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Ane of the indelible puzzles in astrobiology is why we haven't found whatever aliens all the same — and it'due south worth another await, in light of Breakthrough Listen'south latest (and unprecedented) attempt to find some in a new survey of millions of stars in our galaxy. Astronomers estimate there are betwixt 200 and 400 billion stars in the Milky way. The Kepler telescope has proven that many stars have planets, and if the World is in any mode a typical planet, we should see some testify of life elsewhere in the galaxy after 13.8 billion years. Except, of course, nosotros don't. This is known every bit the Fermi Paradox.

A paper past astrobiologists Aditya Chopra and Charley Lineweaver of the Australian National University has an explanation for why this might be so. It'due south a modification of the so-called "Great Filter" theory and of the Gaia Hypothesis, and it argues that life on most planets may become extinct not long later on it outset evolves. This would partially explain why efforts similar Breakthrough Listen take thus far failed to find the results they seek. Before we tackle the Gaian bottleneck theory, allow's discuss the larger Gaian hypothesis.

The Gaian Hypothesis

Start proposed by James Lovelock, the Gaia hypothesis proposes that living organisms interact with inorganic material on the planet in a manner that perpetuates or regulates the weather for life on the planet.

Lovelock proposes 3 central arguments: 1). The Earth is an extremely favorable habitat for life, 2). Life has greatly altered planetary chemistry and the environs, including both the atmosphere and sea, and 3). Globe's environment has remained fairly stable over a long catamenia of fourth dimension.

Of these three points, #two is absolutely truthful. We know, for instance, that an consequence known as the Corking Oxygenation Upshot killed nigh of the life on Earth when oxygen — produced as waste from anaerobic cyanobacteria — could no longer be absorbed past inorganic sources. Life on Earth is known to take existed for over iii.viii billion years, merely it wasn't until some 2.5B years ago that oxygen began to announced in measurable quantities, and the level of O2 in the temper didn't begin approaching modern concentrations until some 850 million years ago.

1599px-Oxygenation-atm-2

Estimated evolution of atmospheric oxygen. The upper carmine and lower green lines represent the range of the estimates. The stages are: stage 1 (3.85–2.45Gyr ago (Ga)), stage 2 (ii.45–1.85Ga), phase iii (1.85–0.85Ga), Stage 4 (0.85–0.54Ga )and stage v (0.54Ga–present). Image by Wikipedia

Even if the GOE was our but evidence for the potential planetary affect of biological life, the transformation of Earth'south atmosphere is proof that biology can absolutely alter planetary atmospheric condition. But the evidence for the Globe being an extremely favorable habitat for life is undercut past what nosotros now know virtually periods of geologic time where the Earth was well-nigh completely frozen (the so-called "Snowball Earth,") while point #3 — Earth's ecology stability — is weakened by the existence of events similar the Slap-up Oxygenation Event, which wiped out vast numbers of species on Earth. In fact, the production of oxygen may have really caused the Snowball Earth. These events rule out strong Gaia, or the idea that life optimizes the biosphere to meet its ain needs.

The "weak" Gaia model, on the other manus, has more than staying power. In the image above, Coevolutionary Gaia and and Influential Gaia posit that life has a commonage bear upon on Earth's surroundings and that the evolution of life and the evolution of the environment are intertwined. These are much less controversial points and are already well-explored and established in literature. There's debate over whether or non this constitues a "Gaian" hypothetical at all.

It is absolutely possible that the life on a planet could play a part in stabilizing the biosphere and making it more likely that life would continue to exist. But based on what we know of Globe's history, it seems possible that the consequence could also run in the opposite direction. The GOE is idea to have possibly caused the Huronian glaciation that most froze the Earth because information technology led to CO2 replacing methane in the Earth's atmosphere — and CO2 isn't as potent a greenhouse gas. This led to the Earth nearly freezing solid as a result.

Now that we've covered the Gaia hypothesis in a flake more detail and constrained the machinery of its operation, let'southward wait at what these scientists are proposing.

The Bully HEPA Filter

The "Great Filter" theory explains the Fermi Paradox by positing the existence of a specific bottleneck that prevents most species (or life in full general) from reaching the point where the colonization of space is possible. Life might evolve frequently, simply supernovae and gamma ray bursts sterilized many of the planets on which it evolved earlier in our galactic history. Maybe highly intelligent life has a trend to over-swallow the resources of its dwelling house planet, and therefore becomes extinct before developing the applied science to colonize a galaxy. Life might be abundant, but the evolution of high intelligence and efficient tool utilize could be extremely rare.

What Chopra and Lineweaver propose is that life may evolve oftentimes, simply only rarely survives long enough to begin to have an impact on its own planet in a way that creates and maintains an surround amenable to life. They call this the Gaian clogging.

Figure 1 - Gaian hypothesis

The blue area at the lesser of the diagram shows the range of planetary environments that can grade, while the yellow cylinder shows the weather condition required for the formation of an "abiogenesis habitable zone (ABZ)." If a planet's initial environment is within this zone, life can brainstorm to evolve. As life evolves (green area) it creates wider zones of habitability, which allow for other life to evolve in turn.

To understand how this particular filter would function, consider the fates of Earth, Venus, and Mars. We at present have strong evidence that Mars was one time a warmer and wetter place than it is today, with many of the base conditions believed to favor the development of life. There's less information available on Venus' early history, but what data we do have suggests that information technology, too, may accept in one case had liquid oceans and fifty-fifty a short-lived magnetic field. (Mars has remnants of such a field today).

The difference between our iii planets is that both Venus and Mars may take lost the conditions that made them initially habitable more quickly than life on those planets could evolve to go on them habitable. From the newspaper:

Liquid water is not easy to maintain on a planetary surface. The initial inventory and the timescale with which h2o is lost to infinite due to a runaway greenhouse, or frozen due to ice-albedo positive feedback, are poorly quantified, but plausible estimates of futurity trajectories have been made. On World, dissociation of water vapor by UV radiation in the upper atmosphere is ongoing and volition eventually (1–2 billion years from now) lead to the loss of water from the bioshell and the subsequent extinction of life on World.

Gaian hypothesis - Figure 2

Without life, liquid water may be quickly lost

The authors' argue that early life may perform a vital regulating function on an early on planetary surroundings by impacting both the planet's albedo (how much light it reflects from the lord's day) and the greenhouse gases present in its atmosphere. They modeled how apace a planet lost its water based on either a runaway greenhouse effect (Venus) or runaway glaciation (Mars). In both cases, the amount of water remaining on the planet is a fraction of its total by roughly 1 billion years old.

The Gaian bottleneck theory offers a model for how life might be distributed inside the universe, equally shown beneath:

Gaian hypothesis - Figure 3

In an emergent clogging scenario, the weather condition for life are rare, merely in one case life evolves, it persists for billions of years. This assumption helps create the Fermi Paradox, because if even a handful of civilizations be, we should accept seen some evidence of them already.

If in that location is no emergence clogging, life should already be plentiful — think of this as the Star Trek / Star Wars hypothesis, where hundreds of species exist across the entire galaxy.

The Gaian model predicts an initial surge of life within the get-go billion years of planetary development, merely that the vast majority of this life never survives long enough to advance across single-cell organisms. It predicts that the bulk of life we'll notice out amid the stars will be fossilized remnants of archaic creatures, assuming whatever evidence exists to be found at all.

Plainly we won't exist able to bear witness or disprove any of these models until we can actually evaluate the conditions around other planets. Merely if the Gaian bottleneck theory is accurate, nosotros might find prove for information technology on the moons and rocky planets of our ain solar system. It'south virtually impossible to build a probe that tin survive on the surface of Venus for any length of time, but Titan, Mars, Europa, and Ganymede might contain such life — or fossilized evidence of its original existence.