With the number of potentially habitable exoplanets in our galaxy alone estimated to be in the billions, many wonder why we are yet to see signs or hear from intelligent alien life. A pair of astrobiologists from the Australian National University (ANU) Research School of Earth Sciences hypothesize the reason may be that ET could be long dead. According to Aditya Chopra and Charley Lineweaver, conditions on young planets are so volatile that if life doesn't evolve fast enough to stabilize the environment, it will quickly become extinct.

One of the biggest wet blankets in the search for intelligent life is the Fermi Paradox. According to the most reliable accounts, in 1950 physicist Enrico Fermi was having lunch at the Los Alamos National Laboratory with Emil Konopinski, Edward Teller, and Herbert York. During the meal, Fermi asked why is it that in a Universe supposedly teeming with life, we haven't encountered any other civilizations yet. Making some back of an envelope computations, Fermi concluded that even the laziest intelligent species would spread throughout our galaxy in only 50 million years. So, he asked, where are they?

There have been a number of responses to the Fermi Paradox, such as the Rare Earth Hypothesis, which states that the conditions on Earth where life arose are so complicated that they're very unlikely to be duplicated. Another is the Hansen Filter, which speculates that life may be very common in the Universe, but something prevents it from advancing far enough to achieve interstellar contact. This can include asteroid strikes, geological catastrophes, supernovae, gamma ray bursts, nuclear war, environmental mismanagement, or social upheavals such as the Shoe Event Horizon.

Aditya Chopra from the ANU Planetary Science Institute(Credit: Stuart Hay/ANU)

The question that Chopra and Lineweaver posed was, does this filter exist before or after the point where intelligent life develops? They have posited what they call a "Gaian Filter" or Bottleneck, which is based on the fact that the emergence of life on a young, rocky planet requires more than just adequate warmth and liquid water. It also requires, among other things, a certain degree of stability – stability that the life itself helps to establish.

Basically, what the astrobiologists are arguing is that life isn't just a passenger on the planet it inhabits, but that it interacts with its environment and alters it. On Earth, the early environment was extremely hostile to more advanced forms, but as simple organisms developed, they stabilized the environment by doing things like changing the carbon dioxide in the environment into oxygen while removing the methane. Eventually, Earth's biosphere created a dynamic system that regulates the environment and keeps it capable of sustaining life despite asteroids, super volcanoes, and major shifts in climate. This is particularly true in terms of greenhouse gases.

"Most early planetary environments are unstable," says Chopra. "To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable."

Associate Professor Charley Lineweaver from the ANU Planetary Science Institute(Credit: Stuart Hay/ANU)

Chopra and Lineweaver point out that four billion years ago Venus, Earth, and Mars were very similar, yet today Venus is a hellhole of intense pressure, temperatures high enough to melt lead, and sulfuric acid rains, while Mars is a desiccated ball of sand with scarcely any atmosphere that is constantly bombarded with hard radiation. Something in their early history caused them to diverge, with the pair saying that one possibility is that life evolved on Earth fast enough to overcome sudden shifts in the environment and kept the planet habitable. On Venus and Mars, life didn't get a chance to take hold and was wiped out.

"One intriguing prediction of the Gaian Bottleneck model is that the vast majority of fossils in the universe will be from extinct microbial life, not from multicellular species such as dinosaurs or humanoids that take billions of years to evolve," says Lineweaver.

A particularly interesting point about the hypothesis is that it gives Earthlings a real stake in seeking traces of extinct life on Mars. The scientists argue that if fossil microbes are found on the Red Planet, it means that the Great Filter that sieves out life is probably behind us and we have passed the greatest hurdles of our existence. However, if there are no microbes there, it means that the filter may be ahead of us and we have an uncertain future to navigate.

The research was published in Astrobiology

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