The Economist May 28th 2022 71
Science & technology
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Astrobiology
Life, but not as we know it
F
or decades, astronomers searching for
life beyond humanity’s home planet
have had a simple strategy: follow the wa
ter. Water is the sine qua nonof terrestrial
life and as thousands of new planets have
been discovered orbiting faraway stars, the
greatest levels of excitement have usually
been reserved for those in the “habitable
zones” of their systems—in other words
orbiting at a distance where liquid water
could exist on the planet’s surface.
The next step has been to look for bio
signatures—molecules which might be
tray the existence of biological processes.
These could include oxygen or methane in
a planet’s atmosphere. On Earth, those
molecules persist only because living
things constantly regenerate them.
The problem with both these approach
es is obvious—they are restricted to find
ing life as currently known. But, as Natalie
Grefenstette, an astrobiologist at the Santa
Fe Institute in New Mexico, points out, “we
don’t know if other forms of life would
necessarily have the same signatures, if
they would have the same metabolisms, if
they would be based on the same genetic
molecules or any of the same molecules at
all.” Life on Earth could have evolved in the
way that it has because the specific chem
istry of the planet at crucial times gave rise
to selective pressures which might not be
present on other worlds. “And so we’ve
been thinking—if life were different, how
do you even look for that?”
Exotic beasts and how to find them
From May 16th20th, at AbSciCon, a bien
nial astrobiology meeting organised by the
American Geophysical Union and held this
time around in Atlanta, Georgia, astrobiol
ogists including Dr Grefenstette consi
dered that question and discussed ways to
expand their searches in the coming de
cades, so that they might stand a better
chance of recognising more exotic forms
of life than are currently being sought. To
do this, they will need several strategies.
The first begins by imagining the va
rious different chemistries which exotic
forms of life might employ, and using
those to devise a wider set of potential bio
signatures. On Earth, the most important
molecules of life almost all involve carbon
atoms. These are particularly versatile be
cause they can form chemical bonds with
up to four other atoms, including other
carbons, to create complex molecular
structures. Carbon is the fourth most
abundant element in the universe and the
molecules it forms can survive for long pe
riods in the sorts of temperatures and pres
sures found on Earth’s surface.
An exotic lifeform might plausibly,
however, rely on silicon instead of carbon.
Silicon sits just underneath carbon in the
periodic table and thus shares with it the
ability to bond with up to four other atoms.
Familiar examples of the results are most
of the huge diversity of minerals which
make up rocks, for silicon is the second
most common element in Earth’s crust. It
is also the seventh most abundant in the
universe, which means there is plenty of it
available for potential siliconbased life
forms to use.
Alien life might, though, have its roots
in something yet more exotic. In the lab
oratory, metal oxides known as polyoxy
metalates have shown some remarkably
lifelike abilities, such as being able to form
membranes (dubbed “inorganic chemical
cells” by Lee Cronin, a chemist at the Uni
versity of Glasgow) and being able to as
semble, with some chemical help, into
Until now, astrobiologists have been looking for aliens that resemble
life on Earth. That will change