A water budget dichotomy of rocky protoplanets from $^{26}$Al-heating (1902.04026v1)

Abstract: In contrast to the water-poor inner solar system planets, stochasticity during planetary formation and order of magnitude deviations in exoplanet volatile contents suggest that rocky worlds engulfed in thick volatile ice layers are the dominant family of terrestrial analogues among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained, and it is not clear whether the solar system is a statistical outlier or can be explained by more general planetary formation processes. Here we employ numerical models of planet formation, evolution, and interior structure, to show that a planet's bulk water fraction and radius are anti-correlated with initial $^{26}$Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals prior to accretion onto larger protoplanets and yields a system-wide correlation of planet bulk abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models suggest two main scenarios of planetary systems' formation: high-$^{26}$Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of $^{26}$Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%.

A Water Budget Dichotomy of Rocky Protoplanets from $^{26}$Al-Heating

The paper by Lichtenberg et al. investigates the role of the short-lived radionuclide $^{26}$Al in the dehydration of planetesimals and its implications on the water content of rocky protoplanets. Utilizing numerical models of planetary formation and evolution, the authors explore how initial $^{26}$Al levels influence the bulk water fraction and planet radius within the planetesimal-based accretion framework. This paper addresses the broader question of whether the compositional characteristics of our solar system are an anomaly or if they can be accounted for by general planetary formation processes.

Numerical simulations reveal an anti-correlation between a planet's bulk water fraction, radius, and initial $^{26}$Al levels. Heat generated by $^{26}$Al contributes significantly to the internal heating of planetesimals, driving rapid dehydration before accretion onto larger bodies. The analysis suggests that systems with high initial $^{26}$Al concentrations, akin to our solar system, tend to form planets that are small and water-depleted. In contrast, systems with low $^{26}$Al levels predominantly form "ocean worlds," resulting in mean planet radii deviations between these two planetary formation scenarios by approximately 10%.

The paper conducts extensive parameter space exploration with 540,000 simulations, taking into account factors such as planetesimal radius and initial $^{26}$Al abundance. Larger planetesimals and those in high-$^{26}$Al environments undergo faster and more complete dehydration, leading to a dichotomy in planet water content and size. The potential implications for observed exoplanetary systems, such as TRAPPIST-1, are significant; the TRAPPIST-1 planet system's water content and atmospheric composition suggest a formation history enriched in $^{26}$Al. This offers an explanation for the apparent uniformity in their low water mass fractions without requiring intricate planetary accretion scenarios.

Practically, this research implies that observed variations in exoplanet radii might correspond to differences in initial $^{26}$Al levels across planetary systems, detectable with missions like PLATO. Theoretically, the paper proposes a dual-path evolutionary model, characterized by a divergence into water-rich and water-poor systems depending on $^{26}$Al enrichment.

Future research directions include assessing the role of pebble accretion in the deviation of $^{26}$Al-induced dehydration, as well as further constraints on initial $^{26}$Al distributions within protoplanetary disks. This could better illuminate whether or not the attributes of the solar system are typical in the galaxy. The paper's insights hold implications for our understanding of planet formation narratives and the possible diversity of terrestrial exoplanets.