Letter: We're not alone

From Graeme Hill

So Peter Ward believes that the life on our planet is unique and would have
the Earth returned to the centre of the Universe
(18 March, p 46).

Let us examine his main points. The stable supply of stellar energy that life
needs to evolve is not the sole preserve of spectral class G2V stars like our
Sun. There are thousands of stars in our galaxy with lifetimes and stability
characteristics conducive to the emergence of multicellular organisms and
possibly intelligent life.

As for the idea that Jupiter’s orbit—which may help protect Earth from
collisions—is unusual, the kind of extrasolar planets discovered to date
has been biased by the method of discovery. Planet hunters look for Doppler
shifts in a star’s light, which will be most pronounced when the associated
planet has a very short orbital period (in other words, is very close to the
star), a high degree of orbital eccentricity and a mass of several Jupiters.
Caution is thus justified when constructing hypotheses on such a skewed data
set.

As for the notion that the chemical composition of the Earth is “right” for
life—the make-up of any stellar system is derived from the composition of
the interstellar gas cloud, which collapses to form stars and their associated
planets. The composition of the interstellar cloud is, in turn, derived from the
number of supernovae (in which heavy elements are formed) that have occurred in
a galaxy since its formation. This is not a “happy accident”, merely a function
of time and the physics of stellar evolution.

Peter Ward also states that the Earth’s position in our Galaxy is important.
Granted, but many stars occupy the habitable zone some 1500 light years wide and
30 000 light years from the galactic centre. It has been estimated there are 1
billion stars in this zone, of which 100 million may have habitable (stellar)
ecospheres.

I suspect that when better instrumentation and analytical techniques have
been developed, we will find that Earth-like planets are numerous and not a
cosmic quirk.

Willoughby, Warwickshire