8 September 2018 | NewScientist | 41for vegetation, they were probably swamped
by purple mats of single-celled bacteria. If
these tiny organisms were present in great
enough numbers, says Pallé, their effect on
the light reflected from Earth would be similar
to that of vegetation, but shifted toward the
far red end of the spectrum. “You can envision
a whole bunch of colours you can get,” says
Abel Méndez, director of the Planetary
Habitability Lab at the Arecibo Observatory
in Puerto Rico, depending on which bacteria
are most common.
Attractive though this sounds, its signal
could be weak and tough to spot from afar.
Olson and her colleagues think they have
identified two more promising avenues. The
first could be to observe planets over a long
period, instead of just getting a snapshot of
their atmosphere’s composition. Observing
Earth in this way, for example, would reveal
a seasonal change in atmospheric carbon
dioxide levels. That is because plants use more
carbon dioxide during their growing season,
with the northern hemisphere dominating
the effect because of its greater land mass.
“You’d see that it’s kind of wobbly up and
down once a year,” says Reinhard.
On plant-free planets like Proterozoic
Earth, it is not likely that photosynthesis by
microbes would be enough to cause clear
oscillations in carbon dioxide levels. Instead,
respiring organisms might produce similar
seasonal variations in oxygen. And although
oxygen levels themselves might be too
low to spot from afar, the effect of their
fluctuations on levels of other chemicalssuch as ozone could conceivably be picked up.
One downside of using seasonality as a
biosignature is that some worlds don’t have
seasons. Méndez points out that planets
around the smaller, dimmer stars that are
Earth’s closest neighbours must orbit close
to their star to be potentially habitable, but
doing so means they tend to end up keeping
the same face pointing toward their star all
the time. “They are tidally locked,” he says,
“so you won’t have any seasons.”Delicate imbalance
The other idea Olson and her colleagues are
working on could prove more fruitful. It
involves rethinking how life might influence
the make-up of an atmosphere. Much as
methane and oxygen would not persist together
on Earth if all present-day life disappeared,
there are other combinations of gases that
scientists regard as being in disequilibrium –
that is, they would be hard to sustain without
life. During the Archaean, for example, such
was the imbalance of atmospheric methane
with carbon dioxide, nitrogen and water that
it would have been rapidly wiped out as soon
as it stopped being produced.
But it isn’t necessary to look for all of
those gases at once. Olson’s team argues
that you need only see carbon dioxide and a
sufficiently large amount of methane together
in an atmosphere to realise something
biological is probably afoot. And although the
relative abundance of oxygen and methane
would probably not have been measurablefrom afar at any point in Earth’s history, that
of carbon dioxide and methane might be.
“A detectable carbon dioxide and methane
disequilibrium is more likely on a broader
range of planets,” says Olson, “including
those with undetectable levels of oxygen.”
Butterfield thinks looking for these
atmospheric imbalances is an interesting idea,
but cautions against falling for alien life as
the explanation. “Just seeing disequilibrium
is interesting,” he says. “But it doesn’t
necessarily have to be biological activity.”
Although false positives are inevitable,
Olson is hopeful that clues like disequilibrium
and seasonality will help fill in some of those
blind spots in our search for life. For one
thing, says Olson, “they’re not tied to specific
metabolisms”. Alien life wouldn’t necessarily
need to be like us, or even be carbon-based,
for these potential biosignatures to reveal its
existence on a distant planet.
Getting a good enough look to find out,
however, isn’t going to be easy. Figuring out
what is going on in the atmosphere of an
exoplanet means gathering as much light as
possible from it. But of course, planets don’t
produce light; they only reflect it, and that
signal is dwarfed by the light of their star.
To separate them, we will need enormous
telescopes like NASA’s James Webb telescope,
due to launch in 2021, or the next generation
of extremely large, ground-based telescopes.
Even with these devices, the task will be tricky.
Seasonality on an exoplanet will require so
much telescope time, says Pallé, that “we will
not be able to measure that, not as long as you
or I are alive”.
No single measurement is ever going to
be conclusive. By looking at the make-up of a
planet’s atmosphere, how it changes over time
and anything unusual that appears to be going
on at the surface, researchers will instead
build a slowly evolving picture of that world’s
chances of hosting life. “It’s not going to be
like a discovery where you dig something
and you say, ‘That’s it! I found it’,” says Pallé.
“It’s a process where we are slowly choosing
our best candidate.”
Our continuing ignorance of aspects of
Earth’s primordial past could also hamper
the hunt for distant life. What set off the
snowball Earths of the Proterozoic, for
example, is still a mystery. Even if we spot
an exoplanet in an ice age, says Reinhard,
we won’t know how to interpret what we see.
One of the biggest discoveries in history could
elude us once again. ■Kelly Oakes is a freelance writer based in London103
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10 -6Earth through the ages
The characteristic signals of present-day life on Earth include the coexistence of methane
and oxygen in the atmosphere. But life existed on the planet for billions of years before the
atmospheric composition we see today formed
ARCHAEANEarliest signs
of lifeOxygen appears in the atmosphere First land
plants appearPhotosynthesis developsSOURCE: ARXIV.ORG/1803.05967
Relative atmospheric concentrationTODAYGlaciation
”Snowball Earth”Plate tectonics resemble modern EarthPROTEROZOIC PHANEROZOICbyaOxygenMethane4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0