Young Solar System Research Articles

Deepest Limits on Scattered Light Emission from the Epsilon Eridani Inner Debris Disk with HST/STIS

Epsilon Eridani is one of the first debris disk systems detected by the Infrared Astronomical Satellite. However, the system has thus far eluded detection in scattered light with no components having been directly imaged. Its similarity to a relatively young solar system combined with its proximity makes it an excellent candidate to further our understanding of planetary system evolution. We present a set of coronagraphic images taken using the Space Telescope Imaging Spectrograph coronagraph on the Hubble Space Telescope at a small inner working angle to detect a predicted warm inner debris disk inside 1″. We used three different postprocessing approaches—nonnegative matrix factorization (NMF), Karhunen–Loève Image Processing (KLIP), and classical reference differential imaging, to best optimize reference star subtraction—and find that NMF performed the best overall while KLIP produced the absolute best contrast inside 1″. We present limits on scattered light from warm dust, with constraints on surface brightness at 6 mJy as−2 at our inner working angle of 0.″6. We also place a constraint of 0.5 mJy as−2 outside 1″, which gives us an upper limit on the brightness for outer disks and substellar companions. Finally, we calculated an upper limit on the dust albedo at ω < 0.487.

Identification of Hot Gas around Low-mass Protostars

The low carbon content of Earth and primitive meteorites compared to the Sun and interstellar grains suggests that carbon-rich grains were destroyed in the inner few astronomical units of the young solar system. A promising mechanism to selectively destroy carbonaceous grains is thermal sublimation within the soot line at ≳300 K. To address whether such hot conditions are common among low-mass protostars, we observe CH3CN transitions at 1, 2, and 3 mm with the NOrthern Extended Millimeter Array toward seven low-mass and one intermediate-mass protostar (L bol ∼ 2–300L ⊙), as CH3CN is an excellent temperature tracer. We find >300 K gas toward all sources, indicating that hot gas may be prevalent. Moreover, the excitation temperature for CH3OH obtained with the same observations is always lower (∼135–250 K), suggesting that CH3CN and CH3OH have a different spatial distribution. A comparison of the column densities at 1 and 3 mm shows a stronger increase at 3 mm for CH3CN than for CH3OH. Since the dust opacity is lower at longer wavelengths, this indicates that CH3CN is enhanced in the hot gas compared to CH3OH. If this CH3CN enhancement is the result of carbon-grain sublimation, these results suggest that Earth’s initial formation conditions may not be rare.

Insights on the Sun Birth Environment in the Context of Star Cluster Formation in Hub–Filament Systems

Cylindrical molecular filaments are observed to be the main sites of Sunlike star formation, while massive stars form in dense hubs at the junction of multiple filaments. The role of hub–filament configurations has not been discussed yet in relation to the birth environment of the solar system and to infer the origin of isotopic ratios of short-lived radionuclides (SLR, such as 26Al) of calcium–aluminum-rich inclusions (CAIs) observed in meteorites. In this work, we present simple analytical estimates of the impact of stellar feedback on the young solar system forming along a filament of a hub–filament system. We find that the host filament can shield the young solar system from stellar feedback, both during the formation and evolution of stars (stellar outflow, wind, and radiation) and at the end of their lives (supernovae). We show that a young solar system formed along a dense filament can be enriched with supernova ejecta (e.g., 26Al) during the formation timescale of CAIs. We also propose that the streamers recently observed around protostars may be channeling the SLR-rich material onto the young solar system. We conclude that considering hub–filament configurations as the birth environment of the Sun is important when deriving theoretical models explaining the observed properties of the solar system.

SOLIS

Context. The study of the early phases of star and planet formation is important to understand the physical and chemical history of stellar systems such as our own. In particular, protostars born in rich clusters are prototypes of the young Solar System. Aims. In the framework of the Seeds Of Life In Space (SOLIS) large observational project, the aim of the present work is to investigate the origin of the previously inferred high flux of energetic particles in the protocluster FIR4 of the Orion Molecular Cloud 2 (OMC-2), which appears asymmetric within the protocluster itself. Methods. Interferometric observations carried out with the IRAM NOEMA interferometer were used to map the silicon monoxide (SiO) emission around the FIR4 protocluster. Complementary archival data from the ALMA interferometer were also employed to help constrain excitation conditions. A physical-chemical model was implemented to characterise the particle acceleration along the protostellar jet candidate, along with a non-LTE analysis of the SiO emission along the jet. Results. The emission morphology of the SiO rotational transitions hints for the first time at the presence of a collimated jet originating very close to the brightest protostar in the cluster, HOPS-108. Conclusions. The NOEMA observations unveiled a possible jet in the OMC-2 FIR4 protocluster propagating towards a previously measured enhanced cosmic-ray ionisation rate. This suggests that energetic particle acceleration by the jet shock close to the protostar might be at the origin of the enhanced cosmic-ray ionisation rate, as confirmed by modelling the protostellar jet.

Hiding Dust around ϵ Eridani

With a Jupiter-like exoplanet and a debris disk with both asteroid and Kuiper Belt analogs, ϵ Eridani has a fascinating resemblance to our expectations for a young solar system. We present a deep Hubble Space Telescope/Space Telescope Imaging Spectrograph coronographic data set using eight orbit visits and the point-spread function calibrator δ Eridani. While we were unable to detect the debris disk, we place stringent constraints on the scattered light surface brightness of We combine this scattered light detection limit with a reanalysis of archival near- and mid-infrared observations and a dynamical model of the full planetary system to refine our model of the ϵ Eridani debris disk components. Radiative transfer modeling suggests an asteroid belt analog inside of 3 au, an intermediate disk component in the 6–37 au region, and a Kuiper Belt analog colocated with the narrow belt observed in the millimeter (69 au). Modeling also suggests a large minimum grain size requiring either very porous grains or a suppression of small grain production, and a radially stratified particle size distribution. The inner disk regions require a steep power-law slope (s −3.8 where s is the grain size) weighted toward smaller grains and the outer disk prefers a shallower slope (s −3.4) with a minimum particle size of >2 μm. These conclusions will be enhanced by upcoming coronagraphic observations of the system with the James Webb Space Telescope, which will pinpoint the radial location of the dust belts and further diagnose the dust particle properties.

In-depth direct imaging and spectroscopic characterization of the young Solar System analog HD 95086

Context. HD 95086 is a young nearby Solar System analog hosting a giant exoplanet orbiting at 57 au from the star between an inner and outer debris belt. The existence of additional planets has been suggested as the mechanism that maintains the broad cavity between the two belts. Aims. We present a dedicated monitoring of HD 95086 with the VLT/SPHERE instrument to refine the orbital and atmospheric properties of HD 95086 b, and to search for additional planets in this system. Methods. SPHERE observations, spread over ten epochs from 2015 to 2019 and including five new datasets, were used. Combined with archival observations, from VLT/NaCo (2012-2013) and Gemini/GPI (2013-2016), the extended set of astrometric measurements allowed us to refine the orbital properties of HD 95086 b. We also investigated the spectral properties and the presence of a circumplanetary disk around HD 95086 b by using the special fitting tool exploring the diversity of several atmospheric models. In addition, we improved our detection limits in order to search for a putative planet c via the K-Stacker algorithm. Results. We extracted for the first time the JH low-resolution spectrum of HD 95086 b by stacking the six best epochs, and confirm its very red spectral energy distribution. Combined with additional datasets from GPI and NaCo, our analysis indicates that this very red color can be explained by the presence of a circumplanetary disk around planet b, with a range of high-temperature solutions (1400–1600 K) and significant extinction (Av ≳ 10 mag), or by a super-solar metallicity atmosphere with lower temperatures (800–300 K), and small to medium amount of extinction (Av ≲ 10 mag). We do not find any robust candidates for planet c, but give updated constraints on its potential mass and location.

Comets in Context: Comparing Comet Compositions with Protosolar Nebula Models

Comets provide a valuable window into the chemical and physical conditions at the time of their formation in the young solar system. We seek insights into where and when these objects formed by comparing the range of abundances observed for nine molecules and their average values across a sample of 29 comets to the predicted midplane ice abundances from models of the protosolar nebula. Our fiducial model, where ices are inherited from the interstellar medium, can account for the observed mixing ratio ranges of each molecule considered, but no single location or time reproduces the abundances of all molecules simultaneously. This suggests that each comet consists of material processed under a range of conditions. In contrast, a model where the initial composition of disk material is “reset,” wiping out any previous chemical history, cannot account for the complete range of abundances observed in comets. Using toy models that combine material processed under different thermal conditions, we find that a combination of warm (CO-poor) and cold (CO-rich) material is required to account for both the average properties of the Jupiter-family and Oort cloud comets, and the individual comets we consider. This could occur by the transport (either radial or vertical) of ice-coated dust grains in the early solar system. Comparison of the models to the average Jupiter-family and Oort cloud comet compositions suggests the two families formed in overlapping regions of the disk, in agreement with the findings of A’Hearn et al. and with the predictions of the Nice model.

A molecular wind blows out of the Kuiper belt

Context. In this Letter we aim to explore whether gas is also expected in the Kuiper belt (KB) in our Solar System. Aims. To quantify the gas release in our Solar System, we use models for gas release that have been applied to extrasolar planetary systems as well as a physical model that accounts for gas released due to the progressive internal warming of large planetesimals. Methods. We find that only bodies larger than about 4 km can still contain CO ice after 4.6 Gyr of evolution. This finding may provide a clue as to why Jupiter-family comets, thought to originate in the KB, are deficient in CO compared to Oort cloud comets. We predict that gas is still currently being produced in the KB at a rate of 2 × 10−8 M⊕ Myr−1 for CO and that this rate was orders of magnitude higher when the Sun was younger. Once released, the gas is quickly pushed out by the solar wind. Therefore, we predict a gas wind in our Solar System starting at the KB location and extending far beyond with regards to the heliosphere, with a current total CO mass of ∼2 × 10−12 M⊕ (i.e., 20 times the CO quantity that was lost by the Hale-Bopp comet during its 1997 passage) and CO density in the belt of 3 × 10−7 cm−3. We also predict the existence of a slightly more massive atomic gas wind made of carbon and oxygen (neutral and ionized), with a mass of ∼10−11 M⊕. Results. We predict that gas is currently present in our Solar System beyond the KB and that, although it cannot be detected with current instrumentation, it could be observed in the future with an in situ mission using an instrument similar to Alice on New Horizons but with larger detectors. Our model of gas release due to slow heating may also work for exoplanetary systems and provide the first real physical mechanism for the gas observations. Lastly, our model shows that the amount of gas in the young Solar System should have been orders of magnitude greater and that it may have played an important role in, for example, planetary atmosphere formation.

The fate of planetesimals formed at planetary gap edges

The presence of rings and gaps in protoplanetary disks are often ascribed to planet–disk interactions, where dust and pebbles are trapped at the edges of planetary-induced gas gaps. Recent works have shown that these are likely sites for planetesimal formation via the streaming instability. Given the large amount of planetesimals that potentially form at gap edges, we address the question of their fate and their ability to radially transport solids in protoplanetary disks. We performed a series ofN-body simulations of planetesimal orbits, taking into account the effect of gas drag and mass loss via ablation. We considered two planetary systems: one that is akin to the young Solar System and another inspired by the structures observed in the protoplanetary disk around HL Tau. In both systems, the proximity to the gap-opening planets results in large orbital excitations, causing the planetesimals to leave their birth locations and spread out across the disk soon after formation. We find that collisions between pairs of planetesimals are rare and should not affect the outcome of our simulations. Collisions with planets occur for ~1% of the planetesimals in the Solar System and for ~20% of the planetesimals in the HL Tau system. Planetesimals that end up on eccentric orbits interior of ~10 au experience efficient ablation and lose all mass before they reach the innermost disk region. In our nominal Solar System simulation, with a stellar gas accretion rate ofṀ0= 10−7M⊙yr−1andα= 10−2, we find that 70% of the initial planetesimal mass has been ablated after 500 kyr. Since the protoplanets are located further away from the star in the HL Tau system, the ablation rate is lower and only 11% of the initial planetesimal mass has been ablated after 1 Myr using the same disk parameters. The ablated material consist of a mixture of solid grains and vaporized ices, where a large fraction of the vaporized ices re-condense to form solid ice. Assuming that the solid grains and ices grow to pebbles in the disk midplane, this results in a pebble flux of ~10−100M⊕Myr−1through the inner disk. This occurred in the Solar System at a time so early in its evolution that there is not likely to be any record of it. Our results demonstrate that scattered planetesimals can carry a significant flux of solids past planetary-induced gaps in young and massive protoplanetary disks.

A new estimate for the age of highly-siderophile element retention in the lunar mantle from late accretion

Subsequent to the Moon's formation, late accretion to the terrestrial planets strongly modified the physical and chemical nature of silicate crusts and mantles. This alteration came in the form of melting through impacts, as well as the belated addition of volatiles and the highly siderophile elements (HSEs). Even though late accretion is well established as having been an important process in the evolution of the young solar system, its intensity and temporal decline remain subject to debate. Much of this deliberation hinges on what can be inferred about late accretion to the Moon from its computed mantle HSE abundances. Current debate centres on whether the lunar HSE record is representative of its whole late accretion history or alternatively that these were only retained in the mantle and crust after a particular time, and if so, when. Here we employ improved Monte Carlo impact simulations of late accretion onto the Moon and Mars and present an updated chronology based on new dynamical simulations of leftover planetesimals and the E-belt – a now-empty hypothesised inner extension of the asteroid belt (Bottke et al., 2012). We take into account the inefficient retention of colliding material. The source of impactors on both planetary bodies is assumed to be the same, hence we use constraints from both objects simultaneously. We compute the crater and basin densities on the Moon and Mars, the largest objects to strike these planets and the amount of material they accreted. Outputs are used to infer the mass in leftover planetesimals at a particular time period, which is then compared to the lunar HSE abundance. From this estimate we calculate a preferred lunar HSE retention age of ca. 4450 Ma which means that the modelled lunar mantle HSE abundances trace almost all of lunar late accretion. Based on our results, the surface ages of the lunar highlands are at least 4370 Ma. We find that the mass of leftover planetesimals with diameters Di < 300 km at 4500 Ma that best fits the crater chronology is approximately 2 × 10−3 Earth mass (ME) while the mass of the E-belt was fixed at 4.5 × 10−4 ME. We also find that a leftover planetesimal mass in excess of 0.01 ME results in a lunar HSE retention age younger than major episodes of lunar differentiation and crust formation, which in turn violates geochemical constraints for the timing and intensity of late accretion to the Earth (Mojzsis et al., 2019).

“TNOs are Cool”: A survey of the trans-Neptunian region

The goal of this work is to determine the physical characteristics of resonant, detached and scattered disk objects in the trans-Neptunian region, observed mainly in the framework of the “TNOs are Cool” Herschel open time key programme. Based on thermal emission measurements with the Herschel/PACS and Spitzer/MIPS instruments, we determine size, albedo, and surface thermal properties for 23 objects using radiometric modeling techniques. This is the first analysis in which the physical properties of objects in the outer resonances are determined for a notable sample. In addition to the results for individual objects, we compared these characteristics with the bulk properties of other populations of the trans-Neptunian region. The newly analyzed objects show a large variety of beaming factors, indicating a diversity of thermal properties, and in general they follow the albedo-color clustering identified earlier for Kuiper belt objects and Centaurs, further strengthening the evidence for a compositional discontinuity in the young Solar System.

Activity of a Young Solar Analog—the Star with an Exoplanet DS Tuc

We present the results of our analysis of the photospheric and chromospheric activity of the star DS Tuc (DS Tuc A, HD 222259A), which is a member of the Tucana–Horologium (Tuc–Hor) group with an age of 45 Myr. The star DS Tuc may be considered as the prototype of a young Sun, while its planetary system may be considered as the prototype of a young Solar system. Using data from the archive of TESS observations, we have analyzed the activity of this star. According to our estimate, the stellar rotation period is $$P=2.85\pm 0.18$$ days, which coincides with its known estimates. The light curve has been analyzed by the method of solving the inverse problem of reconstructing the maps of surface temperature inhomogeneities. We show that there are concentrations of spots at two longitudes on these maps. The spot positions undergo changes. The fraction $$S$$ of the spotted stellar surface is about 3.3 $$\%$$ . We have studied the object’s position on the $$S$$ –age, $$S$$ –rotation period, and $$S$$ –Rossby number diagrams and concluded that it corresponds to the general pattern of the dependences established previously for young stars with planetary systems and solar-type stars. Based on All Sky Automated Survey observations, we have estimated for the first time the possible activity cycle of DS Tuc to be 1610 days (4.4 years).

The partitioning of the inner and outer Solar System by a structured protoplanetary disk

Mass-independent isotopic anomalies define two cosmochemically distinct regions: the carbonaceous and non-carbonaceous meteorites1, implying that the non-carbonaceous (terrestrial) and carbonaceous (Jovian) reservoirs were kept separate during and after planet formation. The formation of Jupiter is widely invoked to explain this compositional dichotomy by acting as an effective barrier between the two reservoirs2. Jupiter’s solid kernel possibly grew to 20 Earth masses ( $${M}_{\oplus }$$ ) in 1 Myr from the accretion of submetre-sized objects (‘pebbles’), followed by slower accretion via planetesimals. Subsequent gas envelope contraction led to Jupiter’s formation as a gas giant3. Here, we use dynamical simulations to show that the growth of Jupiter from pebble accretion is not fast enough to be responsible for the inferred separation of the terrestrial and Jovian reservoirs. We propose instead that the dichotomy was caused by a pressure maximum in the disk near Jupiter’s location, which created a ringed structure such as those detected by ALMA4. One or multiple such—potentially mobile—long-lived pressure maxima almost completely prevented pebbles from the Jovian region reaching the terrestrial zone, maintaining a compositional partition between the two regions. We thus suggest that our young Solar System’s protoplanetary disk developed at least one and probably multiple rings, which potentially triggered the formation of the giant planets. The early Solar System might have been similar to the ringed protoplanetary disks observed by ALMA. One of the gaps, at Jupiter’s position, could be the cause of the observed dichotomy between carbonaceous and non-carbonaceous material.

The formation of Jupiter’s diluted core by a giant impact

The Juno mission1 has provided an accurate determination ofJupiter's gravitational field2, which hasbeenusedto obtain information about the planet's compositionand internal structure. Severalmodels of Jupiter's structure that fit the probe's data suggest that the planet has a diluted core, with a total heavy-element mass ranging from ten to a few tens of Earth masses(about5 to 15 per cent of the Jovian mass), and that heavy elements(elements otherthan hydrogen and helium)are distributed within a region extending to nearly half of Jupiter's radius3,4. Planet-formation models indicate that most heavy elements are accreted during the earlystages ofa planet's formation to create a relativelycompact core5-7 and that almost no solids are accreted duringsubsequent runaway gas accretion8-10. Jupiter's diluted core, combined with its possible high heavy-element enrichment, thus challenges standard planet-formation theory. A possible explanation is erosion of the initially compact heavy-element core, but the efficiency of such erosion is uncertain and depends on both the immiscibility of heavy materials in metallic hydrogen and on convective mixing as the planet evolves11,12. Another mechanism that canexplain this structure is planetesimal enrichment and vaporization13-15 during the formation process, although relevant modelstypically cannotproduce an extended diluted core. Here we show that a sufficiently energetic head-on collision (giant impact) between a large planetary embryo and the proto-Jupiter could have shattered its primordial compact core and mixed the heavy elements with the inner envelope. Models of such a scenario lead to an internal structure that is consistent with a diluted core, persisting over billions of years. We suggest that collisions were common in the young Solar system and thata similar event may have also occurred for Saturn, contributing to the structural differences between Jupiter and Saturn16-18.

Planet seeding through gas-assisted capture of interstellar objects

Planet formation begins with collisional growth of small planetesimals accumulating into larger ones. Such growth occurs while planetesimals are embedded in a gaseous protoplanetary disc. However, small-planetesimals experience collisions and gas-drag that lead to their destruction on short timescales, not allowing, or requiring fine tuned conditions for the efficient growth of $\sim$ metre-size objects. Here we show that $\sim10^{4}$ interstellar objects such as the recently detected 1I/2017-U1 ('Oumuamua) could have been captured, and become part of the young Solar System, together with up to hundreds of $\sim$ km sized ones. The capture rates are robust even for conservative assumptions on the protoplanetary disc structure, local stellar environment and planetesimal ISM density. 'Seeding' of such planetesimals then catalyze further planetary growth into planetary embryos, and potentially alleviate the main-challenges with the meter-size growth-"barrier". The capture model is in synergy with the current leading planet formation theories, providing the missing link to the first planetesimals. Moreover, planetesimal capture provides a far more efficient route for lithopanspermia than previously thought.

Meteorite’s Hidden Treasure: A Comet

A fragment of a comet found hidden inside a meteorite is offering new insights into the dynamics of our young solar system.

Photolysis of Cometary Organic Dust Analogs on the EXPOSE-R2 Mission at the International Space Station.

We describe the results obtained on a set of organic samples that have been part of the experiment "Photochemistry on the Space Station (PSS)" on the EXPOSE-R2 mission conducted on the EXPOSE-R facility situated outside the International Space Station (ISS). The organic samples were prepared in the Catania laboratory by 200 keV He+ irradiation of N2:CH4:CO icy mixtures deposited at 17 K, on vacuum UV (VUV) transparent MgF2 windows. This organic material contains different chemical groups, including triple CN bonds, that are thought to be of interest for astrobiology. It is widely accepted that materials similar to that produced in the laboratory by ion irradiation of frozen ices could be present in some astrophysical environments such as comets. Once expelled from comets, these materials are exposed to solar radiation during their interplanetary journey. In the young Solar System, some of these processed materials could have reached early Earth and contributed to its chemical and prebiotic evolution. The samples were exposed for 16 months to the unshielded solar UV photons. It was found that, if an interplanetary dust particle (IDP) containing organic material (50% vol) is large enough (>20-30 μm), relevant chemical groups, such as those containing the CN triple bond, can survive for many years (>104 years) in the interplanetary medium.

The Disk Substructures at High Angular Resolution Project (DSHARP). VII. The Planet–Disk Interactions Interpretation

Abstract The Disk Substructures at High Angular Resolution Project (DSHARP) provides a large sample of protoplanetary disks with substructures that could be induced by young forming planets. To explore the properties of planets that may be responsible for these substructures, we systematically carry out a grid of 2D hydrodynamical simulations, including both gas and dust components. We present the resulting gas structures, including the relationship between the planet mass, as well as (1) the gaseous gap depth/width and (2) the sub/super-Keplerian motion across the gap. We then compute dust continuum intensity maps at the frequency of the DSHARP observations. We provide the relationship between the planet mass, as well as (1) the depth/width of the gaps at millimeter intensity maps, (2) the gap edge ellipticity and asymmetry, and (3) the position of secondary gaps induced by the planet. With these relationships, we lay out the procedure to constrain the planet mass using gap properties, and study the potential planets in the DSHARP disks. We highlight the excellent agreement between observations and simulations for AS 209 and the detectability of the young solar system analog. Finally, under the assumption that the detected gaps are induced by young planets, we characterize the young planet population in the planet mass–semimajor axis diagram. We find that the occurrence rate for &gt;5 M J planets beyond 5–10 au is consistent with direct imaging constraints. Disk substructures allow us to probe a wide-orbit planet population (Neptune to Jupiter mass planets beyond 10 au) that is not accessible to other planet searching techniques.

Comprehensive Analysis of HD 105, A Young Solar System Analog

HD~105 is a nearby, pre-main sequence G0 star hosting a moderately bright debris disc ($L_{\rm dust}/L_{\star} \sim 2.6\times10^{-4}$). HD~105 and its surroundings might therefore be considered an analogue of the young Solar System. We refine the stellar parameters based on an improved Gaia parallax distance, identify it as a pre-main sequence star {with an age of 50~$\pm$~16~Myr}. The circumstellar disc was marginally resolved by \textit{Herschel}/PACS imaging at far-infrared wavelengths. Here we present an archival ALMA observation at 1.3~mm, revealing the extent and orientation of the disc. We also present \textit{HST}/NICMOS and VLT/SPHERE near-infrared images, where we recover the disc in scattered light at the $\geq$~5-$\sigma$ level. This was achieved by employing a novel annular averaging technique, and is the first time this has been achieved for a disc in scattered light. Simultaneous modelling of the available photometry, disc architecture, and detection in scattered light allow better determination of the disc's architecture, and dust grain minimum size, composition, and albedo. We measure the dust albedo to lie between 0.19 and 0.06, the lower value being consistent with Edgeworth-Kuiper belt objects.

Constraints from zoom-in simulations on the protostellar accretion process

AbstractStars are embedded in different environments of Giant Molecular Clouds during their formation phase. Despite this fact, it is common practice to assume an isolated spherical core as the initial condition for models of individual star formation. To avoid the uncertainties of initial and boundary conditions, we use an alternative approach of zoom-in simulations to account for the environment in which protostars form. Our models show that injections of 26Al from a close-by supernova into the young solar system were highly unlikely. Moreover, we find that the accretion process of protostars is heterogeneous and environment-dependent.