The frequency of Earth-like planets with enough water to host surface oceans is currently poorly understood, as are the conditions leading to their formation and how they can vary between stellar systems born from the same star-forming region. Classical planet formation theory suggests that planets form from reservoirs of material, the composition and water content of which is determined by whether formation occurs outside the protoplanetary disk snow line or not. In the inner Solar System, this results in the formation of water-poor planetesimals, and eventually the conglomeration of these into water-poor rocky planets. However, recent findings from the fields of geochemistry and cosmochemistry suggest that early planetesimal populations were originally accreted from water-rich material (Schiller et al. 2018; Drazkowska et al. 2023; Perotti et al. 2023; Grant et al. 2023), while the bulk water content of the resultant planets is significantly depleted relative to their parent planetesimals.
This evidence presents two important questions: why did the inner rocky planets not remain water-rich, and – more broadly – how many water-rich planets are there in the galaxy?
As our understanding of the early Solar System has grown over the last few decades, it has become clear that decay heating from short-lived radioisotopes (SLRs) has been the driving mechanism behind planetesimal heating (Grimm and Mcsween 1989; Grimm and McSween 1993). This heating results in rapid devolatilization, a removal of volatiles such as H2O within the planetesimal (Monteux et al. 2018; Lichtenberg et al. 2019). SLR decay heating also drives important processes such as thermal evolution and differentiation, but the melting, vaporization and eventual out-gassing of volatiles such as H2O are of particular interest, and drastically affect the final composition of the rocky bodies of a planetary system. Based on observational evidence from decay products found in calcium-aluminium-rich inclusions (CAIs), the distribution of SLRs was approximately isotropic throughout the early Solar System (Desch et al. 2023). SLR decay heating is a dominant driving force behind the formation of water-poor worlds. Conversely, a lack of this heating can help form water-rich ocean worlds. This desiccation would be consistent within stellar systems, while providing a key differentiating factor between planetary systems born in the same star-forming region; as SLR enrichment can potentially vary significantly between systems depending on the enrichment mechanism (Lichtenberg et al. 2016).
While there has been significant research into constraining the abundances of SLRs in the Solar System based on meteoritic samples, it remains unclear whether the amount of enrichment is typical or anomalous for stellar systems in our local galactic environment (Thrane et al. 2006; Tang and Dauphas 2012; Mishra et al. 2016; Davis 2022). Minimum threshold levels of enrichment can be established through galactic chemical evolution in the case of some SLRs (Tang and Dauphas 2012), but this cannot explain the level of enrichment in the Solar System. Stellar cosmic ray spallation can also produce SLRs, though this would result in an inhomogeneous SLR distribution in the Solar System – which is not observed (Huss et al. 2009; Trappitsch and Ciesla 2015). Disk enrichment can also potentially occur through proximal early-type stars, certain isotopes such as 26Al can be enriched through the continuous, high-mass rate outflow of a Wolf-Rayet wind, with a more rapid enrichment of other SLRs such as 60Fe occurring as the star goes supernova. However, highly proximal interaction between massive stars and disks would disrupt the disk through shocks and photoevaporation, hindering planetary formation. AGB stars can also form significant quantities of certain SLRs such as 26Al and 60Fe in their cores, and expel them in their dense, low-velocity winds. This represents a less damaging enrichment mechanism than supernovae, though these AGB stars must enter the star-forming region as “interlopers” – the likelihood of which is currently poorly constrained (Zwart 2019; Parker and Schoettler 2023; González-Santamaría et al. 2024).
Over the course of my postdoctoral project starting in 2022, I have been studying the influence of various enrichment mechanisms on star-forming regions – as well as on the effect of SLRs on planetesimal evolution – via simulation. I have found that through wind and supernova injection models there is significant variance in SLR enrichment between stellar disks in the same star-forming region, with a rare chance for a disk to be enriched to a similar degree as the Solar System (Eatson et al. 2024a). I have also found that 26Al is a more influential SLR on planetesimal evolution than 60Fe (Eatson et al. 2024b). While Solar System levels of 26Al can result in significant desiccation, 60Fe enrichment multiple orders of magnitude greater than Solar System levels are required to achieve the same effect. 60Fe may be influential outside the confines of a purely thermal devolatilization model which was used in these simulations; a more complex chemical model would benefit from an additional, longer-lasting heat source to prevent re-freezing.
The chief issue for determining where the Solar System stands in terms of its population of water-poor worlds and previous SLR enrichment are due to the difficulties of observation. In particular, determining the level of enrichment for still-forming planetary systems represents an enormous challenge, and as such no survey of any kind or size has been performed. Whilst there is interest in the population statistics of water-poor worlds in the Milky Way (and where the rocky planets of the Solar System fall into this), the sample size of potential ocean worlds is currently small (Cadieux et al. 2024), and not enough to draw any adequate conclusions. Future large-scale exoplanet surveys such as those produced by the PLATO space observatory are planned to produce a statistical ensemble of exoplanetary radii. This would allow properties such as bulk density and estimates of water fraction to be performed, and potentially estimating water-poor planet populations shortly after the conclusion of this project (Rauer et al. 2014; Lichtenberg et al. 2019).