Observational constraints on the likelihood of $^{26}$Al in planet-forming environments

Abstract

Recent work suggests that $^{26}$Al may determine the water budget in terrestrial exoplanets as its radioactive decay dehydrates planetesimals leading to rockier compositions. Here I consider the observed distribution of $^{26}$Al in the Galaxy and typical star-forming environments to estimate the likelihood of $^{26}$Al enrichment during planet formation. I do not assume Solar-System-specific constraints as I am interested in enrichment for exoplanets generally. Observations indicate that high-mass stars dominate the production of $^{26}$Al with nearly equal contributions from their winds and supernovae. $^{26}$Al abundances are comparable to those in the early Solar System in the high-mass star-forming regions where most stars (and thereby most planets) form. These high abundances appear to be maintained for a few Myr, much longer than the 0.7 Myr half-life. Observed bulk $^{26}$Al velocities are an order of magnitude slower than expected from winds and supernovae. These observations are at odds with typical model assumptions that $^{26}$Al is provided instantaneously by high velocity mass loss from supernovae and winds. Regular replenishment of $^{26}$Al especially when coupled with the small age differences that are common in high-mass star-forming complexes, may significantly increase the number of star/planet-forming systems exposed to $^{26}$Al. Exposure does not imply enrichment, but the order of magnitude slower velocity of $^{26}$Al may alter the fraction that is incorporated into planet-forming material. Together, this suggests that the conditions for rocky planet formation are not rare, nor are they ubiquitous, as small regions like Taurus that lack high-mass stars to produce $^{26}$Al may be less likely to form rocky planets. I conclude with suggested directions for future studies.