Have you ever wondered why we haven't seen evidence of alien life? Experts from The Australian National University explained in a new research published in the journal Astrobiology why alien life might have become extinct very quickly.

Based on the recent study, a team of astrobiologists suggested one possible explanation as to why extraterrestrial life cease to exist. Chicago Tribune shared that researchers realized the instability of young habitable planets can lead to either a hellish hothouse or frozen wasteland easily, making the once life-giving oasis unconducive to sustain life.

"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," lead author of the study Dr. Aditya Chopra said. "Early life is fragile, so we believe it rarely evolves quickly enough to survive."

Researchers noted that about four billion years ago, Earth, Venus and Mars may have all been habitable. Unfortunately, a billion years or so after formation, Venus turned into a hothouse while Mars froze into an icebox.

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

Study authors also blamed climate change, saying that the mystery of why we haven't found any signs of extraterrestrial existence may be due to 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."

The new study, which explained the "Gaian Bottleneck" hypothesis, could also be the answer to the infamous Fermi Paradox, Space.com noted. Chopra and co-author Charley Lineweaver explained that the theory suggested that extinction is the cosmic default for most life that has ever emerged on the surfaces of wet rocky planets in the universe.

They also added that rocky planets need to be inhabited to remain habitable. The theory, however, is in contrast to the "emergence bottleneck," which hypothesized the apparent scarcity of life in our universe is due to the low probability of life emerging in the first place, noting "intricacies of the molecular recipe."

So, if this new theory is true, where are all the alien fossils?

Astrobiologists believed any life-forms that failed to keep their planet habitable never grew big enough to leave identifiable remains.

"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," Professor Lineweaver said.

Meanwhile, it remains unclear which of these hypotheses better represents reality or these theories represent reality well at all. While there are ways to test such ideas out, researchers said it will be difficult and time-consuming.

For now, these theories are speculations. However, the observations of the study also suggested that another existential threat still looms. Unless we find a way of reversing the damage humans caused to the environment, it appears that there's a possibility that life on Earth may quickly go extinct.