44 | New Scientist | 31 August 2019
An obvious place to start is with our own
planet. If alien astronomers were observing
Earth from a remote star system, would
anything about it grab their attention?
Compared with our rocky neighbours Mars,
Venus and Mercury, the distinctive mix of
oxygen and methane in Earth’s atmosphere
would be sure to trigger interest. Oxygen
makes up 21 per cent of the atmosphere
now and is entirely due to life, entering the
atmosphere from photosynthetic bacteria
and plants that convert sunlight into energy.
We aren’t sure when exactly oxygenic
photosynthesis evolved, but there are
clear signs that our atmosphere filled
with oxygen 2.33 billion years ago.
Methanogens, the microbes that
produce methane, existed even earlier.
Despite the biological origins of both
gases, neither on its own is a sure sign of life.
Methane, for example, is also produced by
volcanoes and hydrothermal vents, although
methane with an organic origin has a higher
carbon-12 to carbon-13 isotope ratio. Oxygen
could be formed when radiation from an active
star splits molecules of water into hydrogen
and oxygen, with the lighter hydrogen
escaping from the planet’s atmosphere. In
combination, however, methane and oxygen
tell a story of a planet swarming with life.
In the 1960s, astronomers realised that the
existence of each gas was fatal to the other.
Without large quantities of both oxygen and
methane being continuously pumped into
the atmosphere, these gases would quickly
react and destroy each other. Individually
you might expect a lifeless planet to contain
either oxygen or methane. But geology alone
doesn’t provide a way to maintain both.
This means that finding oxygen and
methane coexisting in appreciable quantities
on a distant planet is a pretty good indicator
of life. What’s more, life on Earth produces
thousands of other molecular gases that
seem to be unique. Methyl chloride, dimethyl
sulphide and nitrous oxide have all been
proposed as promising biosignature targets.
What if our search for all of these gases
comes up empty? Does that mean a planet is
an arid ball of rock? Not necessarily. Life on a
distant world may be totally different to that
on Earth. It could be hiding under the surface,within solid rock or in hidden seas, where it
would be effectively invisible. More radical
alternatives are also possible. It could be based
on silicon, for example, rather than carbon, or
run on unknown metabolisms that use a liquid
other than water. For these types of weird life,
synthetic biology and research into alternative
biochemistries could help us understand what
unique chemicals to look for.
Sara Seager at the Massachusetts Institute
of Technology is trying to tackle this problem,
working her way through all the molecules
whose presence might indicate the existence
of life. One of my favourite ideas comes from
another MIT researcher, Clara Sousa-Silva,
who says we should look for phosphine as a
sign of life. Phosphine is a gaseous compound
of phosphorus and hydrogen that is producedWhat does life look like on other planets?
Hear Geraint Lewis speaking at New Scientist Live
newscientistlive.com“ Finding no life elsewhere may lead us
to take better care of our own world”
In 1950, physicist Enrico Fermi
was having lunch with his
colleagues when he asked a
profound question: where is
everybody? He wasn’t referring
to the emptiness of the
university cafeteria, but why,
if we calculated that the
universe should be filled with
extraterrestrial life, none had as
yet crossed our radar. Over the
decades since then, various
creative solutions to Fermi’s
paradox have been proposed.THEY ARE ALREADY HERE
This solution remains surprisingly
popular, positing an international
conspiracy to cover up the evidence
of alien contact.THEY DON’T WANT TO DISTURB
Perhaps aliens have some “prime
directive”, as fictional space explorers
in the TV and film series Star Trek
do, to not interfere with the
development of less advanced
cultures on other worlds.
Or maybe extraterrestrials regard
us as a sort of national park
or zoological garden, watching
our movements but hiding their
presence.THEY WON’T LIVE LONG ENOUGH
TO GET IN TOUCH
The depressing possibility exists
that no advanced civilisation
survives long enough to still be
around when its neighbours are
thriving. This idea is called the
Great Filter. We may have already
unknowingly passed through the
filter unscathed, or it may be
looming, in which case threats
such as nuclear war and climate
change might spell our doom.WE ARE ALONE IN THE UNIVERSE
Might the simplest answer
be the best, after all?Solutions to the
Fermi paradox