Astronomers map ‘habitable’ galactic zone

Astronomers have mapped the galactic ‘‘street’’ where ET is most likely to live.

They have identified a ‘‘habitable zone’’, a thin ring of stars covering 1% of the Milky Way where there is the best chance of finding Earth-like planets.

Our galaxy, the Milky Way, is shaped like a Catherine wheel with a dense core and arms spiralling out into space.

The habitable zone is at a point not too close to the core, and not too flung out towards the edge.

Only in this region are there likely to be rocky planets with enough liquid water to support life, say the scientists.

The US team, led by Guillermo Gonzalez at the University of Washington in Seattle, first looked at the composition of the clouds of dust and gas from which stars and planets form.

They realised that these primordial clouds must contain the right mixture of heavy and metallic elements if they are going to create Earth-like worlds.

With too few metallic elements, any rocky planets that formed would be small and cool down quickly, developing a crust too thick for plate tectonics the movement of land masses to occur.

Shifting tectonic plates play an important role in controlling atmospheric temperatures.

Also, the gravity of a small planet would be too weak to retain a viable atmosphere.

The composition of star and planet-making clouds depended on their location.

The astronomers found that suitable Earth-making material was concentrated in a thin ring around the galactic core. It is here that the Sun is situated.

New Scientist magazine, which reported the research last week, said: ‘‘Stars further out are metal deficient, and those nearer the core have too high a metallicity.

‘‘Also, as you get nearer to the centre of the galaxy, the stars are closer together, and planets there would have to endure supernovae fallout and bursts of gamma rays from the core which would kill life.’’

Seth Shostak, of the SETI (Search for Extra Terrestrial Intelligence) Institute in Mountain View, California, said the analysis might affect future attempts to detect radio signals from extraterrestrials.

But he added that the current programme was only targeting Sun-like stars within 150 light years, which is well within the habitable zone.