Mystery of Extraterrestrial Life –“Unlike Earth, It Rarely Evolves Quickly Enough to Survive” (Weekend Feature)

Earth from Space

 

“The mystery of why we haven’t yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces,” said Aditya Chopra at Australia National University. “Early life is fragile, so we believe it rarely evolves quickly enough to survive.”

In short, life on other planets would likely be brief and become extinct very quickly, say astrobiologists from The Australian National University (ANU). In research aiming to understand how life might develop, the scientists realized new life would commonly die out due to runaway heating or cooling on their fledgling planets. The answer, the ANU team realized was found in James Lovelock’s Gaia Theory.

In the 1970s, the chemist Lovelock and the biologist Lynn Margulis developed the idea that our Earth may be like a living organism, a self-regulating entity that employs feedback loops to keep conditions just right for life. They christened the potentially living planet “Gaia,” from the Greek for Mother Earth.

The search for ‘other earths’ is in many ways a search for ‘other Gaias’, and NASA’s plans for the detection of other Earth-like planets very much depends on the understanding of the relationship between life and the Universe developed by Lovelock in the context of Gaia theory.

Life got a grip on Earth with almost indecent haste. When Earth was young, it was bombarded by debris left over from the formation of the Solar System, creating an extreme environment in which life could not get a grip that lasted some 600 million years after the formation of the Solar System. But there is evidence that as soon as the bombardment ended, life began.

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The Earth’s orbit is in a quite extraordinarily lucky part of the Solar System, observed John Gribbin, author of Alone in the Universe, as far as the prospects for the evolution of intelligence. But the situation isn’t quite as clear cut as it seems at first sight. The presence of life on Earth plays a part in regulating the temperature of our planet, through the greenhouse effect. Gases such as carbon dioxide act to warm the surface of the Earth by trapping heat that would otherwise escape into space.

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Today, this natural greenhouse effect keeps the Earth about 33 °C warmer than the surface of the airless Moon, even though the Earth and the Moon are at very nearly the same distance from the Sun. When the Earth first formed, writes Gribbin, the atmosphere was richer in such greenhouse gases, preventing it from freezing even though the Sun was cooler. As the Sun warmed and life developed on Earth, carbon dioxide was drawn down out of the air by living things and deposited as carbonate rocks, reducing the strength of the greenhouse effect. Life alters the amount of carbon dioxide in the air, through feedback processes which keep the planet warm when the Sun is cool and prevent it overheating as the Sun warms up.

This is the basis of Gaia theory, developed by James Lovelock, which gives us the means to search for life beyond the Solar System. Lovelock’s great question was: what it is that makes the Earth special. “The air we breathe can only be an artifact, maintained in a steady state far from chemical equilibrium by biological processes. Living things,” he concluded, “must be regulating the composition of the atmosphere, not just today but throughout the history of life on Earth – literally for billions of years.”

But the puzzle then becomes, why didn’t a runaway greenhouse effect set in as the Sun warmed, searing the surface of the planet, as seems to have happened on Venus? The answer, Lovelock saw, is that life has regulated the composition of the atmosphere, removing the carbon dioxide gradually as the Sun warmed, keeping temperatures on Earth comfortable for life.

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The ANU scientists think the reason we haven’t found signs of advanced technological life might be because all the aliens went extinct. “Extinction is the cosmic default for most life that has ever emerged,” the authors of the study write.

“The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens,” said Chopra from the ANU Research School of Earth Sciences and lead author on the paper, which is published in Astrobiology. “Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable.”

About four billion years ago Earth, Venus and Mars may have all been habitable. However, a billion years or so after formation, Venus turned into a hothouse and Mars froze into an icebox.

Early microbial life on Venus and Mars, if there was any, failed to stabilize the rapidly changing environment, said co-author Associate Professor Charley Lineweaver from the ANU Planetary Science Institute. “Life on Earth probably played a leading role in stabilizing the planet’s climate,” he observed.

Wet, rocky planets, with the ingredients and energy sources required for life seem to be ubiquitous, however, as physicist Enrico Fermi pointed out in 1950, no signs of surviving extra-terrestrial life have been found.

A plausible solution to Fermi’s paradox, say the researchers, is near universal early extinction, which they have named the Gaian Bottleneck. “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,” said Lineweaver.

“Can a planet, in a sense, become alive?” asked NASA astrobiologist David Grinspoon. It’s not the first time he puts the concept forward. In his 2003 book Lonely Planets, Grinspoon introduced the “Living World” hypothesis, a slight variant the well-known Gaia hypothesis.

The idea has since been hotly debated, mostly pegged as more philosophical than scientific. Still, many researchers agree that the concept has helped Earth system science move forward, allowing us to realize that many of Earth’s cycles—the water, nitrogen, and carbon cycles; plate tectonics; and the climate—are deeply interconnected, and is modulating and being modulated by life on Earth.

“Gaia may just be a nice metaphor,” Grinpsoon says, “but I wonder if it may be fruitful to think of life as something that happens not just on a planet, but as something that happens to a planet.”

“You cannot easily separate the living and the non-living parts of Earth,” he adds. “Life has made Earth the way it is to a large extent. That’s the general meaning of the Gaia hypothesis, and the Living Worlds hypothesis is simply extending the idea to other planets.”

“The idea of an origin of life separated from the birth of a living world has interesting implications for life elsewhere,” Grinspoon writes in Lonely Planets: “If self-regulating Gaia is responsible for Earth’s life longevity, then we need to find other places where this kind of global organism has evolved, not merely places where the origin of life might once have occurred.”

In other words, our search for life should then target places with active geological and meteorological cycles, the potential tell-tales of a vibrant biosphere.

We’ve now found nearly 2,000 planets orbiting distant stars, and counting. While these worlds may be too far for us to find any direct evidence for life in the near future, researchers are becoming increasingly proficient at making out the composition of their atmosphere. That ability could perhaps one day allow us to distinguish between “failed biospheres” and potentially living worlds.

“The Case for a Gaian Bottleneck: The Biology of Habitability,” Aditya Chopra & Charles H. Lineweaver, 2016 Jan. 20, Astrobiology [http://online.liebertpub.com/doi/10.1089/ast.2015.1387, PDF: http://adi.life/pubs/ChopraLineweaver2016.pdf].

The Daily Galaxy via NASA Astrobiology and John Gribbin, John. Alone in the Universe: Why Our Planet Is Unique

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