A planet’s habitability is determined by a confluence of many factors. So far, our explorations of potentially habitable worlds beyond our solar system have focused exclusively on their position in the “Goldilocks Zone” of their solar system, where their temperature determines whether or not liquid water can exist on their surface, and, more recently, what their atmospheres are composed of. That’s in part due to the technical limitations of the instruments available to us - even the powerful James Webb Space Telescope is capable only of seeing atmospheres of very large planets nearby. But in the coming decades, we’ll get new tools, like the Habitable Worlds Observatory, that are more specifically tailored to search for those potentially habitable worlds. So what should we use them to look for? A new paper available in pre-print on arXiv by Benjamin Farcy of the University of Maryland and his colleagues, argues that we should look to how a planet formed to understand its chances of harboring life.

To be clear, HWO won’t be able to see into the past - at least not anymore than would be allowed by how far the light from these worlds has to travel to get to us. However, it can glean insights into how the planet was formed based on current measurable values. Dr. Farcy and his co-authors describe four different aspects of a planet that are determined early in its formation that have a major impact on its ability to harbor complex life down the road.

The first major theme is the bulk composition - mainly of the four major elements that make up 93% of terrestrial planets. These are magnesium, iron, silicon, and oxygen. Ultimately, the ratio of these elements determine whether or not the planet has plate tectonics, which are necessary to maintain a relatively stable environment over millions of years. Conveniently, it’s also possible to determine the ratios of these elements in a planet by looking at the ratios in the planet’s host star - they should be equivalent since they were both formed out of the same available matter.