If aliens existed in the past or do exist now, this existence would be fleeting, according to astrobiologists at the Australian National University. In their search to understand how life would develop on other planets, they concluded that new life would likely die out fast due to runaway heating or cooling.
"The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said Aditya Chopra, lead author of the paper, in a press release. "Early life is fragile, so we believe it rarely evolves quickly enough to survive."
"Most early planetary environments are unstable," he added. "To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable."
For example, around four billion years ago, Earth, Venus and Mars were likely all habitable, but a billion years later, Venus became extremely hot and Mars froze over, meaning the destruction of any early microbial life on these planets, if any existed in the first place.
"Life on Earth probably played a leading role in stabilizing the planet's climate," said Charley Lineweaver, who participated in the research.
"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," added Chopra.
The researchers propose an almost universal early extinction, called the Gaian Bottleneck, to explain the fact that even wet, rocky planets with all of the ingredients for life have not showed an signs of extraterrestrial life so far.
"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.
The findings were published in the Jan. 20 issue of Astrobiology.