NC), and those which accreted outside of its orbit where thermal processing occurred under oxidizing conditions, termed 'carbonaceous' (CC); the difference in redox conditions is attributed to differences in the ice, dust, and gas abundances (see also open access papers in EPSL by Budde et al. [2016], LPSC by Worsham et al. [2018 #2720], and Space Science Reviews by Kleine et al. [2020]). These two reservoirs were segregated by the rapid accretion of proto-Jupiter and reflect differences in the contribution (i.e., susceptibility to thermal processing) of proton (p-) process, rapid neutron capture (r-) process, and slow neutron capture (s-) process isotopes inherited as dust ejecta from explosive stellar nucleosynthesis—core-collapse supernovae, neutron star mergers, and low mass (1.5–4 M_⊙) AGB stars, respectively (Poole et al., 2017; Bermingham et al., 2018; Ek et al., 2020). In a study of W, Mo, and Ru isotopes in iron meteorites, Kruijer et al. (2017) recognized that both of these reservoirs were coeval and remained spatially separated within the protoplanetary disk for a prolonged period (~ 3.6–4.8 m.y., inferred from timing of CR and CB parent body accretion, respectively) associated with the rapid growth of proto-Jupiter, attaining an ~30 M_⊕ core at 2.9–3.2 AU within the first 0.6 m.y. (Desch et al., 2018).

Additional constraints on the timing of the growth of Jupiter were provided by Weiss and Bottke (2021) based on isotopic and paleomagnetic data. They determined that the core accretion model rather than the gravitational instability model is most consistent with both their data and observations by the Atacama Large Millimeter/submillimeter Array (ALMA) (see diagram A below).

Weiss and Bottke (2021) and Weiss et al. (2022) proposed a plausible scenario and timing sequence for the formation of Jupiter and evolution of the solar nebula as follows (see diagram B above):

• Accretion of Jupiter begins with the formation of a metal-rich core, then reaching its isolation mass (M_I) of ~10–25 M_⊕ before 1.3 m.y. after CAIs
• The growing planet created a gap (pressure bump) in the protoplanetary disk separating the NC and CC isotopic reservoirs.
• Jupiter reached its crossover mass (M_X) of ~20–50 M_⊕ by 3.46 m.y. after CAIs.
• Jupiter experienced runaway gas accretion at very rapid rates over a short period of <0.5 m.y., attaining its final mass of 318 M_⊕.
• The solar nebula in the non-carbonaceous reservoir dissipated between 1.22 and 3.94 m.y. after CAIs, the latter discerned from angrite paleomagnetic data.
• The solar nebula in the carbonaceous reservoir dissipated between 2.51 and 4.89 m.y. after CAIs, the latter discerned from CV chondrite paleomagnetic data.

  CO chondrite paleomagnetic data obtained by Borlina et al. (2022) suggest that the carbonaceous reservoir gas dissipated between 2.71 and 5.1 m.y. after CAIs, with a best resolved timing of ~4.2 m.y. after CAIs. They also determined that the nebula dissipation was most likely the result of magnetically driven winds and/or photoevaporation.

It has been posited that after Jupiter had grown to a massive size (>50 M_⊕) by ~4 m.y. at an initial heliocentric distance of ~3 AU, it underwent a chaotic migration in a 2:1 (or 3:2) mean motion resonance with Saturn—first inward for ~100 t.y. to ~1.5–2.0 AU while clearing the inner disk of planetesimals, and then outward for 500 t.y. ultimately ending at its current location near 5.2 AU (and Saturn near 7.1 AU); this is known as the 'Grand Tack' scenario (Walsh et al. 2011, #2585, 2012; Johnson et al., 2016; Brasser et al., 2016). In accord with the 'Nice Model' (Gomes et al., 2005; Tsiganis et al., 2005; Morbidelli et al., 2005; named after the French city where proposed), an orbital instability caused the final outward migration of the four (or initially five to six before their ejection; Nesvorný and Morbidelli, 2012) gas planets, ultimately settling into their current orbits. The most plausible timing of this early instability is now considered to be ~30–100 m.y. after nebula dissipation, coeval with the Moon-forming impact (Clement et al., 2021 and references therein). This instability event led to the scattering of existing planetesimals from the carbonaceous reservoir into the inner Solar System where some of this material acquired orbital stability in the outer asteroid belt (Pierens et al., 2014). This event can also explain other characteristics of the current Solar System such as the relatively small mass of both Mars and the asteroid belt, as well as the dynamically cold orbits of the current terrestrial planets (Clement et al., 2018, Clement et al., 2019).

Kruijer et al. (2017) determined the ε182W values for representative iron meteorites, which reflect the timing of metal–silicate differentiation on their respective parent bodies. Based on coupled Mo- and W-isotopic diagrams (see below), they demonstrated that irons from groups IAB, IC, IIAB, IIE, IIG, IIIAB, IIIE, and IVA (and IIG; Wasson and Choe, 2009; A. Rubin, 2018) are associated with the non-carbonaceous reservoir (NC), in which iron parent bodies experienced earlier accretion (<0.4 m.y. after CAIs) and core formation (~ 0.3–1.8 m.y. after CAIs). By contrast, irons from groups IIC, IID, IIF, IIIF, and IVB are associated with the carbonaceous reservoir (CC), and their parent bodies experienced a later accretion (~1 m.y. after CAIs) and core formation (~ 2.2–2.8 m.y. after CAIs).

Rubin (2018 article, abstract) recognized that the CC irons are enriched in refractory siderophile elements and attributed this to the inheritance of refractory metal nuggets initially present in CAIs. The enrichment in Ni observed in CC irons compared to NC irons was considered to reflect in part the higher oxidizing conditions that existed in the outer Solar System where CC irons accreted. However, in an expanded sampling of ungrouped irons conducted by Spitzer et al. (2019, #2592), no significant difference in Ni content was found to exist between irons from the two reservoirs. It was also demonstrated by Rubin (2018) that CC irons generally have significantly higher CRE ages than NC irons, consistent with their location in the outer asteroid belt and thus greater distance traveled prior to intersection with the Earth. He also found that there are generally fewer members in the CC iron groups compared to the NC iron groups, which is likely a factor of (i) a smaller core volume for CC iron asteroids due to later accretion, less differentiation, and perhaps most importantly, a higher oxidation state resulting in less production of metal, and (ii) greater fragmentation experienced by CC iron meteoroids given their longer transit times in space. Furthermore, he posited that most ungrouped magmatic irons would derive from the CC reservoir, as are the two which had been adequately analyzed up to then, Mbosi and Grand Rapids. In their analysis of Mo, W, and Pt isotopes for ungrouped irons, Spitzer et al. (2019 #2592) found support for this latter hypothesis of Rubin (2018), as they determined that more of the ungrouped irons originated from the CC region compared to the NC region reflecting a ratio of 8:5. In addition, they recognized that those originating from the CC region generally have negative Δ17O values while those from the NC region have positive values.

A subsequent Mo isotope study conducted by Budde et al. (2019) and utilizing a more comprehensive sampling of meteorite groups led to a refinement in the slopes and intercepts for the NC and CC groups (see revised coupled Mo isotope diagram below). The mixing lines of both groups have identical slopes with a value of 0.596, while the offset in the y-axis intercept between the CC and the NC groups reflects an excess of r- and possibly p-process Mo in the CC region of the protoplanetary disk. Further details about their study to constrain the origin of the Moon-forming impactor 'Theia' can be found on the NWA 032 page.

The Mo-isotopic dichotomy that exists between the CC and NC meteorites was also investigated by Yokoyama et al. (2019) employing an extremely precise measurement technique. Their study included ten irons (IIAB [4], IVB [4], IVB-like, IIE-like), two carbonaceous chondrites (CK4, CK5), five ordinary chondrites (H4, H5, LL5 [2], LL6), and two rumuruti chondrites (R3.9, R4). While the previous study by Budde et al. (2019) utilized a broader sampling of meteorite groups for their revision of the 95Mo–94Mo isotope diagram after Kruijer et al. (2017), the new study by Yokoyama et al. (2019) used improved values for irons that are corrected for neutron-capture modification due to galactic cosmic ray (GCR) exposure during prolonged transit times in space. Based on these more accurate values for iron meteorites, along with new values for other meteorite groups combined with recent literature data (e.g., Kruijer et al., 2017; Budde et al., 2019), they derived new slopes and intercepts for the coupled Mo isotope diagram as shown below.

The new slope derived by Yokoyama et al. (2019) for the CC meteorites is consistent with that obtained in previous studies, with the Mo-isotopic variability among samples being attributed either to differences in the abundances of s-process-enriched presolar grains or to parent body processing (e.g., oxidation, thermal metamorphism). The new slope derived for the NC meteorites can be explained by the existence of two end-member components, NC-A and NC-B, which are resolved from each other by variations in s-process and r-process Mo isotope depletions. The NC-B component is represented most closely by the R chondrite and ACA–LOD groups, which exhibit the highest depletion in r-process Mo with no depletion in s-process Mo. Yokoyama et al. (2019) posit that the NC-B component could be a residual fine-grained phase that is isotopically complementary to the the type B CAI nebular reservoir, as established by a thermally-induced destruction of an r-process-rich carrier in the early protoplanetary disk (see schematic diagram by Yokoyama et al., 2019). Thereafter, the NC-A component was formed either by 1) another thermal processing event that destroyed an s-process-depleted carrier phase, or 2) the removal of a metal component from (thus increasing the ηs and ηr values), or the addition of a matrtix component to (thus decreasing the ηs and ηr values) an initial NC-B-like reservoir (see schematic illustration below).

After accounting for cosmic-ray exposure effects on Mo isotopes, Spitzer et al. (2020) investigated the Mo isotope variations for a suite of iron meteorites from the NC region of the protoplanetary disk. In the diagram below, they derived a shallower s-process mixing line slope for NC meteorites compared to CC meteorties, which delineate a slope of 0.528 (±045) compared to 0.596, respectively. This difference in slope is attributed to an additional r-process contribution in the NC meteorties.

Improved slope and intercept values were obtained by Bermingham et al. (2024) in a Mo-isotopic study of multiple NC irons representing a variety of chemical groups, along with terrestrial rock samples of various ages from diverse locations (e.g., molybdenite from Isua, Greenland dated at 3.81 b.y.) using an advanced multi-dynamic negative thermal ionization mass spectrometry (N-TIMS) methodology. They found that the slope and intercept values calculated in earlier studies (see initial study and revisions I-III above) would overlap within error the newly acertained NC and CC µ94Mo vs. µ95Mo correlations (non-parallel) if certain aspects of the previous laboratory analytical procedures and/or sample components had been different (e.g., exclusion of leachates in the regression, exclusion of silicate meteorites potentially containing altered presolar components, attaining complete digestion of presolar components). The improved Mo isotopic values were utilized to estimate the proportions of NC and CC material accumulated by Earth (Bulk Silicate Earth [BSE]) during its late-stage accretion (i.e., the final 10 to 20 wt% of growth). The results of the study indicate the CC contribution at this time was minimal (<10%), and that the Moon-forming impactor 'Theia' was likely an NC-type body. A similar conclusion was presented by Kleine et al. (2024 #2537; see diagram), finding that only a minor CC-rich (volatile-enriched) component was delivered to Earth late in its accretionary phase but prior to the Moon-forming event.

Nucleosynthetic isotope anomalies created during supernovae events were heterogeneously distributed within the molecular cloud, and this heterogeneity was imparted to the protoplanetary accretion disk throughout the early and late infall stages (Kruijer et al., 2019). Nucleosynthetic anomalies for many elements support the dichotomy between the NC and CC regions of the protoplanetary disk, including Cr, Ti, Mo, W, Hf, Sr, Nd, Ca, Ni, Sm, Ru, Ba, and Os (e.g., Burkhardt et al., 2019), Zr (e.g., Render et al., 2021), and Fe (e.g., Hopp et al., 2021). The isotopic offset between the CC and the NC groups demonstrated by most of these elements is attributed by Burkhardt et al. (2019) to primordial variability in the admixture of CAIs and CAI-like material on a molecular cloud scale. This isotopic heterogeneity was on the order of only ~0.1% (Kruijer et al., 2019). Burkhardt et al. (2019) suggest that during the cloud collapse phase the isotopic composition of the infalling dust evolved from CAI-like (termed 'IC' for Inclusion-like Chondritic component) to NC-like (more depleted in neutron-rich isotopes). Consequently, this led to an NC-like isotopic dilution of the inner nebular region, while at the same time, the outward expansion of the accretionary radius led to an enrichment of the outer nebular region in unprocessed, primitive, refractory- and volatile-rich material. The rapid formation of Jupiter to ~10–20 M_⊕ within <0.5 m.y. created a barrier to further mixing between the NC and CC reservoirs (Kruijer et al., 2019). A possible inward-then-outward migration of Jupiter and Saturn ('Grand Tack') and/or the creation of an instability due to a mean motion resonance configuration between Jupiter and Saturn ('Nice Model') can explain many characteristics of the Solar System, including the distribution of NC and CC material in the asteroid belt, the volatile budget of Earth, and the small size of Mars. See two schematic illustrations of this scenario below.

In a related study, Nanne et al. (2019) analyzed the Ni isotopes in irons from several CC-region chemical groups and found an ~60 ppm 58Ni excess for these irons compared to those from the NC-region chemical groups (see top diagrams below). They also assert that the NC–CC dichotomy reflects the late infall to the inner protoplanetary disk of material depleted in supernova-derived nuclides such as 54Cr, 50Ti, 58Ni, and r-process Mo at a time when the Jupiter gap prevented efficient exchange between the two isotopically-distinct disk regions. Thus, the present CC region in the outer protoplanetary disk preserves for the most part the initial disk isotopic composition that was established through an enrichment in supernova-derived nuclides, which were hosted by the earliest formed nebular solids—CAIs.

Boss (2022 and previous articles) employed multidimensional hydrodynamics codes to further investigate the model illustrated above (e.g., Burkhardt et al., 2019; Kruijer et al., 2020) for the formation of the NC–CC dichotomy. A physical mechanism was posited to explain the isotopic change that occurred—from an enrichment in Type II supernova (SN II) isotopes before ~100,000 years which denotes the early infall phase, to a depletion in SN II isotopes thereafter which denotes the late infall phase ending ~1 m.y. years later. Their model demonstrated that a SN II shock wave triggered the gravitational collapse of the presolar molecular cloud core, injected short-lived radioisotopes (e.g., 60Fe, 26Al; the early infall phase), and accelerated this newly-formed protoplanetary disk to a velocity of ~3.2 km/s as it traversed the giant molecular cloud into a region lacking any supernova-derived nuclides (the late infall phase).

Render and Brennecka (2021) conducted a high-precision investigation of the nucleosynthetic isotope anomalies associated with the lithophile elements Nd and Zr in several angrites and eucrites. The rendered isotopic compositions for Nd and Zr, together with literature isotopic compositions for the moderately siderophile Mo and the highly siderophile Ru, demonstrate that a coherent trend exists in the distribution of s-process nuclides among different meteorite parent bodies, i.e., a progressive increase in s-deficits with increasing heliocentric distance. These nucleosynthetic signatures enabled them to model the heliocentric order of accretion of several major meteorite groups in the early protoplanetary disk (see top diagram below). Render et al. (2021) conducted a similar high-precision investigation of the nucleosynthetic isotope anomaly associated with the highly refractory, fluid-immobile, lithophile element zirconium (Zr) in most meteorite groups, which also demonstrates the NC–CC dichotomy (see bottom diagram below).

Using Mo, Ru, and Ni isotope systematics, Spitzer et al. (2020 #3040, 2021) identified the first meteorite that has a mixed NC–CC isotopic composition—the 4.6 kg ungrouped iron Nedagolla that fell in India in 1870. The formation of Nedagolla is best modeled by a hit-and-run collision between a small differentiated CI-like body and a larger, likely differentiated EH-like body that resulted in mixing of core material from both. This scenario is consistent with the plot of Nedagolla on a ε94Mo–ε95Mo isotope diagram located between the NC and CC lines (see diagram below). The chemical composition of Nedagolla reflects depletions in Mo and W indicative of more oxidizing conditions during core formation, in character with a CC origin. On the other hand, its composition reflects enrichments in Si and Cr indicative of more reducing conditions, in accord with an NC origin. The collision that produced the mixed melt parental to Nedagolla can be reasonably dated to later than ~7 m.y. after CAIs by Hf–W systematics, and the event may be associated with a dynamical instability related to the growth of Jupiter. A subsequent smaller impact could have ejected the Nedagolla fragment toward Earth orbit. Notably, a second ungrouped iron, the ~12.9 kg sulfide-rich Sirjan 001, has been determined by Tornabene et al. (2022 #6095) to also have a mixed NC–CC Mo-isotopic composition; further analyses are ongoing.

Hopp et al. (2022) investigated the comparative nucleosynthetic anomalous isotopes (O, Cr, Ti, Fe) of CI chondrites and samples of the Cb-type asteroid Ryugu returned by the JAXA Hayabusa2 mission. They found that the µ⁵⁴Fe values of both CI chondrites and Ryugu samples are distinct from those of either NC or CC meteorites, indicating the existence of a third isotopic reservoir in the outer protoplanetary disk and thereby defining a trichotomy (see diagrams below). They suggest that CI chondrites and Ryugu originally formed in this distant reservoir and may even represent a common precursor body that experienced fragmentation after its implantation into the inner Main Belt. Previously, computer modeling by Fernández and Ip (1981) demonstrated that close planetary encounters with Uranus and Neptune would have dispersed such cometesimals both outward into the cometary reservoir (Oort cloud) and inward into the inner Main Belt.

Spitzer et al. (2024 abs, article) conducted a study similar to that of Hopp et al. (2022), in which they analyzed nucleosynthetic Ni isotopes in CI chondrites and Ryugu samples. Their results define a distinct compositional field for CI chondrites and Ryugu rather than an isotopic endmember (see their Fig. 1). Therefore, they argue that the segregation of the CI group/Ryugu from the other CC groups based on nucleosynthetic Ni isotopes can be attributed to a shared loss of some FeNi-metal content among the non-CI carbonaceous chondrites and/or a more efficient accretion of µm-scale FeNi-metal by the CI group/Ryugu subsequent to nebula dissipation, rather than to the existence of an isotopic trichotomy. With respect to the predicted mean µ54Fe for both the CI group/Ryugu and the other CC groups, Yap and Tissot (2023 #1902) found that there is substantial overlap between them, which also supports a disk isotopic dichotomy rather than trichotomy. Spitzer et al. (2024) contend that all CC groups, including the CI group/Ryugu, are comprised of the same four initial components: refractory inclusions, chondrules, FeNi-metal, and fine-grained matrix. In addition, they demonstrated that the ~15% variation in Fe/Si ratios among carbonaceous chondrites can be attributed to the ~5 wt% variation in FeNi-metal abundances of their respective parent bodies.

Based on mineralogical analyses initially conducted by Ikeda (1992), and then by King et al. (2015) and others, a potential new group was proposed which was assigned the working name of 'CY chondrites'. However, it has now been demonstrated that the original six Yamato meteorites composing the 'CY' group actually originate from multiple source parent bodies (Ebihara et al., 2025; Schrader et al., 2025). Zhu et al. (2025) have verified through petrographic, geochemical, and isotopic data that the 'CY' members represent two distinct asteroids: 1) the CYi members are thermally metamorphosed CI lithologies produced in an impact shock event, during which both lithologies were ejected, and 2) the CYm members represent similar heating/dehydration processes on a parent body unique from that of CM chondrites. See the Dhofar 1988 page for current updates on the status of the CY group.

Furthermore, Schrader et al. (2025) have shown that the isotopic hiatus previously observed in the disk has now become a continuum with the inclusion of Cr and Ti isotopic data for these heated meteorites and other anomalous CC members, negating any evidence for the existence of a trichotomy (see diagram below).

Genetic relationships among the meteorite parental source bodies were established at the time of their accretion within isotopically distinct disk reservoirs. A first-order taxonomic division was identified by Warren (2011), and the two subgroups were termed the 'non-carbonaceous' (NC) and the 'carbonaceous' (CC) superclans. This dichotomy is attributed by Rüfenacht et al. (2023) to a combination of snow line processes, thermal processing, and gravitationally-generated pressure bumps and gaps. They recognized that the isotopic compositional variability inherent among different meteorite groups could have been established through a number of possible mechanisms, including temporal isotopic heterogeneity of infalling material from the molecular cloud (e.g., Kruttasch, 2024 [#2096]), aerodynamic size- and density-sorting processes within the protoplanetary disk, and the selective thermal processing/destruction of interstellar medium dust and presolar grains. Utilizing high precision 50Ti, 54Cr, and 48Ca isotopic data for a wide variety of meteorite groups, Rüfenacht et al. (2023) distinguished two sub-reservoirs within the NC reservoir, each of which are comprised of isotopically distinct parent body clusters: 1) Vesta-like (ε50Ti = –1.23 [±0.14]), comprising HEDs, mesosiderites, angrites, brachinites, acapulcoites, lodranites, and MG pallasites, and 2) Mars-Earth-like (ε50Ti = –0.34 [±0.46]), comprising martian and terrestrial samples, aubrites, ECs, OCs, RCs, winonaites, and IAB silicates. In addition, a compelling case for the existence of a third sub-reservoir encompassing the ureilites has been presented by Kruttasch et al. (2025; see Figs. 4, 5). Rüfenacht et al. (2023) attribute these isotopically distinct clusters to secondary ring-like pressure maxima and gap substructures, thought to be generated and sustained by the spiral arms of a rapidly growing proto-Jupiter. The pressure maxima promote the aggregation of planetesimals through gravitational scenarios such as that involving the streaming instability (see diagrams below).

Spitzer et al. (2025) obtained Fe and Ni isotopic data for 27 ungrouped CC-type iron meteorites, 4 ungrouped CC-type achondrites, and 5 established iron groups (IIC, IIF, IIIE, IIIF, and IVB). Using both µ54Fe vs. µ64Ni and µ60Ni vs. µ64Ni coupled diagrams, they demonstrated that Ryugu and CI chondrites plot in a unique space compared to all other CC- and NC-type meteorites. They also found that no CI chondrites experienced a differentiation process, which they attribute to late formation after dissipation of the nebular gas. In addition, they demonstrated that CR-related meteorites constitute a genetic cluster distinct from Ryugu/CI chondrites and other CC-type meteorites based on Fe–Ni isotopic compositions, thus establishing three statistically significant CC-type clusters (see diagram below). Notably, all CR-related meteorites (e.g., chondrites, achondrites, and irons) derive from an early, isolated and sustained reservoir, and represent those CC planetesimals which are the most dissimilar to CI chondrites.

Anand and Mezger (2025) conducted Cr isotopic analyses on 9 chondrules from the CR2 chondrite Shişr 033. In agreement with Δ17O values obtained in a previous study by Marrocchi et al. (2022), ε54Cr values for the Shişr 033 chondrules exhibit a statistically-confirmed bimodal distribution (see the Renazzo page). Anand and Mezger (2025) propose that secondary disk substructures are likely responsible for the existance of two isotopically distinct CR chondrule populations, as previously suggested by Rüfenacht et al. (2023). Anand and Mezger (2025) inferred that a relatively small abundance of low-ε54Cr CV–CO–CM-like chondrules drifted outwards across a disk gap and into the high-ε54Cr CR sub-reservoir prior to parent body accretion (see diagram below).

The formation of the primary substructure of protostellar disks, namely gaps and rings, was described by astrophysicist A. Prof. Kurt Liffman (2024 #1048). As micron-sized dust particles are ejected from the proto-Sun in bipolar jet flows and/or disk winds, they fall back onto the outer regions of the protostellar disk. These infalling inner-disk particles carry less angular momentum than does the outer-disk material, which results in a reduction of angular momentum in the outer-disk region and a corresponding radial flow inwards toward the protostar (at a speed of ~30 km per year). Because the mass of the infall material is temporally correlated with its radial disk speed, an inhomogeneous infall mass will produce gaps (higher infall mass) and rings (lower infall mass) over time. As this process continues, the radial flow of material in the gaps will ultimately reach the outer edge of the rings, thus increasing the local density and promoting the growth of planetesimals (see schematic diagrams below).

A high abundance of sub-spherical objects (SSOs) were observed by Genge et al. (2025) in a highly altered sample (A0180) of asteroid Ryugu. These SSOs, observed primarily through Nano X-ray Computed Tomography, have a mean diameter of ~7 µm and occur at a modal volume >350 ppm. They consider that the SSOs represent microchondrules in which the core silicates have been completely altered to phyllosilicates, and the original FeNi-metal rims have been transformed to iron sulfide crystals by an S-rich fluid. They contend that these microchondrules have been aerodynamically size-sorted and concentrated through intense turbulence within small, high-vorticity eddies near the sunward inner edge of the pressure bump of proto-Jupiter, where such conditions may have generated streaming instabilities that initiated planetesimal accretion. This fine-grained, unprocessed material would have been conveyed to this unique location in part through an inwards meridional flow, and it would have remained chemically and isotopically (e.g., 16O- and 15N-enriched) primitive (see schematic diagram below).

Insights into the earliest stages of planetary accretion can also be made through studies of nearby protoplanetary disks. The Disk Substructures at High Angular Resolution Project (DSHARP) survey was instituted to characterize the substructures of a select group of 20 protoplanetary disks associated with young stars (Andrews et al., 2018). These stars have median ages of ~1 m.y., with some as young as a few hundred thousand years, and they have masses of ~0.2–2 M_⊕. Utilizing the very high resolution data (spatial resolution of ~5 AU) from the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, the team investigated small-scale disk features, including concentric bright rings, dark gaps, spiral patterns, and asymmetries (e.g., crescent/arc structures [vortex scenario of Pérez et al., 2018]; Isella et al., 2018), which are associated with the earliest stages of planet formation. The rings are likely formed through dust trapping at the outer edge of a pressure bump produced by one or more embedded planets (Dullemond et al., 2018; Guzmán et al., 2018; Bodénan et al., 2021 #1954). In-depth analyses of the disk features enabled the identification of potential planets with Neptune- to Jupiter-scale masses (Zhang et al., 2018; see diagram below).

The compositional dichotomy between the NC and CC regions of the protoplanetary disk has been attributed to the rapid growth of Jupiter to a size of ~20 M_⊕ in ~1 m.y., at which point it became a sustained barrier to the inward streaming of dust and pebbles due to gas drag. An alternative model for the creation of this barrier was proposed by Brasser and Mojzsis (2020). Based on data from their dynamical simulations, they contend that the accretion of Jupiter to the minimum size necessary to establish an effective barrier would have taken a longer time, and therefore would not have prevented proto-Mars from growing into a much more massive body than it did. They posit instead that the protoplanetary disk had a ringed structure with coherent pressure maxima and minima (bright and dark annuli, respectively) which enabled the accretion of Jupiter and other gas giants and which provided an effective barrier near 5 AU separating the NC and CC regions.