Better to dry a rocky planet before use

12 February 2019

Better to dry a rocky planet before use

Dr Tim Lichtenberg (@tim_lichtenberg, a postdoctoral fellow working in AOPP) and colleagues from ETH Zurich, Universities of Bayreuth, Bern, and Michigan have published a new paper describing a new theoretical model of how the water abundances of planets similar to the Earth may be altered during the formation process.

If a planetary system, such as the solar system, is born in a massive star-forming region, the planetary building blocks are enriched in the powerful short-lived radionuclide Al-26, which dries them out before they can accrete onto the planet, and so may enable clement surface conditions like on Earth. If, on the other hand, no Al-26 is present when the system forms, ocean worlds very much unlike the solar system terrestrial planets will form.

Aluminium-26-Tim-Lichtenberg_Final LR(1)_0.jpg

[Illustration by Thibaut Roger]

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 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 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: high26Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of 26Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%.

If you’d like to find out more, you can read the paper here
More information in this blog post
The original press release by NCCR PlanetS can be found here.