Sky & Telescope - May 2021 - 36

Looking for Life

In looking for biosignatures on distant worlds, astrobiologists are mostly focusing at the moment on chemical compounds in a planet's atmosphere. Find the right molecules in
the proper context, and it could be circumstantial evidence
for life.
But to get to this point, astrobiologists must ﬁrst construct
a methodology for identifying reliable, resilient, and detectable biosignatures. That might sound boring, but it's crucial:
We can't ﬁnd life's ﬁngerprints if we don't know what to look
for or how to interpret it once we ﬁnd it. Hints of methane on
Mars (S&T: Jul. 2019, p. 9) and the recent tentative detection
of phosphine in the clouds of Venus (S&T: Mar. 2021, p. 9)
remain contentious in part because we don't fully understand what could create these signals. As exciting as it is to
reach out into the solar system and the galaxy, the work of
understanding what we ﬁnd begins back here on Earth, with
fundamental research in three ﬁelds: quantum chemistry,
molecular biology, and atmospheric science.

A Work in Progress
There are probably plenty of exo-Earth candidates in our line
of sight. And the next generation of instruments will give us
unprecedented views of these worlds' atmospheres. But before
we get too excited about any potential biosignatures, we need
a better understanding of the basic nature and behavior of
molecules under a wide range of temperatures. This is the
purview of quantum chemistry, the ﬁrst pillar of the biosignature framework. Scientists do this work both in the lab and
computationally.
Biology is the next pillar. Speciﬁcally, we need to know
all the gases that all the various lifeforms on Earth produce.
There are a few groups tackling this monumental task, utilizing both analog methods (graduate students) and machine
learning to scroll through a vast agglomeration of scientiﬁc
literature and compile useful databases.
Then comes atmospheric science, which takes a litany of
chemical information from the above-mentioned efforts and

Key Constituents of Earth's Atmosphere,
Relative Abundances
Groundtruth Earth

Galileo
Value

Thermodynamic
Equilibrium

0.78

0.21

0.19 +/- 0.05

0.21
(ignoring the crust's
under-oxidized state)

H2O

0.03-0.001

CH4

1.6 ×

Molecule

10 -6

0.01-0.001
3 +/- 1.5 ×

10 -6

0.03-0.001
<10 -35

S TOO MUCH METHANE These data from a 1993 paper by Carl Sagan
and others highlight the disparity between the methane level expected
in a steady-state atmosphere and that found in Earth's atmosphere.
Methane should not survive more than a decade or so in our oxygen-rich
atmosphere without being replaced, they noted.

M AY 2 0 2 1 * S K Y & T E L E S C O P E

EXO-EARTHS
Based on Kepler data, astronomers estimate
that there should be four rocky planets in the
habitable zones around G and K dwarfs within
30 light-years of the Sun.

plugs it into a computer code that simulates how everything
would behave in a planetary context. This allows scientists to
model gases' abiotic production, abundances, lifetimes, movements, and altitudes.
The ﬁrst time these specialties were brought together
to look for signs of life from space, the effort was a stunning success. The year was 1990, the spacecraft was NASA's
Galileo, and the planet, Earth. (We'd stacked the deck.) At
a distance of about 1,000 kilometers (600 miles), Galileo
detected an unstable combination of methane and oxygen in
our atmosphere, which researchers interpret as one of Earth's
particular life signs. A similar chemical cocktail on another
world might also be a biomarker.
But 30 years later, astronomers are still trying to understand the total context and composition of Earth's biosphere,
both in the present and throughout geologic time - and then
use that to aid in the search for life elsewhere.
" It's not just about ﬁnding liquid water, or oxygen, or even
methane and oxygen together, " Sarah Rugheimer (Oxford
University, UK) says. " There's the whole planet to consider,
and with that comes so much potential for false positives.
On top of all this is the fact that we don't really understand
exotic chemistry very well, which I think is what Venus is
showing us. "
Venus has taken center stage in the biosignatures hunt
thanks to the 2020 announcement by Jane Greaves (Cardiff
University, UK) and others that there's phosphine in the cool
cloud deck of our sister planet. On Earth, phosphorus and
hydrogen don't tend to get together to make phosphine unless
life forces their hand. On Jupiter and Saturn, the chemical
forms thanks to the extremely hot, high-pressure environments deep below. But the conditions on our planet's evil
twin are nothing like those distant cousins, the gas giants. So,
could phosphine form on Venus abiotically? We don't know
yet because, until recently, we had no reason to ﬁnd out.

Quantum Chemistry, Lab Edition
There is a bias in exoplanet data towards very hot worlds
- worlds like WASP-79b, a gas giant where the clouds are
1500°C (2700°F). This is because both of the most successful
detection techniques used by astronomers (radial velocity and
transit) tend to ﬁnd planets very close to their stars.
The transit method allows us to observe the way the starlight changes when the planet passes in front of its host. The
speciﬁc wavelengths that the planet's atmosphere absorbs give
us information about the chemicals therein. But interpreting
these data has proven difﬁcult.

Sky & Telescope - May 2021

Table of Contents for the Digital Edition of Sky & Telescope - May 2021