The EconomistFebruary 13th 2021 BriefingAstrobiology 672
1chemical disequilibriumwith their sur-
roundings could be a sign of life. Oxygen is
a reactive gas that would not build up in
Earth’s atmosphere in normal conditions.
As a by-product of photosynthesis, how-
ever, it is being replenished continuously.
Given the presence of so much oxygen, the
simultaneous persistence of methane in
Earth’s atmosphere is also inexplicable
without lifeforms that keep producing the
gas. Normal abiotic chemistry would oth-
erwise quickly deplete it.
Other gases likely to be biosignatures
include nitrous oxide, methyl chloride,
isoprene, ammonia and phosphine. In-
deed, Sara Seager, an astrobiologist and as-
trophysicist at the Massachusetts Institute
of Technology, has identified more than
14,000 small, volatile molecules, of which
a quarter are produced by life and others
potentially so. This greatly increases the
number of potential quarry for future as-
trobiologists to hunt. Meanwhile, laborato-
ry experiments and computer models that
can characterise the sources and life cycles
of these gases in different types of atmo-
spheres will help the understanding of fu-
ture data collected about exoplanets.The edge of reason
Other biosignatures might come from a
planet’s surface. On Earth, a phenomenon
called the red edge is a sign of oxygenic
photosynthesis. Chlorophyll, the plant
pigment that captures the light which pro-
vides the energy for photosynthesis, ab-
sorbs most visible frequencies emitted by
the sun but reflects the longer wavelengths
of infrared light. This sharp change in re-
flectance can be spotted easily from space.
With due acknowledgment that photo-
synthetic life on other planets would al-
most certainly employ other pigments,
tuned to absorb the electromagnetic fre-
quencies emitted from their parent stars in
the way that chlorophyll is tuned to sun-light, this method could be adopted to look
for “plants” elsewhere. Near an m-type star,
for example, some astrobiologists’ models
suggest that planetary vegetation tuned to
local conditions could reflect yet longer
wavelengths than those reflected by Earth.
Alternatively, a planet might be dominated
by light-harvesting organisms similar to
Earth’s purple bacteria, which thrive in an-
oxic conditions and produce sulphur as the
waste product of their photosynthesis,
rather than oxygen. Yet other pigments,
each with its own spectral signature, might
have jobs beyond photosynthesis, such as
protection against harsh radiation.
None of this study will be easy, particu-
larly when the molecules under investiga-
tion are dozens or hundreds of light-years
away. The James Webb telescope will begin
by looking for biosignatures in the atmo-
spheres of planets around m-type stars, but
may struggle to do the same for those orbit-
ing brighter g-types. Examining the sur-
faces of planets, understanding atmo-
spheric dynamics, looking for continents
and detecting surface biosignatures will
have to wait until direct-imaging technol-
ogy is sensitive enough to reach across the
light-years and record something useful.
That might happen by the mid 2030s,
when three ground-based telescopes with
mirrors 25-40 metres wide, which should
start operating later this decade, get into
full swing. These are the Giant Magellan
Telescope, in Chile, the Extremely Large
Telescope, also in Chile, and the Thirty Me-
tre Telescope, proposed for Hawaii. Their
observations may be complemented by im-
ages taken by two proposed nasaspace-
craft, luvoirand HabEx. If approved, these
could fly in the late 2030s. luvoirwould be
a general-purpose successor to the James
Webb. HabEx would be designed specifical-
ly to take pictures of habitable planets.
Finding one particular chemical on an-
other planet will never be a clear cut indica-tor of life. Volcanoes also produce some of
the molecules associated with biology, so
the risk of false positives is high. Even oxy-
gen is not foolproof. It can be generated
abiotically when water molecules are split
into their constituents by high-energy ra-
diation from a parent star. Conversely, a
planet with life on it may not yet have de-
tectable levels of oxygen in its atmosphere
(which was indeed the case with Earth for
much of its early history). For astronomers,
this means placing potential detections of
biomolecules into the wider context of the
planet under study.
Building catalogues of non-biological
sources of gases will help to fine-tune
models and give astrobiologists a better
chance of weeding out false positives. But
Charles Cockell, an astrobiologist at Edin-
burgh University, says a more robust ap-
proach would be to collect spectrographic
data from lots of exoplanets and thereby
create better statistical confidence in indi-
vidual detections. If astronomers had at-
mospheric data from tens of thousands of
them, for example, and a thousand showed
strong signals for oxygen, that would build
confidence about the oxygen being from
biological sources, rather than the results
simply being false positives.
This, though, is to assume the process
of detection itself is robust. An instructive
tale here is the recent debate over whether
or not there is phosphine in the atmo-
sphere of Venus—for this is a gas which, on
Earth, is created only by living organisms
(some of them, admittedly, human chem-
ists). In September, a group of astronomers
announced that Venusian air contained 20
parts per billion of phosphine. Others who
subsequently scrutinised these results
raised red flags. Some questioned the way
the original team had processed the data.
Some tried to find evidence for phosphine
in independent data sets, and failed. Partly
in response to those criticisms, the origi-
nal team later reanalysed the data them-
selves, and concluded there was, after all,
only a tentative detection of one part per
billion of phosphine present on Venus.Venus sky trap
Spectroscopic analysis of Venus’s atmo-
sphere in this way can be viewed as a test-
bed for the harder task of doing the same to
the atmospheres of exoplanets. In the case
of Venus, though, if the signs do end up
looking good, it will be possible to go and
check directly—the second of the broad ap-
proaches to astrobiology. That the Venu-
sian atmosphere may have a biomarker in
it came as a surprise even to those optimis-
tic about finding life elsewhere. Most such
eyes are turned to Mars, with a side-bet on
the icy moons of Jupiter and Saturn.
Mars has already generated a couple of
intriguing results. One was from the so-
called labelled-release experiments car-The geysers of Enceladus