New Scientist - USA (2019-08-31)

31 August 2019 | New Scientist | 45

Mars. Alternatively, some have suggested that

life could reside on Saturn’s moon Titan,

swimming in its lakes of liquid methane.

Whatever we find on these nearby worlds,

I am confident life exists elsewhere in the

universe. But confidence isn’t enough. Over

the next few years, our searches are going to

become more accurate, more thorough and

capable of looking further than before.

The answers we find stand to fundamentally

shift our understanding of the universe and

our place in it. As the science fiction writer

Arthur C. Clarke put it: “Two possibilities

exist: either we are alone in the universe

or we are not. Both are equally terrifying.”

To my mind, finding alien life would humble

our apparently exalted status in the cosmos.

We would be just one more example of life as

a planetary process, crystallising out of the

molecules that make up our universe.

Searching widely and finding nothing would

be equally sobering, however, indicating that

even in environments we think of as habitable,

the chasm between chemistry and simple life

is vast. Hopefully, such an appreciation of

life’s rarity would lead us to protect all forms

of existence on our own world, reminding us

that Earth is the only home we have.

The next two decades will witness a

revolution in exoplanetary science. We have

already found dozens of potentially habitable

worlds and the next technological advancement

in observations will be able to detect potential

biosignatures in their atmospheres. Now we

need to watch – and wait. ❚

molecules respond to different wavelengths

of light, and by separating the light we collect

in our telescope into different wavelengths,

we could see the telltale spectra, or light signals,

produced by substances such as oxygen,

ozone, methane, water and carbon dioxide.

What makes it such an exciting time to work

in this field is the number of missions being

developed to perform this task. The first

of these will be NASA’s James Webb Space

Telescope, scheduled to launch in 2021. This

will be our first hope at identifying molecules

in the atmosphere of a habitable exoplanet.

ARIEL, a European Space Agency mission due

to launch in 2028, will continue this effort.

Another promising technique involves

using large ground-based telescopes to do

the same thing. These include the European

Southern Observatory’s Extremely Large

Telescope, currently being built in Chile and

due to start working in 2025. Observing planet

atmospheres from Earth’s surface is difficult

because you must first remove our planet’s

atmosphere from the signal. Next-generation

ground observatories will be able to do just

that by subtracting its effects from the light

entering the telescope. This detailed technique

can even allow us to distinguish isotopes

on other worlds, subtly different versions

of the same atoms that differ only by the

presence of a single neutron in their nuclei.

That is something I never dreamed would

be possible in my lifetime.

For all the excitement surrounding far-flung

planets, perhaps the first successful detection

of extraterrestrial life will happen closer to

home. Certainly, other places in our solar

system have conditions suitable for life as

we know it, such as in the liquid water ocean

hidden beneath a thick ice layer on Jupiter’s

moon Europa or in the subsurface water on

Sarah Rugheimer is an

astrobiologist at the

University of Oxford, UK

DRAKE EQUATION

Quiet neighbourhood

Frank Drake’s 1961 equation remains the best method to get a rough sense of how many detectable alien

civilisations should exist within our galaxy (N). According to the latest data, that number is somewhere

between 1 – our lonely selves – and an impressive 4 billion

N= R* x fp x ne x fl x fi x fc x L

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on Earth by anaerobic microbes, which don’t
rely on oxygen to survive. Not only would it
be relatively easy to detect in an exoplanet’s
atmosphere, but it is the simplest gas that
can’t be produced by any natural processes
we know of. Detecting phosphine, in other
words, could indicate an anaerobic biosphere.
If coming up with such hypotheses seems
challenging, putting them to the test is
something else entirely. The first step is to
identify candidate exoplanets: those with the
right temperatures to nurture the complex
chemistry needed to sustain life. At present,
finding worlds beyond our solar system is
usually done by looking for the slight dimming
that happens when a planet crosses in front of
its star. It is a process hundreds of times more
difficult than spotting a firefly crossing a
searchlight on the other side of the Atlantic.
This detection method also opens the door
to sensing different types of molecules in the
atmosphere of a temperate and rocky planet.
For example, when light from a star passes
through the air cloaking such worlds it can
reveal the composition of that air. Different