Brussels, 14 Jan 2004
A tenth of the stars in our galaxy are in a 'habitable zone' that – like a galactic city in which we live in the outer suburbs – is teeming with planets that may support life, new research suggests.
After studying the distribution of four key conditions needed for life to form and survive long enough to evolve, Australian astronomers have identified a life-friendly 'galactic habitable zone' for our Milky Way. The ring-shaped zone – roughly 25,000 light years from the centre of the galaxy – is 8 billion years old and includes stars up to 4 billion years of age. The Australian team estimates that 10% of stars in the Milky Way – about 30 billion – could support complex life.
"What we have done for the first time is to quantify carefully where complex life is likely to exist," said the study's lead author Charles Lineweaver from the University of New South Wales in Australia. "Somewhere between the crowded inner Galaxy and the barren outer regions, a habitable zone appeared about 8 billion years ago that broadened with time as more distant regions accumulated heavy elements," said one of Lineweaver's team, Brad Gibson of Swinburne University of Technology, also in Australia.
But other researchers cast doubt on the study saying that we do not possess the knowledge to undertake this kind of mapping. "We hardly understand the origin of life, let alone the evolution of complex life. Until we do, it is extraordinarily difficult to talk about habitable zones," Mario Livio of the Space Telescope Science Institute (US), told Science in which the Australian study appeared.
Life in the suburbs
If this zone were compared to a stellar metropolis, then the Earth would be located somewhere in suburbia. Our own sun is younger – by on average 1 billion years – than 75% of the stars in the habitable zone. This extra growth time may indicate that there are more advanced life forms than our own out there.
Alternatively, it may also suggest that we are alone in the galaxy since, after a decade of trying, scientists believe they have not yet overheard any communications signals ­– such as extraterrestrial soap operas or movies – that would indicate intelligent life from outer space.
Lineweaver was also one of the brains behind an earlier study that calculated the chance that life had emerged in other parts of the universe using a similar approach to that used in working out the probability of someone winning the lottery. In that study, they inferred that the rapid appearance of life on Earth indicated that life was probably quite common in the universe – about one in three Earth-like planets probably supports simple life, they estimated.
Closer to home, the European Space Agency's (ESA) Mars Express – from its orbit around Mars ­– and the American space agency NASA's Spirit Rover – from the surface of the planet – are searching for signs of whether the red planet ever sustained life.