The famous Fermi paradox raises the question: why haven’t we detected signs of alien life, despite high estimates of probability, such as observations of planets in the “habitable zone” around a Sun-like star by the Kepler telescope and calculations of hundreds of billions of Earth-like planets in our galaxy that might support life. Now astrobiologists from Australian National University (ANU) Research School of Earth Sciences say they have the best answer: Because life on other planets would likely be brief and would become extinct very quickly from 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, PhD., lead author on a paper published in Astrobiology. In fact, “early life is fragile, so we believe it rarely evolves quickly enough to survive. 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.”
The Gaian Bottleneck
For example, 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, the authors explain. Early microbial life on Venus and Mars, if there was any, failed to stabilize the rapidly changing environment, while life on Earth probably played a leading role in stabilizing the planet’s climate.