The aliens are silent because they are all extinct »

Life on other planets would likely be brief and become extinct very quickly, say astrobiologists from ANU Research School of Earth Sciences. In research aiming to understand how life might develop, the scientists realised new life would commonly die out due to runaway heating or cooling on their fledgling planets. "The universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said Dr Aditya Chopra, lead author on the paper, which is published in Astrobiology. "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." 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. Early microbial life on Venus and Mars, if there was any, failed to stabilise the rapidly changing environment, said co-author Associate Professor Charley Lineweaver from the ANU Planetary Science Institute.