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Register a new userObservational constraints on the likelihood of $^{26}$Al in planet-forming environments
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Megan Reiter
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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.
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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 planets 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%.
10 Hibonite-pyroxene/glass spherules discovered hitherto are a rare suite of refractory inclusions that show the largest range of exotic isotopic properties (anomalies in neutron rich isotopes (e.g., $^{48}$Ca, $^{50}$Ti), abundance of $^{26}$Al) despite their defining simple spherical morphology and mineralogy consisting predominantly of few hibonites nestled within/with glassy or crystallised calcium, aluminium-rich pyroxene. $^{26}$Al-$^{26}$Mg chronological studies along with petrography and mineralogy of a relatively large (~120 micron diameter), found in Allan Hills 77307 (CO3.03) has been performed. Uniquely, both hibonite and pyroxene show discordant abundance of short-lived now-extinct radionuclide $^{26}$Al that suggest disparate and distinct regions of origin of hibonite and pyroxene. The pristine petrography and mineralogy of this inclusion allow discernment of their genesis and trend of alteration in hibonite-pyroxene/glass spherules.
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Our understanding of protoplanetary disks has greatly improved over the last decade due to a wealth of data from new facilities. Unbiased dust surveys with Spitzer leave us with good constraints on the dust dispersal timescale of small grains in the terrestrial planet forming region. In the ALMA era, this can be confronted for the first time also with evolutionary timescales of mm grains in the outer disk. Gas surveys in the context of the existing multi-wavelength dust surveys will be a key in large statistical studies of disk gas evolution. Unbiased gas surveys are limited to ALMA CO submm surveys, where the quantitative interpretation is still debated. Herschel gas surveys have been largely biased, but [OI] 63 mic surveys and also accretion tracers agree qualitatively with the evolutionary timescale of small grains in the inner disk. Recent advances achieved by means of consistent multi-wavelength studies of gas AND dust in planet forming disks reveal the subtleties of the quantitative interpretation of gas surveys. Observational methods to determine disk masses e.g. from CO submm lines require the knowledge of the dust properties in the disk. Understanding not only the gas evolution, but also its chemical composition will provide crucial input for planet formation models. Kinetic chemical results give profoundly different answers than thermodynamic equilibrium in terms of the C/O ratios as well as the water ice/rock ratios. Again, dust has a key impact on the chemical evolution and composition of the gas. Grain growth for example affects freeze-out processes and strongly increases the cosmic ray induced UV field.
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