Life may be easier to find on planets that correspond to a previous Earth

We are getting closer to reliably detecting biosignatures on distant planets. Much of the focus is on determining which chemicals indicate the presence of life.

But life can also create free energy in a system, and excess energy can create chemical imbalance. This is what happened on Earth when life began. Could chemical imbalance be a biosignature?

When a system has excess energy, this manifests as a chemical equilibrium. Each planet is a system and life can generate excess energy. So, if we detect chemical equilibrium, are we detecting a biosignature? Are we detecting life? Perhaps. Especially if we are looking at an exoplanet very similar to early Earth.

New research addresses this question. The title of the study is “Inferring chemical disequilibrium biosignatures for Proterozoic Earth-like exoplanets.” The lead author is Amber Young of the Department of Astronomy and Planetary Sciences at Northern Arizona University. The paper can be found on the preprint server arXiv.

“When trying to infer whether a distant world is inhabited, chemical imbalance is a potential indicator of life that has a long history of study in planetary environments in the solar system,” the authors write in their paper.

When methane (CH₄) and oxygen (O_two) are present in an atmosphere, it is an indication that life is in action. This is because, in an oxygen environment, methane only lasts about 10 years. Its presence indicates imbalance. To be present, it has to be continually replenished in quantities that only life can produce.

The Gibbs free energy concept attempts to capture this idea. When a system reaches chemical equilibrium, the thermodynamic potential is minimized. The further from chemical equilibrium a system is, the more Gibbs free energy there is.

“A primary metric for quantifying chemical imbalance involves calculating the difference in chemical energy associated with an observed system and that system’s theoretical equilibrium state,” the authors explain. Researchers are exploring how they can use Gibbs free energy to understand the worlds in our solar system. Most importantly, researchers are working to understand how this can be applied to Earth’s history.

This research focuses on the Proterozoic Eon, the third of Earth’s four eons. It ranged between 2.5 billion years ago and 541 million years ago and covers two critical events in Earth’s history. Free oxygen appears in Earth’s atmosphere at the beginning of the Proterozoic, and the Proterozoic ends shortly before the appearance of complex life.

The obstacle to using the Gibbs metric as a biosignature is that we don’t know what the observational uncertainties are when we try to understand it on Earth-like exoplanets. In this research, Earth-like means “an Earth-sized, oceanic world with Earth-like surface pressures and temperatures and an atmosphere dominated by N_twoH_twoO and CO_two with traces of CH₄ and varying levels of O_two“, explain the authors.

Scientists understand a lot about Earth during the Proterozoic eon, although, of course, there are many unanswered questions. In their effort to understand some of the observational uncertainties, the researchers modeled two different scenarios for Earth and one for Mars.

Each scenario contains a different amount of free atmospheric energy. They then explored how the atmospheres of each of these simulated planets would reflect light in different scenarios: high-, medium-, and low-biosignature gases in the atmospheres.

The result was light spectra that we can observe in the atmospheres of exoplanets that mimic three different cases of the Proterozoic Earth.

“Constraining the available Gibbs free energy is a promising characterization strategy that has good synergy with established techniques for biosignature gas detection,” the authors conclude. But to realize this potential, we need better telescopes with better signal-to-noise (SNR) performance. And those, hopefully, are on the way.

“For a 6 m class space telescope with noise properties modeled on the LUVOIR-B concept, the high SNR cases explored here could be achieved for an Earth-like target around a solar host at distances of 5-7 pc (16 to 23 light years) with an investment of two to four weeks of observation time”, explain the authors.

Although this may seem like a long observation time, it is in line with the expected target observation times with the HabEx telescope concept. And a potential Proterozoic Earth-like exoplanet is a high-value target, worthy of so much dedicated observation time. Is there anything else that space telescopes should prioritize? It is unlikely.

“From an observational perspective, characterizing CH₄ and the_two abundances is essential for inferring the signal of atmospheric chemical imbalance from Earth analogs throughout most of its evolutionary history,” the authors write.

While we tend to normalize everything we see around us, the current state of the Earth is hardly “normal.” Earth was very different for most of its history. It makes sense to look for planets similar to the way Earth was during the Proterozoic.

The Proterozoic lasted two billion years, and life actively shaped its atmosphere throughout the period. If we are lucky enough to discover another life-supporting exoplanet, then, by pure chance, it is likely to look more like Proterozoic Earth than modern Earth.