杏吧原创

EARTH is probably a youngster among planets of its kind, says an astronomer
in Australia. This may mean that any intelligent beings that have evolved on
other Earth-like planets will be so highly advanced that to them we seem little
better than bacteria. No wonder they haven鈥檛 been in touch yet.

Charles Lineweaver of the University of New South Wales in Sydney says that
Earth-like planets orbiting other stars will be on average about 1.8 billion
years older than Earth. He deduced this figure by cleverly combining a host of
factors that determine the formation and destruction of terrestrial planets.

One such factor is the way in which heavy atoms such as iron have become more
plentiful since the big bang. The early Universe contained only the lightweight
elements hydrogen and helium. All other atoms were made in the thermonuclear
furnaces of stars, and elements heavier than iron are only released into
interstellar space when old, massive stars explode as supernovas. Rocky planets
cannot form around a star unless there are enough heavy atoms in the dust it is
made of, so there were none when the Universe was young.

However, a brew too rich in heavy elements would lead to 鈥渉ot
Jupiters鈥濃�攇iant planets orbiting so close to their parent stars that they
destroy newborn earths.

In a paper submitted to the journal Icarus, Lineweaver concludes
that three-quarters of all Earth-like planets must have been around longer than
the Earth and that the average age is 6.4 billion years, compared with Earth鈥檚
4.6 billion years. 鈥淭his analysis gives us an age distribution for life on such
planets and a rare clue about how we compare to other life which may inhabit the
Universe,鈥� he says.

But neither Lineweaver nor anyone else will commit themselves on what this
tells us about the prospects of finding ETs. 鈥淭he odds against simple life are
still completely uncertain,鈥� says Martin Rees of the University of Cambridge.
鈥淲e don鈥檛 know the chance of any extraterrestrial intelligence evolving
anywhere, nor what variety of evolutionary tracks are possible.鈥�

However, Lineweaver鈥檚 calculations could offer a hint about why we haven鈥檛
picked up any radio signals from alien beings. Intelligent life on older planets
may have advanced as far beyond us as we are beyond bacteria, and such beings
would be unlikely to communicate via a medium as primitive as radio waves.

鈥淐ertainly this can be a way of explaining the `Great Silence鈥�,鈥� says ET
hunter Paul Schuch, executive director of the SETI League in New Jersey. 鈥淏ut I
rather think our lack of success is more related to the fact that, on the cosmic
clock, we just started looking yesterday. These things take time, and require
great patience.鈥�