42 | New Scientist | 31 August 2019
Is there
anybody
out there?
After millennia of guesswork, we can
finally start finding out for certain,
says astrobiologist Sarah Rugheimer
I
T IS the biggest question in the universe:
are we alone? Philosophers have
debated the question for millennia.
When 16th-century Italian astronomer and
Dominican friar Giordano Bruno declared that
the cosmos contained “an infinity of worlds of
the same kind as our own”, he was directly
contravening religious dogma. He was later
burned at the stake during the Inquisition, in
part for daring to question Earth’s unique status.
The debate continues, in more restrained
fashion, to this day. For some, the sheer
size of the universe makes it unlikely that
life formed only once. For others, the
remarkable complexity of life on Earth
is testament to its uniqueness.
Until recently, vague philosophical
answers of this kind were the best science
could do. The signs of life were far too
ambiguous to pin down for certain, and
our nearest potentially habitable worlds
were too small and distant to test.
But for the first time in human history we
are reaching the technological sophistication
needed to provide a genuine answer. Powerful
telescopes are letting us study planets in other
solar systems, giving us a glimpse into their
atmospheres and a flavour of what type of life
might be living on their surfaces. At the same
time, improved analysis of our own planet is
allowing us to redefine what life might look
like from afar, and is helping us to distinguish
the signs of a flourishing alien civilisation fromthe mere geological rumblings of a lifeless
world. With these tools at our disposal,
answers are finally within our grasp.
To understand my optimism, it is worth
revisiting the work of astronomer Frank Drake.
In 1961, Drake devised a formula to estimate
how many advanced civilisations were capable
of signalling their presence in the Milky Way.
His eponymous equation depends on breaking
down that big unknowable quantity into a
number of more tractable ones that can be
multiplied together, such as the number of
stars in the galaxy and the fraction of those
likely to have planets (see “Quiet
neighbourhood”, page 45).
Even with pessimistic values, the existence
of millions of technological civilisations seems
likely. The main bottleneck on that apparent
explosion of life, however, is in Drake’s final
term: the average lifetime of a communicating
civilisation. Humans have been broadcasting
radio signals that escape into space for only
about a century, and, in the current geopolitical
climate, who is to say how many more years
we have left. If you take the pessimistic
assumption that intelligent life destroys itself
rather quickly, the Drake equation suggests
that statistically we are alone in the galaxy.
If intelligent civilisations survive for millions,
or even billions of years, however, then the
Milky Way should be teeming with aliens.
This calls for optimism, but also caution.
After all, if there are millions of alien BE
THHOEC
KE
LFeatures Big question