In contrast to the water-poor planets of the inner Solar System, stochasticity during planetary formation1,2 and order-of-magnitude deviations in exoplanet volatile contents3 suggest that rocky worlds engulfed in thick volatile ice layers4,5 are the dominant family of terrestrial analogues6,7 among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained3, 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 use numerical models of planet formation, evolution and interior structure to show that a planet’s bulk water fraction and radius are anti-correlated with initial 26Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals8 before their accretion onto larger protoplanets and yields a system-wide correlation9,10 of planetary bulk water abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets11. Qualitatively, our models suggest two main scenarios for the formation of planetary systems: high-26Al systems, like our Solar System, form small, water-depleted planets, whereas those devoid of 26Al predominantly form ocean worlds. For planets of similar mass, the mean planetary transit radii of the ocean planet population can be up to about 10% larger than for planets from the 26Al-rich formation scenario.
This is a preview of subscription content, access via your institution
Open Access articles citing this article.
Scientific Reports Open Access 29 July 2022
Nature Astronomy Open Access 27 June 2022
Experimental Astronomy Open Access 21 May 2021
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$29.99 / 30 days
cancel any time
Subscribe to this journal
Receive 12 digital issues and online access to articles
$119.00 per year
only $9.92 per issue
Rent or buy this article
Prices vary by article type
Prices may be subject to local taxes which are calculated during checkout
Code and data availability
The data that support the plots within this paper, precompiled versions of the custom computer codes used, and other findings of this study are available from the corresponding author upon reasonable request.
Alibert, Y. & Benz, W. Formation and composition of planets around very low mass stars. Astron. Astrophys. 598, L5 (2017).
Raymond, S. N. & Izidoro, A. Origin of water in the inner Solar System: planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion. Icarus 297, 134–148 (2017).
Kaltenegger, L. How to characterize habitable worlds and signs of life. Annu. Rev. Astron. Astrophys. 55, 433–485 (2017).
Kuchner, M. J. Volatile-rich Earth-mass planets in the habitable zone. Astrophys. J. Lett. 596, L105–L108 (2003).
Léger, A. et al. A new family of planets? ‘Ocean-planets’. Icarus 169, 499–504 (2004).
Tian, F. & Ida, S. Water contents of Earth-mass planets around M dwarfs. Nat. Geosci. 8, 177–180 (2015).
Ramirez, R. M. & Levi, A. The ice cap zone: a unique habitable zone for ocean worlds. Mon. Not. R. Astron. Soc. 477, 4627–4640 (2018).
Grimm, R. E. & McSween, H. Y. Heliocentric zoning of the asteroid belt by aluminum-26 heating. Science 259, 653–655 (1993).
Millholland, S., Wang, S. & Laughlin, G. Kepler multi-planet systems exhibit unexpected intra-system uniformity in mass and radius. Astrophys. J. Lett. 849, L33 (2017).
Weiss, L. M. et al. The California-Kepler survey. V. Peas in a pod: planets in a Kepler multi-planet system are similar in size and regularly spaced. Astron. J. 155, 48–60 (2018).
Dorn, C., Mosegaard, K., Grimm, S. L. & Alibert, Y. Interior characterization in multiplanetary systems: TRAPPIST-1. Astrophys. J. 865, 20–37 (2018).
Fu, R. R. & Elkins-Tanton, L. T. The fate of magmas in planetesimals and the retention of primitive chondritic crusts. Earth. Planet. Sci. Lett. 390, 128–137 (2014).
Lichtenberg, T., Golabek, G. J., Gerya, T. V. & Meyer, M. R. The effects of short-lived radionuclides and porosity on the early thermo-mechanical evolution of planetesimals. Icarus 274, 350–365 (2016).
Monteux, J., Golabek, G. J., Rubie, D. C., Tobie, G. & Young, E. D. Water and the interior structure of terrestrial planets and icy bodies. Space Sci. Rev. 214, 39–72 (2018).
Benz, W., Ida, S., Alibert, Y., Lin, D. & Mordasini, C. in Protostars and Planets VI (eds. Beuther, H. et al.) 691–713 (Univ. Arizona Press, Tucson, 2014).
Ansdell, M. et al. ALMA survey of Lupus protoplanetary disks. I. Dust and gas masses. Astrophys. J. 828, 46–61 (2016).
Johansen, A., Mac Low, M.-M., Lacerda, P. & Bizzarro, M. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion. Sci. Adv. 1, e1500109 (2015).
Schiller, M., Bizzarro, M. & Fernandes, V. A. Isotopic evolution of the protoplanetary disk and the building blocks of Earth and the Moon. Nature 555, 507–510 (2018).
Alibert, Y. et al. The formation of Jupiter by hybrid pebble-planetesimal accretion. Nat. Astron. 2, 2397–3366 (2018).
Lichtenberg, T., Parker, R. J. & Meyer, M. R. Isotopic enrichment of forming planetary systems from supernova pollution. Mon. Not. R. Astron. Soc. 462, 3979–3992 (2016).
Lugaro, M., Ott, U. & Kereszturi, Á. Radioactive nuclei from cosmochronology to habitability. Prog. Part. Nucl. Phys. 102, 1–47 (2018).
Delbo, M., Walsh, K., Bolin, B., Avdellidou, C. & Morbidelli, A. Identification of a primordial asteroid family constrains the original planetesimal population. Science 357, 1026–1029 (2017).
Kuffmeier, M., Frostholm Mogensen, T., Haugbølle, T., Bizzarro, M. & Nordlund, Å. Tracking the distribution of 26Al and 60Fe during the early phases of star and disk evolution. Astrophys. J. 826, 22–47 (2016).
Noack, L., Snellen, I. & Rauer, H. Water in extrasolar planets and implications for habitability. Space Sci. Rev. 212, 877–898 (2017).
Unterborn, C. T., Desch, S. J., Hinkel, N. R. & Lorenzo, A. Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions. Nat. Astron. 2, 297–302 (2018).
Alibert, Y. On the radius of habitable planets. Astron. Astrophys. 561, A41 (2014).
Rauer, H. et al. The PLATO 2.0 mission. Exp. Astron. 38, 249–330 (2014).
Marcus, R. A., Sasselov, D., Stewart, S. T. & Hernquist, L. Water/icy super-Earths: giant impacts and maximum water content. Astrophys. J. Lett. 719, L45–L49 (2010).
Inamdar, N. K. & Schlichting, H. E. Stealing the gas: giant impacts and the large diversity in exoplanet densities. Astrophys. J. Lett. 817, L13 (2016).
Grimm, S. L. et al. The nature of the TRAPPIST-1 exoplanets. Astron. Astrophys. 613, A68 (2018).
de Wit, J. et al. Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1. Nat. Astron. 2, 214–219 (2018).
Bourrier, V. et al. Temporal evolution of the high-energy irradiation and water content of TRAPPIST-1 exoplanets. Astron. J. 154, 121–138 (2017).
Ciesla, F. J., Mulders, G. D., Pascucci, I. & Apai, D. Volatile delivery to planets from water-rich planetesimals around low mass stars. Astrophys. J. 804, 9–20 (2015).
Ormel, C. W., Liu, B. & Schoonenberg, D. Formation of TRAPPIST-1 and other compact systems. Astron. Astrophys. 604, A1 (2017).
Drażkowska, J. & Dullemond, C. P. Planetesimal formation during protoplanetary disk buildup. Astron. Astrophys. 614, A62 (2018).
Lichtenberg, T. et al. Impact splash chondrule formation during planetesimal recycling. Icarus 302, 27–43 (2018).
Ikoma, M., Elkins-Tanton, L., Hamano, K. & Suckale, J. Water partitioning in planetary embryos and protoplanets with magma oceans. Space Sci. Rev. 214, 76–104 (2018).
Kite, E. S. & Ford, E. B. Habitability of exoplanet waterworlds. Astrophys. J. 864, 75–102 (2018).
Gerya, T. V. & Yuen, D. A. Robust characteristics method for modelling multiphase visco-elasto-plastic thermo-mechanical problems. Phys. Earth Planet. Inter. 163, 83–105 (2007).
Golabek, G. J., Bourdon, B. & Gerya, T. V. Numerical models of the thermomechanical evolution of planetesimals: application to the acapulcoite–lodranite parent body. Meteorit. Planet. Sci. 49, 1083–1099 (2014).
Crameri, F. et al. A comparison of numerical surface topography calculations in geodynamic modelling: an evaluation of the ‘sticky air’ method. Geophys. J. Int. 189, 38–54 (2012).
Lodders, K. Solar System abundances and condensation temperatures of the elements. Astrophys. J. 591, 1220–1247 (2003).
Castillo-Rogez, J. et al. 26Al decay: heat production and a revised age for Iapetus. Icarus 204, 658–662 (2009).
Nicholson, R. B. & Parker, R. J. Supernova enrichment of planetary systems in lowmass star clusters. Mon. Not. R. Astron. Soc. 464, 4318–4324 (2017).
Costa, A., Caricchi, L. & Bagdassarov, N. A model for the rheology of particle-bearing suspensions and partially molten rocks. Geochem. Geophys. Geosys. 10, Q03010 (2009).
Siggia, E. D. High Rayleigh number convection. Annu. Rev. Fluid. Mech. 26, 137–168 (1994).
O’Brien, D. P., Izidoro, A., Jacobson, S. A., Raymond, S. N. & Rubie, D. C. The delivery of water during terrestrial planet formation. Space Sci. Rev. 214, 47–71 (2018).
Castillo-Rogez, J. & Young, E. D. in Planetesimals: Early Differentiation and Consequences for Planets (eds Elkins-Tanton, L. T. & Weiss, B. P.) 92–114 (Cambridge Univ. Press, Cambridge, 2017).
Fu, R. R., Young, E. D., Greenwood, R. C. & Elkins-Tanton, L. T. in Planetesimals: Early Differentiation and Consequences for Planets (eds Elkins-Tanton, L. T. & Weiss, B. P.) 115–135 (Cambridge Univ. Press, Cambridge, 2017).
Machida, R. & Abe, Y. Terrestrial planet formation through accretion of sublimating icy planetesimals in a cold nebula. Astrophys. J. 716, 1252–1262 (2010).
Larsen, K. K. et al. Evidence for magnesium isotope heterogeneity in the solar protoplanetary disk. Astrophys. J. Lett. 735, L37 (2011).
Schiller, M., Connelly, J. N., Glad, A. C., Mikouchi, T. & Bizzarro, M. Early accretion of protoplanets inferred from a reduced inner Solar System 26Al inventory. Earth. Planet. Sci. Lett. 420, 45–54 (2015).
Alibert, Y., Mordasini, C., Benz, W. & Winisdoerffer, C. Models of giant planet formation with migration and disc evolution. Astron. Astrophys. 434, 343–353 (2005).
Mordasini, C., Alibert, Y. & Benz, W. Extrasolar planet population synthesis. I. Method, formation tracks, and mass–distance distribution. Astron. Astrophys. 501, 1139–1160 (2009).
Mordasini, C., Alibert, Y., Benz, W. & Naef, D. Extrasolar planet population synthesis. II. Statistical comparison with observations. Astron. Astrophys. 501, 1161–1184 (2009).
Mordasini, C., Mollière, P., Dittkrist, K.-M., Jin, S. & Alibert, Y. Global models of planet formation and evolution. Int. J. Astrobiol. 14, 201–232 (2015).
Shakura, N. I. & Sunyaev, R. A. Black holes in binary systems. Observational appearance. Astron. Astrophys. 24, 337–355 (1973).
Clarke, C. J., Gendrin, A. & Sotomayor, M. The dispersal of circumstellar discs: the role of the ultraviolet switch. Mon. Not. R. Astron. Soc. 328, 485–491 (2001).
Matsuyama, I., Johnstone, D. & Hartmann, L. Viscous diffusion and photoevaporation of stellar disks. Astrophys. J. 582, 893–904 (2003).
Sasselov, D. D. & Lecar, M. On the snow line in dusty protoplanetary disks. Astrophys. J. 528, 995–998 (2000).
Weidenschilling, S. J. Aerodynamics of solid bodies in the solar nebula. Mon. Not. R. Astron. Soc. 180, 57–70 (1977).
Dittkrist, K.-M., Mordasini, C., Klahr, H., Alibert, Y. & Henning, T. Impacts of planet migration models on planetary populations. Effects of saturation, cooling and stellar irradiation. Astron. Astrophys. 567, A121 (2014).
Inaba, S., Tanaka, H., Nakazawa, K., Wetherill, G. W. & Kokubo, E. High-accuracy statistical simulation of planetary accretion: II. Comparison with N-body simulation. Icarus 149, 235–250 (2001).
Inaba, S. & Ikoma, M. Enhanced collisional growth of a protoplanet that has an atmosphere. Astron. Astrophys. 410, 711–723 (2003).
Fortier, A., Alibert, Y., Carron, F., Benz, W. & Dittkrist, K.-M. Planet formation models: the interplay with the planetesimal disc. Astron. Astrophys. 549, A44 (2013).
Genda, H. & Abe, Y. Survival of a proto-atmosphere through the stage of giant impacts: the mechanical aspects. Icarus 164, 149–162 (2003).
Schlichting, H. E., Sari, R. & Yalinewich, A. Atmospheric mass loss during planet formation: the importance of planetesimal impacts. Icarus 247, 81–94 (2015).
Burger, C., Maindl, T. I. & Schäfer, C. M. Transfer, loss and physical processing of water in hit-and-run collisions of planetary embryos. Celest. Mech. Dyn. Astron. 130, 2–32 (2018).
Bollard, J. et al. Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules. Sci. Adv. 3, e1700407 (2017).
Connelly, J. N. & Bizzarro, M. Lead isotope evidence for a young formation age of the Earth–Moon system. Earth. Planet. Sci. Lett. 452, 36–43 (2016).
Pollack, J. B. et al. Formation of the giant planets by concurrent accretion of solids and gas. Icarus 124, 62–85 (1996).
Alibert, Y. et al. Theoretical models of planetary system formation: mass vs. semi-major axis. Astron. Astrophys. 558, A109 (2013).
Demircan, O. & Kahraman, G. Stellar mass–luminosity and mass–radius relations. Astrophys. Space. Sci. 181, 313–322 (1991).
Alibert, Y., Mordasini, C. & Benz, W. Extrasolar planet population synthesis. III. Formation of planets around stars of different masses. Astron. Astrophys. 526, A63 (2011).
Kennedy, G. M. & Kenyon, S. J. Stellar mass dependent disk dispersal. Astrophys. J. 695, 1210–1226 (2009).
Mordasini, C. et al. Characterization of exoplanets from their formation. II. The planetary mass–radius relationship. Astron. Astrophys. 547, A112 (2012).
Bodenheimer, P. & Pollack, J. B. Calculations of the accretion and evolution of giant planets: the effects of solid cores. Icarus 67, 391–408 (1986).
Saumon, D., Chabrier, G. & van Horn, H. M. An equation of state for low-mass stars and giant planets. Astrophys. J. Suppl. 99, 713–741 (1995).
Seager, S., Kuchner, M., Hier-Majumder, C. A. & Militzer, B. Mass–radius relationships for solid exoplanets. Astrophys. J. 669, 1279–1297 (2007).
Guillot, T. On the radiative equilibrium of irradiated planetary atmospheres. Astron. Astrophys. 520, A27 (2010).
Jin, S. et al. Planetary population synthesis coupled with atmospheric escape: a statistical view of evaporation. Astrophys. J. 795, 65–87 (2014).
Jin, S. & Mordasini, C. Compositional imprints in density–distance–time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation. Astrophys. J. 853, 163–186 (2018).
Adams, F. C., Fatuzzo, M. & Holden, L. Distributions of short-lived radioactive nuclei produced by young embedded star clusters. Astrophys. J. 789, 86–104 (2014).
Gounelle, M. The abundance of 26Al-rich planetary systems in the Galaxy. Astron. Astrophys. 582, A26 (2015).
Pfalzner, S. et al. The formation of the Solar System. Phys. Scr. 90, 068001 (2015).
Parker, R. J., Lichtenberg, T. & Quanz, S. P. Was Planet 9 captured in the Sun’s natal star-forming region? Mon. Not. R. Astron. Soc. 472, L75–L79 (2017).
Dwarkadas, V. V., Dauphas, N., Meyer, B., Boyajian, P. & Bojazi, M. Triggered star formation inside the shell of a Wolf–Rayet bubble as the origin of the Solar System. Astrophys. J. 851, 147–161 (2017).
Kita, N. T. et al. 26Al–26Mg isotope systematics of the first solids in the early Solar System. Meteorit. Planet. Sci. 48, 1383–1400 (2013).
Klahr, H. & Schreiber, A. Linking the origin of asteroids to planetesimal formation in the solar nebula. Proc. IAU 10 (S318), https://doi.org/10.1017/S1743921315010406 (2016).
Simon, J. B., Armitage, P. J., Youdin, A. N. & Li, R. Evidence for universality in the initial planetesimal mass function. Astrophys. J. Lett. 847, L12 (2017).
Tsirvoulis, G., Morbidelli, A., Delbo, M. & Tsiganis, K. Reconstructing the size distribution of the primordial Main Belt. Icarus 304, 14–23 (2018).
Meng, H. Y. A., Rieke, G. H., Su, K. Y. L. & Gáspár, A. The first 40 million years of circumstellar disk evolution: the signature of terrestrial planet formation. Astrophys. J. 836, 34–53 (2017).
Kral, Q., Clarke, C. & Wyatt, M. in Handbook of Exoplanets (eds Deeg, H. J. & Belmonte, J. A.) https://doi.org/10.1007/978-3-319-30648-3_165-1 (Springer Living Reference, Springer, Cham, 2017).
Andrews, S. M., Wilner, D. J., Hughes, A. M., Qi, C. & Dullemond, C. P. Protoplanetary disk structures in Ophiuchus II. Extension to fainter sources. Astrophys. J. 723, 1241–1254 (2010).
Hunter, J. D. Matplotlib: a 2D graphics environment. Comput. Sci. Eng. 9, 90–95 (2007).
Jones, E. et al. SciPy: open source scientific tools for Python. http://www.scipy.org (2001).
Van Der Walt, S., Colbert, S. C. & Varoquaux, G. The NumPy array: a structure for efficient numerical computation. Preprint at https://arXiv/abs/1102.1523 (2011).
McKinney, W. Data structures for statistical computing in Python. In Proc. 9th Python in Science Conference (eds van der Walt, S. & Millman, J.) 51–56 (SciPy, 2010).
Waskom, M. et al. Seaborn v0.9.0. https://doi.org/10.5281/zenodo.1313201 (2018).
The authors thank R. Parker for helping to initiate this project, E. Kite, T. Roger and C. Dorn for comments and discussions, and M. Bizzarro for comments that helped to improve the manuscript. T.L. was supported by ETH Zürich Research Grant ETH-17 13-1 and acknowledges partial financial support from the Swiss Society for Astrophysics and Astronomy through a MERAC travel grant. Y.A. acknowledges support from the Swiss National Science Foundation (SNSF). C.M. acknowledges support from the SNSF under grant BSSGI0_155816 ‘PlanetsInTime’. The numerical simulations in this work were partially performed on the EULER computing cluster of ETH Zürich. Parts of this work have been carried out within the framework of the National Center of Competence in Research PlanetS supported by the SNSF.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Lichtenberg, T., Golabek, G.J., Burn, R. et al. A water budget dichotomy of rocky protoplanets from 26Al-heating. Nat Astron 3, 307–313 (2019). https://doi.org/10.1038/s41550-018-0688-5
This article is cited by
Nature Astronomy (2022)
Scientific Reports (2022)
Nature Astronomy (2022)
Experimental Astronomy (2022)