杏吧原创

WANT to know where in the Galaxy you might find extraterrestrial life? Help
is at hand from astronomers who have mapped out a 鈥済alactic habitable zone鈥�, a
narrow region of the Milky Way where suitable terrestrial planets are most
likely to exist.

Astronomers already map out the habitable zone around individual stars, which
they define as the region where a rocky planet could retain enough liquid water
to support life. Now Guillermo Gonzalez and colleagues at the University of
Washington in Seattle have widened their sights to define a habitable zone for
the entire Galaxy.

To do this, the researchers looked at the composition of the clouds of dust
and gas from which stars and planets form. They reasoned that these clouds must
have the right mixture of heavy and metallic elements if they are going to
create Earth-like worlds. If this 鈥渕etallicity鈥� is too low, any rocky planets
that formed would be small and would cool down quickly, developing a crust that
was too thick for plate tectonics to occur. The movement of tectonic plates
plays 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 the clouds depended on their location in the Galaxy.
Gonzalez鈥檚 team found that suitable Earth-making material was concentrated in a
thin ring鈥攚hich contains our Sun鈥攃entred on the galactic core. 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.

Gonzalez believes that his team鈥檚 analysis narrows down the places where life
might arise to around 1 per cent of the Milky Way鈥檚 stars. Seth Shostak of the
SETI Institute in Mountain View, California, says such an analysis may affect
SETI searches. But, he adds, the current programme is only targeting Sun-like
stars within 150 light years, well inside the habitable zone.