The universe we live in is incredibly vast. As human technology and knowledge expand over the years, our search for other living species outside our own planet has increased and intensified. So with all of this increase in our technologies and instruments, why haven't we found any sign of life elsewhere?

Even using our lowest and most pessimistic estimates, there should be billions upon billions of intelligent civilizations running around in the universe. With so many alien civilizations running around, it stands to reason that humans should have been contacted. Or at least we should have found promising evidence  by now, perhaps some radio emissions or other signs of intelligent life.

We should have seen them. But we haven’t.

To get a bit more specific, there are some 100 billion planets in the Milky Way, and even if just a mere fraction are habitable (a modest estimate), that still means that there should be millions of planets with alien life and hundreds of thousands that have intelligent alien life.

Why this life seems to be missing is the very heart of Fermi’s paradox, which addresses this apparent contradiction—the difference between our estimates and the evidence (or lack thereof) for alien life.

Now, new research from the Australian National University proposes the possibility that we have not been able to find aliens because they are all dead.

The hypothesis is based on what we know about the first billion years or so after a new planet forms - its environments are extremely unstable, fluctuating in temperature and atmospheric composition so wildly that the chances of a new life form evolving quick enough to cope are very slim.

"The Universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said lead researcher, Aditya Chopra from the Australian National University. "Early life is fragile, so we believe it rarely evolves quickly enough to survive."

So how did the Earth survive?

The research team points out that 4 billion years ago, Venus and Mars were fairly habitable when they first formed and may have indeed been home to life beyond Earth. However, evidence shows that a billion years after they formed, these two planets may have killed off any existing life forms due to its changing temperature.

With all of these in mind, why has our planet continued to nourish its inhabitants?

Apparently, the answer is the inhabitants themselves. It was suggested that the Earth's success is causes by the stabilizing effect the inhabitants had on the environment. "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."

The researchers simulated this scenario using a model they call the "Gaian bottleneck," which basically means that if life can’t evolve fast enough to stabilize its environment, it dies out. If we’re looking at examples such as Venus and Mars, the Gaian bottleneck states that if you don’t make it through that brief period of rough but possible habitability, you’ve probably missed your chance.

From previous research, it is a known fact that life on Earth managed to evolve so fast, it ended up regulating greenhouse gas emissions on a planetary scale, and this appeared to have a favorable effect on what’s known as our planet’s albedo - the ratio of reflected radiation to absorbed radiation. This is important because when Earth was just forming, the Sun was up to 25 percent less luminous than it is now, but all the evidence points to the oceans being liquid, or at least not completely frozen. This scenario is also known as the faint young Sun paradox.

The interaction between these large patches of frozen and liquid water on the surface of Earth interacted with the radiation, and established the planet’s albedo, which ultimately determined its surface temperature.

"In extremely rare cases - like on Earth - the relatively rapid evolution from single- to multicellular organisms to complex life forms did not produce enough greenhouses gases to cause runaway negative feedback and heat the planet enough to evaporate all its liquid water," explains Campbell Simpson at Gizmodo. "It’s that particular and so far unique quirk that has kept us alive, if the Gaian bottleneck explanation is accurate."

If this hypothesis is proven to be true, it will answer the problem posed by the Fermi paradox: if the Universe is a colossal space, filled with trillions upon trillions of potentially life-sustaining stars and habitable planets, why haven’t we found any aliens?

"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," says team member astronomer Charles Lineweaver.

The research has been published in the journal Astrobiology.

If their hypothesis turns out to be accurate, it is quite saddening that we will never meet other life forms outside of our own planet. At the same time, this gives us more reason to appreciate our planet and work together to ensure its existence by focusing on the environment.

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