New Scientist 2018 sep

8 September 2018 | NewScientist | 39

F

EW discoveries could be bigger than

detecting life on another planet. Whether

it is a rocky ball or a giant cloud of gas,

hot, cold, or somewhere in between, we aren’t

picky: so long as a world has life, we want

to find it.

For as long as we have searched, we have had

one image in mind: Earth. It might seem like

vanity, but our focus makes a certain amount

of sense. After all, Earth is the only planet in

the universe that we know for a fact supports

life. Even if faraway exo-Earths don’t have

oceans, continents, rainforests, deserts and

polar caps, the long-standing assumption is

that they will still be familiar in certain ways.

There will be water on the surface, oxygen in

the air, possibly even vegetation on the land.

But Earth hasn’t always looked the way it

does now. In the 4.5 billion years our planet

has existed, it has experienced dramatic

transformations: ice ages and warming

periods, times when the atmosphere was

twice around Earth, using the planet’s

gravitational pull as a slingshot to power it

on its way to its ultimate goal, Jupiter. The

astronomer Carl Sagan suggested taking the

opportunity to point our best alien-finding

equipment at Earth itself. If it found nothing,

he reasoned, that meant life elsewhere might

also escape our attention.

Galileo was to be the first of several probes

to use their instruments in this way, and they

confirmed that our technological civilisation

should be detectable by a distant, similarly

advanced alien civilisation.

Where there are no hints of technology

to home in on, by far the best life sign to

latch on to is oxygen. Its abundance in our

atmosphere – 21 per cent – would be difficult

to sustain without plants pumping it out

constantly. Methane, released by bacteria

as well as flatulent livestock, is a more

ambiguous biosignature, given that a number

of non-biological mechanisms can produce

the gas.

A bigger giveaway than either oxygen

or methane on their own, however, is the

presence of both. That is because the

combination is combustible and releases

energy when it reacts, forming carbon dioxide

and water. “If these two gases are left to

their own devices, they’ll usually annihilate,”

says Chris Reinhard at the Georgia Institute

of Technology. “The fact that they’re both

present at relatively high abundance

suggests that they’re being pumped into

the atmosphere at very high rates, potentially

by biology.”

Another reliable sign of life is the light

reflected by plants. Although our planet’s

vegetation absorbs most visible wavelengths,

with the exception of green, it absorbs far less

infrared light. The upshot is an abrupt jump –

known as the red edge – in Earth’s reflectance

spectrum, a phenomenon that would be hard

to replicate without living organisms.

Searching for high levels of oxygen,

methane coexisting with oxygen, or a red

edge would be a good way of picking up

planets that look like Earth now. But Earth has

been inhabited – not to mention inhabitable –

for far longer than it has displayed any of

these features (see diagram, page 41).

The earliest known life forms emerged

about 4 billion years ago, when the planet’s

crust was cooling to form rocks and the

beginnings of continents. At this time, known

as the Archaean, Earth’s atmosphere would

have been dominated by carbon dioxide

produced by active volcanoes. In this hostile

environment, primitive microbes emerged

Lessons from

early Earth

Our deepest history could hold the key to the

search for life on other planets. Kelly Oakes reports

>

impossible to breathe, when large areas were

desert, or when lush tropical forests hugged

the poles. Throughout the vast majority of this

turbulent history, life has somehow clung on.

If, armed with a spotters’ guide to the world

we inhabit today, we found exoplanets

resembling those early Earths, would we even

recognise them for what they were? Maybe

not. We know how to seek comparatively

advanced signs of intelligent life, such as

cacophonous radio transmissions and the

bright lights of megacities. If a planet has less

sophisticated inhabitants, however, we must

rely on identifying other signatures of life.

That is why scientists interested in filling in

the blind spots of our search for intelligent

life have started a lot closer to home. They

want to imagine how early Earth would look

if seen from far outside the solar system.

The first time we tried to study our planet

from afar was when the Galileo mission

launched in 1989. It was programmed to orbit