LONDON, Jan 1 PA - A tenth of the stars in the Milky Way may have planets that support advanced life,Australian scientists said today.
Astronomers have plotted a ring-shaped region of the galaxy where there might be Earth-like worlds old enough for life to have reached a high level of evolution.
The Sun exists in this Galactic Habitable Zone'', which contains about 10 per cent of all the Milky Way's stars. Stars within the band have enough heavy elements to form Earth-like planets, are a safe distance from catastrophic supernova explosions, and have existed forat least four billion years. The Australian team, led by Charles Lineweaver from the University of New South Wales, used a chemical evolution model of the galaxy to identify the region. Three quarters of the stars in the zone were older than the Sun, ranging in age between four and eight billion years. The astronomers wrote in the journal Science: The Galactic Habitable Zone (GHZ)… is an annular region lying in the plane of the galactic disk possessing the heavy elements necessary to form terrestrial planets and a sufficiently clement environment over several billion years to allow the biological evolution of complex multicellular life.’’
A critical factor was the amount of heavy metallic''elements in the host star. Analysis of known planetary star systems showed a strong correlation between close-orbiting massive planets and a high proportion of heavy elements. In our solar system, large gas planets such as Jupiter and Saturn keep safely away from the centre. But if they migrated inwards -- as they appeared to in the heavy’’ systems – they could destroy Earth-like planets on the way or disrupt their orbits.
Thus, there is a Goldilocks zone of metallicity,'' wrote the scientists. With too little metallicity, Earth-mass planets are unable to form; with too much metallicity, giant planets destroy Earth-mass planets.’’
Supernovae – colossal explosions caused by the death of massive stars – presented another obstacle to the emergence of life.
The explosions triggered blast waves and released lethal cosmic, gamma and X-rays.
Early intense star formation towards the centre of the galaxy provided the heavy elements necessary for life, but the region was also a minefield of supernovae.
The scientists said they were not assuming that complex life was probable.
But they concluded: ``We have identified the space-time regions most likely to harbour complex life.This result depends on the assumption that the terrestrial time scale for biological evolution is representative of life elsewhere.’’
Imagine how many scientists the universe holds…