To date, astronomers have identified more than 4,000 confirmed exoplanets — planets orbiting stars other than our Sun — but only a fraction have the potential to sustain life. Now, new research published in the Astrophysical Journal Letters from an international collaboration of scientists, is using the geology of early planet formation to help identify those that may be capable of supporting life.
We typically hope to find these planets in the so-called ‘goldilocks’ – or habitable zone, where they are the right distance from their stars to support liquid water on their surfaces. However, while this is a great way to sort through the thousands of candidate planets, it’s not quite enough to say whether the planet is truly habitable. “Just because a rocky planet can have liquid water, that doesn’t mean it does, says lead author Dr. Brendan Dyck, Assistant Professor at the University of British Columbia.
“For a planet to remain hospitable for life, it must retain significant surface water for very long periods of time; around 4 billion years on Earth,” says Dr. Jon Wade, Associate Professor at Oxford, and co-author on the study. “Unfortunately for Mars, its surface water appears to have vanished early in its history, proving catastrophic for any Martian life. So why has Earth, unlike Mars, kept its oceans? Our new study shows that the amount of iron contained in the rocky part of the planet – the mantle – is of critical importance, and is a legacy of the processes that occurred when the planet was initially accreted.”
Within any given planetary system, the smaller rocky planets all have one thing in common — they all have the same proportion of iron as the star they orbit. What differentiates them is how much of that iron is contained in the mantle versus the core. This is a feature of a planet called the core-mass fraction, or CMF (Fig. 1), that can be estimated from various observational data.
Figure 1. Histogram showing the diversity of core-mass fraction (CMF) for extrasolar planets, with information for rocky bodies in our solar system shown for reference. For data sources, please see Dyck et al. (2021) and references therein.
“The chemical makeup of a planet’s interior imparts a fundamental control on the minerals that form at depth below the surface,” says Dr. Richard Palin, Associate Professor at Oxford, and co-author on the study. “Different exoplanets with different mantle mineral assemblages will each have a different capacity to melt at any given pressure and temperature conditions. It is this melting process that leads to formation of a crust at the surface, just like we can observe on Earth today at mid-ocean ridges or above deep-seated mantle plumes. Our understanding of the thermodynamic properties of minerals means that we can reliably model the changes that would occur in an exoplanet’s crust and mantle over their lifetimes.”
“The findings from our study show that as planets form, those with a larger core will form thinner crusts, whereas those with smaller cores form thicker iron-rich crusts like Mars,” says Dr. Dyck. “This divergence is controlled by the oxidation state at the time of planetary accretion,” adds Dr. Wade. “So, if a planet forms from relatively oxidized material, it forms a small core and thick crust, but the opposite is true for a planet forming from relatively reduced material.” The thickness of the planetary crust will then dictate whether the planet can support plate tectonics and how much water and atmosphere may be present – key ingredients for life as we know it.
Figure 2. Summary diagram showing how the oxidation state during planetary accretion and differentiation controls the core size and thus future thickness and composition of surface crusts (Dyck et al., 2021).
Later this year, in a joint project with NASA, the Canadian Space Agency and the European Space Agency, the James Webb Space Telescope (JWST) will launch. The research team describes this as the golden opportunity to put his findings to good use. “One of the goals of the JWST is to investigate the chemical properties of extra-solar planetary systems,” says Dr. Dyck. “It will be able to measure the amount of iron present in these alien worlds and give us a good idea of what their surfaces may look like and may even offer a hint as to whether they’re home to life.”
“We’re on the brink of making huge strides in better understanding the countless planets around us and in discovering how unique the Earth may or may not be. It may still be some time before we know whether any of these strange new worlds contain new life or even new civilizations, but it’s an exciting time to be part of that exploration.”
Article adapted from the University of British Colombia Press Release by Nathan Skolski. You can read the full version here.
Featured image: Photo courtesy of NASA/Goddard Space Flight Center.