Metals are critical to life. We should screen exoplanets for them

Life is complicated, and not just in a philosophical sense. But one simple thing we know about life is that it requires energy, and to get that energy it needs certain fundamental elements. A new paper published in The Open Journal of Astrophysics by Giovanni Covone and Donato Giovannelli from the University of Naples discusses how we might use that constraint to narrow our search for stars and planets that could potentially harbor life. To put it simply, if it doesn’t have many of the constituent parts of the “building blocks” of life, then life probably doesn’t exist there.

So how does one go from needing energy to needing elements? Life gets much of its energy from a physical phenomenon called a “thermodynamic disequilibria”—basically a fancy way of saying that a “system” in nature has some potential energy, whether that’s thermal, mechanical, chemical, or radiative. One of the most common ways for life to take advantage of a disequilibria is through a process called a reduction-oxidation (redox) reaction.

Redox reactions are common in chemistry, and usually involve the transfer of an electron, which itself involves a release of energy. That energy is what life uses to power itself, and to facilitate these types of reactions, it uses proteins called oxidoreductases. Each of these proteins requires at least one metal as part of their chemical structure.

To clarify, these are metals in the chemistry sense, not the astronomical one, which classifies any element higher on the periodic table than hydrogen as a “metal.” For example, nickel and iron are key components of proteins that take electrons from hydrogen, whereas copper is a key component of proteins that redox oxygen.

Archaeologists have noticed that the availability of these metals has affected the course of life on Earth. Their availability changes based on events like plate tectonics, volcanism like the Deccan Traps, or the “Great Oxidation Event” of 2.3 billion years ago, when cyanobacteria released so much oxygen into Earth’s atmosphere that it dramatically changed the planet’s biosphere. That change included a massive extinction event, but also gave life the ability to develop aerobic respiration, eventually paving the way for the development of animals.

Given the known impact of the availability of these elements on the evolution of life, Drs. Covone and Giovannelli put forward a reasonable argument—if they are so important, why don’t we check stars and planets to see if they have these elements in abundance as a way of pre-screening them for astrobiological investigation.

There are thousands of exoplanets that could make interesting targets for those investigations, and likely millions more that we’ll uncover as we continue our survey of the galaxy. Sorting and prioritizing them becomes increasingly important as humanity is limited in the number of observatories that are capable of checking for concrete biosignatures.

Typically that screening process is done by looking at the availability of three things: free energy, liquid water, and CHNOPS (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur) elements. However, the authors argue that those are all relatively abundant in the galaxy, and the constraints on higher atomic number metals, like those found in the proteins used for redox reactions, are actually much more of a constraint than the three typical search parameters.

By further constraining their search to stars and planets that have an abundance of these critical materials, they could save scarce observational resources for targets that have a higher likelihood of actually harboring life.

Luckily, missions like ESA’s upcoming PLATO observatory will already be checking the spectroscopies of exoplanets for CHNOPS, and doing so for the biometals discussed in the paper would be collected in the same dataset. All the scientists would have to do is add a further screening category to any systems marked for a follow-up.

However, that is getting into a complex realm, as we have reported on other papers that show a higher “metallicity” star tends to have less UV radiation, causing less development of critical ozone layers. There are many complex factors that go into the search for life, and continually looking for them is the best way to keep refining them. This paper adds one more, particularly unique, consideration to the mix.