Having cold feet: The fate of extrasolar systems starting with less 26Al

The short-lived radioactive nuclide Aluminum-26 (²⁶Al) played a crucial role in the early solar system, serving both as a major heat source and a chronometer. With a half-life of about 0.717 Myr, its decay to ²⁶Mg released enough energy to heat small planetesimals, driving their melting, differentiation into cores and mantles, and early volcanic activity (e.g. [1]). This internal heating influenced the thermal evolution and geologic processes of these early bodies, as, notably, any planetesimals above a few kilometers in diameter formed during the first million year of our solar system experienced melting (e.g., [2]). Additionally, the ²⁶Al–²⁶Mg decay system is an important radiometric dating tool to determine the relative ages of early solar system materials such as calcium-aluminum-rich inclusions (CAIs) and chondrules (e.g., [3]).

From an astrophysical point of view, the production site of ²⁶Al is unclear. Currently, ²⁶Al can be observed using γ-ray telescopes [4], indicating that its production is ongoing, outflows of Wolf–Rayet massive stars and core-collapse supernovae accounting for 70% of its production (e.g., [5]). Measurements in meteorites and their early components (CAI and chondrules), show that the initial (²⁶Al/²⁷Al)₀ in the early solar system was (5.11 ± 0.14) × 10^-5 [3]. That amount of ²⁶Al, however, surpasses what is expected by steady-state nucleosynthesis (e.g. [6]). Various models have predicted a concomitant injection of ²⁶Al from winds of massive stars (M>40-50 Msun;  [7] and references therein), with the collapse of the presolar nebula in a dynamic stellar environment (e.g. [8]).

While the probability of solar systems forming with elevated (²⁶Al/²⁷Al)₀ has already been investigated [9], here we investigate the fate of solar systems that start off with a lower complement of ²⁶Al, and track the thermal cooling of planetesimals accreted in the first few million years. Planetary differentiation in a context of low ²⁶Al and thus cold conditions will be discussed. We also present plausible upper and lower bounds on the abundance of ²⁶Al and the ⁶⁰Fe/²⁶Al ratio in the galaxy’s solar annulus, thus parameterizing the initial thermal states of rocky exoplanets through time.

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