Letter to the Editor

Observational constraints on the likelihood of ²⁶Al in planet-forming environments

UK Astronomy Technology Centre, Blackford Hill, Edinburgh EH9 3HJ, UK
e-mail: megan.reiter@stfc.ac.uk

Received: 1 September 2020
Accepted: 17 November 2020

Abstract

Recent work suggests that ²⁶Al 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 ²⁶Al in the Galaxy and typical star-forming environments to estimate the likelihood of ²⁶Al 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 ²⁶Al with nearly equal contributions from their winds and supernovae. Observed ²⁶Al 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 million years, which is much longer than the 0.7 Myr half-life. Observed bulk ²⁶Al velocities are an order of magnitude slower than expected from winds and supernovae. These observations are at odds with typical model assumptions that ²⁶Al is provided instantaneously by high velocity mass loss from supernovae and winds. The regular replenishment of ²⁶Al, especially when coupled with the small age differences that are common in high-mass star-forming complexes, may significantly increase the number of star- and planet-forming systems exposed to ²⁶Al. Exposure does not imply enrichment, but the order of magnitude slower velocity of ²⁶Al 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 such as Taurus, that lack high-mass stars to produce ²⁶Al may be less likely to form rocky planets. I conclude with suggested directions for future studies.