Planetary Interiors a Key to Habitability

Interdisciplinary approaches to new data offer a robust way to see past the conventions of a specialized field, noting connections that provide perspective and deepen understanding. That idea is sound across many disciplines, but it is getting new emphasis with an essay in Science asking whether we have not been too blinkered in our approach to astrobiology. After all, reams have been written about studying exoplanet atmospheres for biomarkers, but shouldn’t we be studying how atmospheres couple to planetary interiors?

“We need a better understanding of how a planet’s composition and interior influence its habitability, starting with Earth,” says Anat Shahar (Carnegie Institution for Science), one of the paper’s four authors. “This can be used to guide the search for exoplanets and star systems where life could thrive, signatures of which could be detected by telescopes.”

Thus the paper’s call for merging data from astronomical observations, mathematical modeling and simulations, and laboratory experiments on planetary interiors. We can assume key building blocks of rocky planets like those similar to Earth, knowing to expect silicon, magnesium, hydrogen, iron, oxygen and carbon. But each planet will have its own specific abundances, its own history shaped by its position in its stellar system and its interior chemistry, all of which will help to determine whether or not it has oceans, their size, and the nature of its atmosphere.

Shahar, along with Carnegie’s Peter Driscoll, Alycia Weinberger, and George Cody, proceed to explain the significance of understanding these factors if we want to make the call on habitability, citing the range of outcomes possible from different compositions:

Composition determines the internal material properties associated with heat and mass transport, like melting temperature, thermal and electrical conductivity, viscosity, and the abundance and partitioning of radiogenic isotopes. These properties control the heat budget and thermal evolution of a planet. The amount of water accreted during formation will affect the ocean volume at the surface, which in turn is influenced by water cycling between the surface and the deep Earth. The composition and subsequent partitioning of elements in the interior will determine the oxidation state of the mantle and therefore whether the species that are outgassed to the atmosphere are enriched or reduced (11). The physical parameters of high-pressure phases of rock that might exist in deep exoplanetary mantles control their water capacity, rate of heat transfer, likelihood of global convection, and rate of core cooling.

This figure from the paper illustrates the significance of plate tectonics:

Image Credit: N. Desai/Science.

The contingent nature of planetary evolution is clear as we study what can happen to a world over billions of years in the evolution from protoplanet through differentiation of the interior, impact history and the emergence of plate tectonics and development of a magnetic field. What the authors are arguing is that coherent research on these matters is not the work of a single discipline. Indeed:

Observations of stellar, disk, and planetesimal compositions must be combined with experimental studies of mineral physics and melting behavior to serve as inputs to planet formation and geodynamic models. In turn, the results of those modeling efforts will provide feedbacks into the observations and experiments by making predictions and identifying the compositions and material properties that are most important for habitability.

So as we learn about exoplanetary atmospheres, and we are on the edge of great strides in this area with the next generation of large ground- and space-based telescopes, we’ll need to put what we learn in the context of planetary interiors and their role in evolving a life-sustaining atmosphere. The idea that habitability is hugely influenced by planetary interiors is sensible, even obvious — think of the Earth without plate tectonics — but our approach to these habitability questions will surely be enriched by crossover studies of the kind the authors describe.

After all, as opposed to straight characterization of an atmosphere, learning about the interior planetary processes needed for life will be difficult. We can make the first call based on our evaluation of planet densities, available through combined transit and radial velocity studies. But density gives us only a crude insight into planetary composition. Our best recourse, then, is the combination of modeling, experimentation, and observations that will help us learn whether planets unlike our own may still have internal processes that can support and sustain life.

The paper is Shahar et al., “What makes a planet habitable?” Science Vol. 364, Issue 6439 (03 May 2019), pp. 434-435 (full text).