The early solar system contained live 26^{26}Al (half-life ≈0.7\approx 0.7 Myr) at 26^{26}Al/27^{27}Al ≈5×10−5\approx 5\times10^{-5} when its oldest solids formed, and its decay heat shaped planetesimal thermal evolution and planetary water budgets. Sawada, Taki, Kurokawa and Suwa (2024) showed that the leading 'disk injection' scenario — a nearby supernova implanting 26^{26}Al-bearing dust directly into the already-formed protosolar disk — is severely strained: it requires an ejected 26^{26}Al mass ≳2.1×10−3 M⊙(ηd/0.2)−1\gtrsim 2.1\times10^{-3}\,M_\odot(\eta_{\rm d}/0.2)^{-1}, an order of magnitude above typical yields, and even then shock heating drives the disk to ∼500\sim 500 K, in tension with meteoritic and cometary records. The open problem is the self-consistent physical account of how the solar system acquired its fresh 26^{26}Al (and other short-lived radionuclides): whether any direct disk-injection configuration survives multidimensional ejecta–disk modeling, what dust mass fraction supernova ejecta realistically provide, whether the associated thermal processing is compatible with the solar-system record, and which alternative channel — molecular-cloud inheritance, local irradiation, cloud-core or filament injection, asymptotic-giant-branch interlopers (dying Sun-like stars passing through the Sun's birth cluster), or the newly proposed cosmic-ray 'immersion' mechanism — can reproduce the meteoritic abundances without destroying the disk.

Background

Aluminum-26 (26^{26}Al) is a short-lived radioactive isotope (half-life t1/2≈0.7t_{1/2}\approx0.7 Myr) that was present, alive, in the early solar system. Its former presence is recorded in calcium–aluminum-rich inclusions (CAIs, the oldest dated solar-system solids, formed about 4.57 Gyr ago) as excesses of its daughter isotope 26^{26}Mg, implying an initial 26^{26}Al/27^{27}Al ratio of about 5×10−55\times10^{-5}. Decay of this 'fresh' 26^{26}Al released enough heat to melt and differentiate many planetesimals and is thought to have driven water loss from rocky-planet building blocks, so its abundance directly conditions terrestrial-planet composition. Because 26^{26}Al decays quickly on geological timescales, its abundance at CAI formation requires a nucleosynthetic origin shortly before or during solar-system formation; the isotope is produced mainly in massive stars and their core-collapse supernovae (SN). Three families of origin hypotheses have been debated for decades: inheritance (the Sun's parent molecular cloud was already enriched, e.g. by winds of massive Wolf–Rayet stars (hot, luminous stars in a late evolutionary phase that shed nuclearly processed material) and earlier supernovae), irradiation (26^{26}Al synthesized locally inside the disk by accelerated particles from the young Sun), and injection (fresh 26^{26}Al delivered from a nearby dying star). The injection family splits into injection into the molecular-cloud core or filament before collapse, and direct injection into the already-formed protoplanetary disk. Earlier direct-injection work — notably the two-dimensional simulations of Ouellette, Desch and Hester (2007, 2010), which tracked supernova dust grains through the shocked disk gas — treated disk survival, disk structure and 26^{26}Al capture with restricted setups, and the two requirements pull in opposite directions: enough injection demands a very close supernova (≲0.3\lesssim0.3 pc), while disk survival by ram-pressure balance demands a more distant one. Sawada, Taki, Kurokawa and Suwa (2024) closed this loop self-consistently for a face-on disk hit by isotropic ejecta in the thin-shell approximation (the shocked ejecta layer is thin compared with the disk scale, so the momentum flux is set by the blast-wave ram pressure). For a range of disk masses and structures, supernova 26^{26}Al yields and explosion energies, and large-grain dust mass fractions ηd\eta_{\rm d} (the fraction of ejecta condensed into grains large enough, ≳μ\gtrsim\mum, to penetrate the disk gas ballistically rather than being deflected around it), they find the disk radius that survives disruption and the 26^{26}Al mass it can capture simultaneously. They conclude that direct injection requires an ejected 26^{26}Al mass MSN(26Al)≳2.1×10−3 M⊙(ηd/0.2)−1(Eexpl/1051 erg)2/3M_{\rm SN}({}^{26}{\rm Al})\gtrsim2.1\times10^{-3}\,M_\odot(\eta_{\rm d}/0.2)^{-1}(E_{\rm expl}/10^{51}\,{\rm erg})^{2/3} — roughly an order of magnitude above their 1.3×10−4 M⊙1.3\times10^{-4}\,M_\odot reference yield and hard to reconcile with observed dust fractions (ηd≲0.2\eta_{\rm d}\lesssim0.2 from SN 1987A and SN 1980K at the relevant decades after explosion) and with modern supernova explodability constraints (many massive progenitors likely collapse quietly to black holes, further limiting yields). Moreover, radiative equilibrium of the shocked layer heats the disk to Tdisk,eq≈537T_{\rm disk,eq}\approx537 K at the self-consistent injection distance — hot enough to pyrolyze pristine organic mantles (removed above ≃250\simeq250–400 K) and to convert amorphous cometary ice to crystalline ice, both in tension with meteoritic and cometary (e.g. Rosetta measurements at comet 67P) records. Their summary: direct disk injection is 'quite challenging', so the fresh 26^{26}Al 'should have been synthesized/injected in other ways'. The authors explicitly flag what remains to be done: comprehensive studies of supernova dust formation — simulations of multidimensional supernova explosions with dust formation, and observations of dust emission in supernovae several decades after the explosion — since their ηd≲0.2\eta_{\rm d}\lesssim0.2 estimate assumes spherical symmetry and non-extreme conditions, while higher ηd\eta_{\rm d} values are possible in clumpy structures; and a more detailed analysis of the heating and cooling processes of disks during supernova exposure and the physicochemical impacts on the disk materials. Their thin-shell and isotropic-ejecta approximations are adopted for simplicity, and their inclination analysis shows that tilt only makes injection harder, the face-on disk giving the most conservative condition for the injection scenario; the multidimensional question they leave open concerns clumpy ejecta structure rather than disk inclination.

Problem Statement

Determine, self-consistently, how the early solar system acquired its fresh 26^{26}Al (with the other short-lived radionuclides), given the strong constraints placed by Sawada, Taki, Kurokawa and Suwa (2024) on direct supernova disk injection. The open questions include: (i) whether any direct disk-injection configuration can deliver the meteoritic abundance (26^{26}Al/27^{27}Al ≈5×10−5\approx 5\times10^{-5}) without disrupting the disk — in particular whether multidimensional ejecta–disk coupling, clumpy ejecta geometries or grain-size-dependent injection efficiencies relax the required ejected mass MSN(26Al)≳2.1×10−3 M⊙(ηd/0.2)−1(Eexpl/1051 erg)2/3M_{\rm SN}({}^{26}{\rm Al})\gtrsim2.1\times10^{-3}\,M_\odot(\eta_{\rm d}/0.2)^{-1}(E_{\rm expl}/10^{51}\,{\rm erg})^{2/3} (the 2024 analysis adopted the thin-shell and isotropic-ejecta approximations for simplicity and argued that a face-on disk is the most conservative condition for injection, larger inclination only making injection harder); (ii) what realistic large-grain mass fractions ηd\eta_{\rm d} supernova ejecta provide, from self-consistent modeling of dust formation and survival at the decades-long post-explosion timescales relevant to disks reached by the blast wave; (iii) whether the thermal processing implied by any successful injection or synthesis channel — time-dependent heating and cooling of the shocked layer and the disk vertical structure, re-condensation, amorphous-to-crystalline ice conversion, organic pyrolysis, and mixing — is compatible with meteoritic and cometary constraints on pristine organics and amorphous ice; and (iv) which alternative source channels can reproduce the meteoritic short-lived-radionuclide abundances within the updated constraints on supernova explodability, yields, and birth-cluster dynamics: those named in the 2024 analysis (inheritance from the parent molecular cloud, local irradiation, injection into the molecular-cloud core or filament) and those proposed subsequently (asymptotic-giant-branch interlopers — dying Sun-like stars whose winds carry freshly synthesized nuclei — and in-situ cosmic-ray nucleosynthesis during immersion in a supernova shock). The four sub-questions (i)–(iv) above are jointly required, and none of them is independently sufficient: a self-consistent account requires a coupled treatment in which abundance delivery, disk survival, thermal processing and discrimination among the candidate source channels are satisfied simultaneously, rather than treating them separately.

References

  1. Sawada, R., Taki, T., Kurokawa, H., & Suwa, Y. (2024). Self-consistent Conditions for 26Al Injection into a Protosolar Disk from a Nearby Supernova. The Astrophysical Journal 963, 68. DOI: 10.3847/1538-4357/ad1e62; arXiv:2312.01948. https://arxiv.org/abs/2312.01948
  2. Sawada, R., Kurokawa, H., Suwa, Y., Taki, T., Lee, S.-H., & Tanikawa, A. (2025). Cosmic-ray bath in a past supernova gives birth to Earth-like planets. Science Advances 11(50), eadx7892. DOI: 10.1126/sciadv.adx7892; arXiv:2512.09660. https://arxiv.org/abs/2512.09660
  3. Eatson, J. W., Parker, R. J., & Lichtenberg, T. (2024). Toward a unified injection model of short-lived radioisotopes in N-body simulations of star-forming regions. The Astrophysical Journal 977, 13. DOI: 10.3847/1538-4357/ad8642; arXiv:2410.17163. https://arxiv.org/abs/2410.17163
  4. Eatson, J. W., & Parker, R. J. (2025). Short-lived radioisotopic enrichment from AGB interlopers in low-mass star-forming regions. Monthly Notices of the Royal Astronomical Society 544, 343-354. DOI: 10.1093/mnras/staf1768; arXiv:2510.12079. https://arxiv.org/abs/2510.12079
  5. Desch, S., & Miret-Roig, N. (2024). The Sun's Birth Environment: Context for Meteoritics. Space Science Reviews 220, 76. DOI: 10.1007/s11214-024-01113-x; arXiv:2409.10638. https://arxiv.org/abs/2409.10638
  6. Shimonishi, T., Sano, H., Furuya, K., & Oya, Y. (2026). Survival of Molecular Complexity under Recent Supernova Feedback: Detection of Hot Cores in RX J1713.7-3946. The Astrophysical Journal 1005, 142. DOI: 10.3847/1538-4357/ae6fba. https://doi.org/10.3847/1538-4357/ae6fba
  7. Ouellette, N., Desch, S. J., & Hester, J. J. (2007). Interaction of Supernova Ejecta with Nearby Protoplanetary Disks. The Astrophysical Journal 662, 1268-1281. DOI: 10.1086/518102; arXiv:0704.1652. https://arxiv.org/abs/0704.1652
  8. Ouellette, N., Desch, S. J., & Hester, J. J. (2010). Injection of Supernova Dust in Nearby Protoplanetary Disks. The Astrophysical Journal 711, 597-612. DOI: 10.1088/0004-637x/711/2/597. https://doi.org/10.1088/0004-637x/711/2/597