One tenth of the stars in our galaxy might provide the right conditions to support complex life, according to a new analysis by Australian researchers. And most of these stars are on average one billion years older than the Sun, allowing much more time, in theory, for any life to evolve.
The concept of a “galactic habitable zone” (GHZ) for the Milky Way was first proposed in 2001. Now Charles Lineweaver of the University of New South Wales and colleagues have defined a life-friendly GHZ using a detailed model of the evolution of the Milky Way to map the distribution in space and time of four major factors thought essential for complex life.
“We’re looking at what we think are the most robust and conservative pre-requisites for life – but they are very, very basic,” Lineweaver says.
The researchers conclude that a ring-shaped habitable zone emerged about eight billion years ago, roughly 25,000 light years from the core of the Milky Way. The zone has expanded slowly and includes stars born up to about four billion years ago. It encompasses close to ten per cent of all stars ever born in this galaxy.
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But other researchers say that too little is known about the prerequisites for life for this kind of mapping to have a great deal of meaning.
“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 in Maryland, US, told Science.
Gas giants
Lineweaver stresses that his team is not arguing that complex extra-terrestrial life is probable, or even exists, within their GHZ. “What we’re saying is that this is the region that has the most potential for the formation of complex life,” he says.
The first factor the team considered in mapping the GHZ was the presence of host stars for a solar system. The second was the presence of sufficient heavy elements to form terrestrial planets. The third was a sufficiently safe distance from exploding supernovae. And the fourth was enough time for biological evolution. The team set this figure at a minimum of four billion years, since this was the amount of time it took for complex life to emerge on Earth.
Future work will test the importance of some of these factors, the team adds. Only about 100 extra-solar planets have been spotted to date, and these are all Jupiter-like gas giants. But the launch of NASA’s Terrestrial Planet Finder telescope in about 2013 will mark the start of a major search for nearby planets that could harbour life.
Journal reference: Science (vol 303, p59)
