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Register a new userA water budget dichotomy of rocky protoplanets from $^{26}$Al-heating
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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%.
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The existence of water in extrasolar planetary systems is of great interest as it constrains the potential for habitable planets and life. Here, we report the identification of a circumstellar disk that resulted from the destruction of a water-rich and rocky, extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs.
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.
We predict magnitudes for young planets embedded in transition discs, still affected by extinction due to material in the disc. We focus on Jupiter-size planets at a late stage of their formation, when the planet has carved a deep gap in the gas and dust distributions and the disc starts being transparent to the planet flux in the infrared (IR). Column densities are estimated by means of three-dimensional hydrodynamical models, performed for several planet masses. Expected magnitudes are obtained by using typical extinction properties of the disc material and evolutionary models of giant planets. For the simulated cases located at $5.2$ AU in a disc with local unperturbed surface density of $127$ $mathrm{g} cdot mathrm{cm}^{-2}$, a $1$ $M_J$ planet is highly extincted in J-, H- and K-bands, with predicted absolute magnitudes $ge 50$ mag. In L- and M-bands extinction decreases, with planet magnitudes between $25$ and $35$ mag. In the N-band, due to the silicate feature on the dust opacities, the expected magnitude increases to $40$ mag. For a $2$ $M_J$ planet, the magnitudes in J-, H- and K-bands are above $22$ mag, while for L-, M- and N-bands the planet magnitudes are between $15$ and $20$ mag. For the $5$ $M_J$ planet, extinction does not play a role in any IR band, due to its ability to open deep gaps. Contrast curves are derived for the transition discs in CQ Tau, PDS70, HL Tau, TW Hya and HD163296. Planet mass upper-limits are estimated for the known gaps in the last two systems.
We review the state of knowledge on the origin of Earths water. Empirical constraints come from chemical and isotopic measurements of solar system bodies and of Earth itself. Dynamical models have revealed pathways for water delivery to Earth during its formation; most are anchored to specific models for terrestrial planet formation. Meanwhile, disk chemical models focus on determining how the isotopic ratios of the building blocks of planets varied as a function of radial distance and time, defining markers of material transported along those pathways. Carbonaceous chondrite meteorites -- representative of the outer asteroid belt -- match Earths water isotopes (although mantle plumes have been measured at lower D/H). But how was this connection established -- did Earths water originate among the asteroids (as in the classical model of terrestrial planet formation)? Or, more likely, was Earths water delivered from the same parent population as the hydrated asteroids (e.g., external pollution, as in the Grand Tack model)? We argue that the outer asteroid belt -- the boundary between the inner and outer solar system -- is the next frontier for new discoveries. The outer asteroid belt contains a population of icy bodies with volatile-driven activity seen on twelve main belt comets (MBCs); seven of which exhibit sublimation-driven activity on repeated perihelion passages. Measurements of the isotopic characteristics of MBCs would provide essential missing links in the chain between disk models and dynamical models. Finally, we extrapolate to rocky exoplanets. Migration is the only mechanism likely to produce very water-rich planets with more than a few percent water by mass (and even with migration, some planets are pure rock). While water loss mechanisms remain to be studied in more detail, we expect that water should be delivered to the vast majority of rocky exoplanets.
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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