The last few years has seen an explosion of exoplanet discoveries. Some of those worlds are in what we deem the “habitable zone,” at least in preliminary observations. But how many of them will have life-supporting, oxygen-rich atmospheres in the same vein as Earth’s?

A new study suggests that breathable atmospheres might not be as rare as we thought on planets as old as Earth.

Earth took a long time to develop the oxygenated atmosphere that we enjoy now. Until about 2.4 billion years ago, our planet had much less oxygen in its atmosphere and oceans. That all changed when a major oxygenation event took place; the first of three that shaped the Earth.

The three-step model of Earth’s oxygenation is pretty widely understood and accepted, though it’s not without controversy. The model outlines three major shifts in Earth’s history, with each one substantially altering the Earth’s atmosphere by adding more oxygen.

The three events were:

The Great Oxidation Event occurred about 2.4 billion years ago during the Paleoproterozoic Era. In this event, biologically produced oxygen accumulated in the oceans and atmosphere, likely leading to an initial mass extinction.

The Neoproterozoic Oxygenation Event saw a dramatic rise in oxygen levels, and preceded the Cambrian Explosion about 540 million years ago.

The Paleozoic Oxygenation Event happened about 400 million years ago and saw oxygen reach its current level of about 21%.

The history of Earth’s oxygenation is complicated. It wasn’t a linear progression. At first, oxygen was produced as a waste by-product by life forms, and much of it was absorbed by the Earth’s crust. Oxygen is highly reactive and it formed all sorts of compounds with other elements and became locked in the crust. In particular, it reacted with iron to produce iron oxide in the geological record, one of our best indicators of when oxygen entered the atmosphere.

A banded iron formation in Australia. The conventional understanding is that these formed in seawater with the release of oxygen during the Great Oxygenation Event. Image Credit: By Graeme Churchard from Bristol, UK – Dales GorgeUploaded by PDTillman, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=30889569

There’s a lot of debate around this model though. According to one understanding of the model, photosynthetic bacteria in the ocean produced much of the early oxygen. Then land-based planets came along hundreds of millions of years later, raising the oxygen level again. There’s also evidence that plate tectonics and massive volcanic eruptions played a role.

An article by the authors of this new study says this model implies that a certain level of luck is required to create an oxygen-rich world. “If one volcanic eruption hadn’t happened, or a certain type of organism hadn’t evolved, then oxygen might have stalled at low levels,” it says.

But maybe that’s not the case.

Their new study is titled “Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling” and the word “inherent” is key here. The authors say that once we had the right microbes and plate tectonics, which were both established 3 billion years ago, it was only a matter of time before we reached the oxygen level we have now. Regardless of volcanoes and land-based plants.

“This research really tests our understanding of how the Earth became oxygen rich, and thus able to support intelligent life.“ Lewis Alcott, Lead Author, Earth Surface Science Institute, Leeds University.

Rather than external forces, it was “a set of internal feedbacks involving the global phosphorus, carbon, and oxygen cycles” that led to the Earth’s oxygenation, as the study says. In fact, those cycles would have “produced the same three-step pattern observed in the geological record.”

Cyanobacteria under a microscope. They are credited with fuelling the Great Oxygenation Event. Image Credit: NASA, http://microbes.arc.nasa.gov/images/content/gallery/lightms/publication/unicells.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5084332

It all comes down to this, from the paper: “We conclude that Earth’s oxygenation events are entirely consistent with gradual oxygenation of the planetary surface after the evolution of oxygenic photosynthesis.”

But how did they arrive at that conclusion?

The researchers are from Leeds University in the UK. The lead author is Lewis J. Alcott, a PhD student based in the Earth Surface Science Institute. Alcott and the other researchers worked with a well-established model of marine biogeochemistry and modified it. They ran that model across all of Earth’s history, and found that it produced the three main oxygenation events all by itself.

In a press release Alcott said, “This research really tests our understanding of how the Earth became oxygen-rich, and thus able to support intelligent life.”

The dominant thinking behind the Earth’s history of oxygenation relies on a couple broad categories of events to explain it. One is major evolutionary developments in life-forms that produce oxygen. Basically “biological revolutions,” where lifeforms became progressively more complex, and engineered an oxygen-rich environment. The second category is tectonic revolutions: a dramatic and particular increase in tectonic activity, including significant volcanic activity, that altered the crust and led to greater oxygen levels.

Image taken by a crew member of Expedition 13 from the ISS, showing the eruption of Cleveland Volcano, Aleutian Islands, Alaska. According to accepted thinking, significant volcanic events were necessary to produce the oxygen-rich atmosphere Earth has today. But a new study suggests they weren’t necessary. Credit: NASA

There’s been a lot of debate around the exact nature of both of those broad categories, but this new study is giving scientists something more to think about. Rather than relying on “step-wise” events that can be pinpointed in the geological record to explain oxygenation, the new study points to feedback cycles between phosphorous, carbon, and oxygen.

The study also suggests that oxygenation was inevitable.

Study co-author Professor Simon Poulton, also from the School of Earth and Environment at Leeds, said: “Our model suggests that oxygenation of the Earth to a level that can sustain complex life was inevitable, once the microbes that produce oxygen had evolved.”

At the heart of this new model is the marine phosphorous cycle. Their model produced the same three step oxygenation pattern the Earth experienced “when driven solely by a gradual shift from reducing to oxidising surface conditions over time. The transitions are driven by the way the marine phosphorus cycle responds to changing oxygen levels, and how this impacts photosynthesis, which requires phosphorus.”

A simple graphic of the Earth’s Phosphorus Cycle. Image Credit: Biogeochemical cycles: Figure 5 by OpenStax College, Concepts of Biology, CC BY 4.0; modification of work by John M. Evans and Howard Perlman, USGS

“Our work shows that the relationship between the global phosphorus, carbon and oxygen cycles is fundamental to understanding the oxygenation history of the Earth. This could help us to better understand how a planet other than our own may become habitable,” said senior author Dr. Benjamin Mills.

So there’s hope for some of those exoplanets yet.

This study won’t be the final word on the matter. But it’s an intriguing result, and if it stands up to further scientific scrutiny, it may well impact how we characterize the exoplanets we’ve found already, and the thousands more we’ll find with TESS and other future planet-finding telescopes.

More: