Wallace Arthur, author of The Biological Universe, sheds some light on one of humanity's most enduring questions.
The short answer to this question is ‘no’. As a scientist, I have little
doubt about this, which is odd, because so far we have absolutely no evidence
for extraterrestrial life of any kind, anywhere. And this applies to past life
as well as present. The putative fossils described in a meteorite found in
Antarctica in the 1980s are now recognized as having been produced by chemical,
not biological, processes. The sum total of evidence for extraterrestrial life
from about 60 years of searching – from the early days of SETI around 1960 to
the present – is zero, zilch, nothing. A resounding silence. No replies to our
outgoing messages; no incoming signals that have been generated by life-forms.
We appear to be a living speck of dust, or a ‘pale blue dot’, as Carl Sagan
famously referred to the Earth, in a barren expanse of space littered with
lifeless stars and planets.
But that last word – planets – is the key to my odd positivity in the face of the overwhelming lack of evidence. The ancient Greeks knew of six planets: Mercury, Venus, Earth, Mars, Jupiter and Saturn. Then, in the 1700s and 1800s, the number went up to eight, with the discovery of Uranus and Neptune. For more than a century, the number remained the same, apart from the transient recognition of Pluto as a planet, a phase that ended in 2006 with its declassification by the International Astronomical Union, the reason for which we can ignore here, because we have bigger fish to fry.
The planet Jupiter, photographed by the Hubble Space Telescope. It’s lifeless, though some of its moons might not be. The many recently-discovered ‘hot Jupiters’ are also lifeless. Image courtesy of NASA.
In the 1990s, the number of known planets increased again, but in a different way. During that decade, the planets discovered were the first ‘exoplanets’ – those orbiting other stars than our local one, the Sun. By the turn of the millennium, more than 20 exoplanets had been discovered. By 2010, the number had risen to a few hundred, and today it’s a few thousand. As these figures suggest, this increase in human discovery of planets roughly fits an exponential pattern – it just keeps accelerating. When will it end? Actually, it probably never will – prior to our extinction.
The Transiting Exoplanets Survey Satellite, or TESS for short, has discovered Earth-like exoplanets. Image courtesy of NASA.
Here’s the reason why we will keep on discovering exoplanets ‘forever’.
Apologies in advance for the astronomical numbers (in both senses of that
term). There are about 400 billion stars in our galaxy, the Milky Way. Almost
all of these are thought to have planets orbiting them. The number varies, but
on a conservative estimate of the average – three – the number of planets in
the galaxy is over a trillion. But the number of galaxies in the observable
Universe is also over a trillion. So our best guestimate of the total number of
planets is more than a trillion trillion, or ten to the 24th power
for the mathematicians among you.
That’s an awful lot of planets. Some, such as the huge ‘hot Jupiters’ orbiting within a whisker of their star, are uninhabitable by life as we know it, and probably by life as we don’t. Others, such as those that orbit volatile ‘flare stars’, may be equally uninhabitable. But a small fraction of planets orbit in the habitable zones of quiescent Sun-like stars, and it seems inconceivable that none of these have life. After all, a small fraction of a huge number is a very large number indeed. It’s probably at least a billion trillion, or 10 to the power 20. That many potential Earths and only one of them with life? Nonsense! But as I said at the start, that’s only the short answer. The longer one is here:
Wallace Arthur is an evolutionary biologist with two main areas of interest: the interplay between evolution and development ('evo-devo'), and the question of how evolution might operate on inhabited exoplanets (part of the broad field of astrobiology)....
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