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Advances in Astrobiology
RICHARD BLAUSTEIN
Collaboration, new technologies deepen understanding of life’s origins.
D
uring a summer lunch in 1950 at
Los Alamos National Laboratory,
physicists Emil Konopinski, Edward
Teller, Herbert York, and Enrico Fermi
reputedly discussed the possibility of
advanced extraterrestrial life, which
was a focus in the US media that
week. “So where are they?” Fermi
was quoted as asking about these
life forms. The Nobel Prize–winning
Fermi, renowned for his probabilistic thinking, held enough sway that
his reported utterance became known
as the Fermi paradox—a test question that asks, “If the universe is vast
enough for numerous possibilities of
life and civilizations, spanning trillions
of star systems and having existed
for 13.8 billion years, how is it that
humans have not received one incontrovertible signal from intelligent life?”
To answer that question, scientists are
collaborating across disciplines as never
before to methodically think through
the possibilities of how life might evolve
and where in the cosmos it might be
found. The field of astrobiology is looking at the broad probabilities, the geological and astronomical parameters for
life in our galaxy, and Earth history in
order to understand determinants and
signatures for extraterrestrial life.
Spurring progress in the field is the
development of new telescopic technologies that have opened up experimental
possibilities for assessing extraterrestrial life, although there is a very long
way to go. The epochal discovery of
exoplanets—planets that circle suns in
other solar systems—began only in the
1990s. In the past 5 years, the Kepler
Kepler telescope discoveries were far ranging, including Kepler-47, illustrated
here. The image shows two self-orbiting stars sharing a planetary system. The
outer planet fits within the “habitable zone.” Although Kepler-47 probably is
a hot, gaseous planet without life, it may have water. Image: T. Pyle, NASA Jet
Propulsion Laboratory, Caltech.
space telescope has greatly expanded
the discovery, confirming over 1000
planets and listing another 4000 as candidate planets. These discoveries have
encouraged scientists to think about
the parameters for planetary systems
having habitable zones and signaling
biosignatures that might identify a
planet’s having biology at work.
Scientists are also taking stock of
where they are in the field and what
questions need to be addressed. Recent
2015 papers, one by Massachusetts
Institute of Technology (MIT)
astrophysicist Sara Seager and MIT
biochemist William Bains, which
­
focuses on the exoplanet atmospheres
(Science Advances, 2015), and another
by Georgia Tech chemists Nicholas
Hud and Brian Cafferty, which focuses
on the RNA origin of life hypothesis (published in the Israel Journal of
Chemistry), testify to the refinement
going on in these robust inquiries.
Looking inward
As its starting point for understanding the biological possibilities beyond,
astrobiology has supported and integrated investigations of the origins of
life on Earth. Recent RNA-relevant
investigations have widened life-origins possibilities for Earth and other
planets as well.
“I think one of the motivations,
although it’s not always articulated,
BioScience 65: 460–465. © 2015 Blaustein. All rights reserved.
doi:10.1093/biosci/biv043
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for doing something like exploring
Mars and looking for extant or extinct
life there, is precisely the question of
‘what if?’—what if you restarted the
clock on Earth, what if life evolved
under radically different conditions,
what if biology turns out a bit differently?” says Caleb Scharf, Columbia
University astrophysicist and astrobiologist and author of the 2014 book
The Copernicus Complex: Our Cosmic
Significance in a Universe of Planets
and Probabilities. “This is an incredible
test of the stochasticity of evolution at
all levels. We’ve got a single 4-billionyear-long experiment on Earth (as far
as we know)—we’d like to find another
experiment somewhere.”
Scharf also highlights that astrobiology is distinctly an interdisciplinary
effort. The field encompasses work on
theory, origins of life research, physics,
astronomy, and engineering advances
in telescopes and other means of
detection. So, to the challenge of the
Fermi paradox, astrobiology appears
determined to do a generation or more
of serious science.
Starting point: Origins of life
on Earth
Biological research into the origins
of life is central to astrobiology. The
NASA Astrobiology Program, a leader
in the field, has six departments and
funds various investigations, including
origins of life research. The terrestrial
explorations include those of the biology of extreme environments, such
as the microbial communities buried
deep below the surface; South African
mines, where microbes acquire heat
through geothermal radiation; and
the Antarctica subsurface lakes, which
might shed light on the moons of
Jupiter that are thought to have oceans
beneath their frozen surfaces.
Microbiologist Mary Voytek, who
heads NASA’s Astrobiology Program,
explains that the term astrobiology
is all inclusive and suggests that the
chemistry that spawns the biology of
Earth is the same found throughout
the universe. Voytek says her own
biological work on life in extreme
environments, which included a
http://bioscience.oxfordjournals.org
Astrobiologist Caleb Scharf of Columbia University asks, “What if life
evolved under radically different conditions? What if biology turns out a bit
differently?” Photograph: Albert Chau, Grumpy Bert.
dissertation on Antarctic life and work
on the Chesapeake Crater, got her
interested in varied possibilities for
life origins. “I am also interested in
biogeochemical cycles, and I have long
wondered how the iteration between
the environment and the biosphere
led to the evolution of systems that we
have in complete ecosystems, as well
as the adaptations we see in microbes,”
Voytek says.
According to Voytek, although these
extreme environments are interesting
in and of themselves, they might best
represent the locales where life originated on Earth. They also serve as analogues for conditions beyond Earth. “I
think it is essential if you are going to
look for life elsewhere to understand
how it evolved and emerged here on
Earth, to understand the diversity that
we observe on Earth. That could lead
us to an understanding of what might
be a universal biology,” Voytek says.
“We have one example of life: the
one we have on Earth,” she adds. “Do
we look for the same kind of molecules
to do the work in extraterrestrial cells,
or is there some way to look at an
environment and still find what we can
define as life and not based 100 percent
on what we have here? But certainly,
our quest for coming up with criteria
for what we look for begins with what
we know about life on Earth.”
Two major focuses for origins of
life research are the metabolism-first
hypothesis and the more well-known
and established RNA-world hypothesis. The metabolism-first idea looks
at how energy was provided to simple monomer organic molecules that
joined in a chain of linkages and transformations that eventually manifested
as DNA and the cell. Independent
inorganic energy sources, such as
hydrogen-energy generation at
extreme environments, may have
formed, been geologically sustained,
and offered the energy to make these
organic linkages. That is one possibility. Another more conventional view
holds that the monomers linked spontaneously, and, eventually, a genetically
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based energy capacity developed in the
early biotic amalgamation. This notion
is integrated within the RNA-world
idea, to some extent. Voytek says that
the metabolism-first investigations are
important for understanding how precursors to cellular life took in energy.
The RNA-world line of thought is
more advanced, having received early
support from prominent scientists
such as Francis Crick in the 1960s.
In the 1980s, the RNA-world view
became dominant in life-origins thinking, when the labs of Tom Cech at the
University of Colorado, in Boulder,
and Sidney Altman of Yale University,
in New Haven, showed that naturally
occurring RNA molecules, which are
much simpler than DNA molecules
are, could catalyze reactions, make
proteins, and store information, a discovery for which the two lab heads
were awarded the Nobel Prize.
This finding established that an
organic polymer—an organic molecule with many monomer linkages and
atoms—could self-organize within the
early Earth chemistry. But there are
questions as to whether the earliest
prebiotic chemistry would have been
conducive to RNA formation, and so
specialists in astrobiology are taking
the investigation further back, exploring the pre-RNA chemistry that characterized prebiotic chemistry.
Georgia Tech University chemist
Nicholas Hud receives NASA support
for his lab’s origins of life research, and
his work is distinguished for exploring the precursor to RNA that might
have been found in the early Earth
environment. These precursors could
have had parts replaced over time,
eventually evolving into RNA. Hud’s
work offers a range of analogues for
life origins that could shed light on the
biochemistry on other planets.
“We are particularly interested in
molecules of polymers that would have
properties similar to RNA but [are]
more likely to have formed spontaneously on early Earth,” Hud explains.
“When we look at the chemistry of
RNA, the chemistry, and the linkages
and molecules involved, it looks difficult from a chemical perspective if
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Shrimp (Rimicaris hybisae) live in symbiosis with bacteria some 2300 meters
underwater. This ecosystem gives scientists insights into what kind of life could
thrive in extreme environments elsewhere, such as on Jupiter’s moon Europa.
Photograph: Chris German, WHOI, NSF, NASA, ROV Jason © 2012 WHOI.
Is Earth Unique?
The rare Earth hypothesis posits that although microbial life found beyond Earth is a
strong possibility, the chances of complex life in our galaxy are remote or nil. The
hypothesis looks at the species-threatening hurdles in our planet’s natural history
and key biological determinants, such as the amount of carbon dioxide in the atmosphere. Paleontologist Peter Ward and astronomer Donald Brownlee prominently
articulated this view in their 2000 book Rare Earth: Why Complex Life Is Uncommon
in the Universe, and science writer and astrophysicist John Gribbin added to this
viewpoint with his 2011 book Alone in the Universe: Why Our Planet is Unique.
Ward and Brownlee hypothesized that microbial life on Earth has persisted for close
to 4 billion years and complex life has endured since 545 million years ago because
of the uniqueness of Earth. Plate tectonics, for example, recycles calcium and silica
that restrict the atmospheric greenhouse gases from either carbon dioxide freezing,
such as on Uranus, or the runaway phenomenon, in which greenhouse gases build
up and prevent the release of surface heat into space, as is thought to happen on
Venus. Plate tectonics also forms continents and rocky terrains surrounded by surface water, ideal habitats for life. And, in our solar system, plate tectonics exists only
on Earth. Moreover, the unique core of the Earth produces its magnetic field, which
fends off cosmic radiation. Earth also has an ozone layer that repels ultraviolet
light. Both cosmic radiation and ultraviolet light would be perilous for complex life.
The rare Earth argument maintains that the fossil record suggests that it took almost
3.5 billion years for the formation of complex life, during and after which time, life
forms had to endure huge extinction events. Other lucky factors for Earth include
its distance from the Sun, allowing it to maintain an atmosphere and surface water;
Jupiter’s shielding many asteroid and meteor impacts; and a moon with the orbit
and gravity force to help lock in a seasonally auspicious tilt to the Earth.
The rare Earth hypothesis concludes that the hurdles of creating complex life are so great
that Earth’s rich biodiversity, including intelligent life, is truly a unique phenomenon.
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you don’t have the assistance of highly
evolved protein enzymes.” Hud says
that the contributions of geologists,
atmospheric physicists, and other
Earth scientists have facilitated the
understanding of the chemical parameters of prebiotic chemistry.
Hud and his team have shown that
molecules that are different from the
nucleic-acid bases of RNA could have
attached to the forms or analogues
of ribose and sugar that resemble the
backbone of RNA. These molecules
were more likely to have been available
in the prebiotic mix early in the Earth’s
history.
For example, Hud has demonstrated
that 81 different molecular possibilities could have been the precursor for
adenine. He explains that this exhibits
the “robustness” that origins of life
biologists seek because it shows that
many self-organizing molecular possibilities could have easily existed within
the restrictive prebiotic mix for this
one life function.
Hud’s lab also examines the solute
context for this beginning of life and
explores contexts in which water is
available but would not swamp the
early monomers, preventing them
from concentrating and amalgamating. Some cycling of dry and wet
conditions (dry days and wet nights)
would support the early linkages.
Scharf finds Hud’s nuanced
approach to life representative of the
maturity of astrobiology. “You have
to have an environment where you go
through these hydration–dehydration
cycles—day night is a pretty good one,
or tidal cycles. For me, the fascinating thing is that it comes from the
astrophysics. You have a planet that
is spinning,” Scharf says. “That is one
example of the beauty of astrobiology, this cross-fertilization of ideas.
Suddenly, something becomes important that you might not have thought
about—spin of a planet, the day length,
for example.”
Voytek agrees with Scharf and further explains the importance of Hud’s
work. “Hud’s and other groups have
done a great job looking at how you
get the precursor… through natural
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Georgia Tech chemist Nicholas Hud explores how life got started on Earth,
which should shed light on possible origins of life on other planets. Photograph:
Georgia Institute of Technology.
Habitable zones are regions where planets have Earth-like properties. The
conservative habitable zone is the more restrictive with regard to climate and
carbon dioxide than is the optimistic habitable zone, according to James Kasting
of Penn State. He and his students look to ancient Mars and recent Venus. In
the image, temperature is expressed in degrees kelvin (K), and the numbers and
letters next to the planets refer to the Kepler planet discovery numbers. Graphic:
Image created by Penn State graduate student Chester Harman with data from
Dr. Ravi Kopparapu, as well as planet images from Planet Habitability Lab at
the University of Puerto Rico–Arecibo and NASA Jet Propulsion Laboratory.
processes, through a system where you
have heating and cooling, or evaporation and rewetting, or you begin with
other small molecules,” Voytek says.
“To me, looking at the parameter space
of prebiotic chemistry gives you many
more options for what might have
occurred someplace else.”
Habitable zones and biosignatures
In the mid-1990s, astronomy, cosmology, and astrobiology (then commonly
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referred to as exobiology) were revolutionized with the conclusive finding
of exoplanets. Penn State planetary
scientist James Kasting recalls the
great surprise in the field when these
discoveries were made by a team in
an observatory in Switzerland and
quickly corroborated by another in the
United States. The findings were all
the more jolting because the planets
were Jupiter size, orbiting close to their
suns in 4-day cycles. The Jupiter in our
solar system has about a 12-year orbit.
“It was a complete surprise. We had no
idea that there were these hot Jupiters
out there,” Kasting says.
A few years before, Kasting published a paper on the habitable zone,
a focus he has returned to this decade.
“The habitable zone doesn’t take into
account all the things you need for
habitability,” explains Kasting. “The
habitable zone, as I like to define it, is
simply the region around a star where
a planet with Earth-like properties—
that [has] water, has [carbon dioxide],
has volcanism—where a planet can
maintain a liquid ocean on its surface
over long geological time periods.”
Kasting explains that, to support
life, a planet would need to have a
rocky, solid surface, which, he argues,
“is an absolute requirement for life to
originate because you have to have
some kind of stable pressure–temperature environment.”
MIT astrophysicist Sara Seager also
studies exoplanets and habitable zones
and is working intensively on the
Transiting Exoplanet Survey Satellite
(TESS) telescope mission, which will
be launched in 2017 and will identify
more exoplanets. The TESS identifications will then indicate good planets
for the James Webb Satellite Telescope,
also to be launched this decade, to
look at and make more refined assessments by means of collecting spectra ­measurements, which are the very
specialized measurements that break
down the light of an object in the
sky and record properties, such as
­temperature and composition.
Seager is working on biosignatures,
which are the telltale signs of biological activity. “We would like to see signs
464 BioScience • May 2015 / Vol. 65 No. 5
Sara Seager is working on the Transiting Exoplanet Survey Satellite, to be
launched in 2017, which is expected to identify more exoplanets.
Photograph: Justin Knight.
of liquid water far away but we [are
unlikely to] see the surface, [for], say,
the next 100 years,” Seager says, “so we
would like to see water vapor in the
atmosphere, and we will take that as a
proxy for water vapor on the surface.”
Seager also points out that other gases
serve well as biosignatures too, such as
methane, nitrous oxide, and methane
chloride.
Seager explains that a life-supporting planet cannot be too large or two
small because of size’s effects on surface temperature. She is conducting
research on the mass–radius figures
for exoplanets, from which a planet’s
density and volume can be ascertained.
This is a first parameter for establishing the suitability, especially in terms
of surface, for the habitability of a
planet.
Seager points out that assessing
greenhouse gases in a planet’s atmosphere is important. “For exoplanets, the planet could have a much
more severe greenhouse effect,” she
says. “We look at the atmosphere for
whether we should look at it further.”
Seager and Kasting note that the
tools for truly understanding the exoplanets in depth are not yet available. Eventually, direct-imaging
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technologies will be able to block
the light of the nearby stars, which
prevents a good view of the planets.
Kasting adds that something like the
Terrestrial Planet Finder mission, a
space-based survey with planets as
its prime focus that NASA considered last decade, will be reconsidered
and, if launched, will produce much
essential information on biological
activity on exoplanets. In the meantime, serious thought is going into
biosignatures. A few fundamentals are
agreed on. “We would be looking for
things like the simultaneous presence
of oxygen and methane,” says Kasting.
Oxygen and methane react together
quickly, and if the two are together
in the atmosphere, as on Earth, it
indicates not only biology at work
but different active biological systems.
“You can’t look for life directly, but you
can look for the gaseous byproducts of
life,” Kasting adds.
In her March article in Science
Advances, Seager and biochemist
William Bains write that biosignature
gases are indeed being refined and
revisited, in part because even the
most biologically associated gas can
have abiotic sources. But the authors
fundamentally agree with the others in
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the field. “All life on Earth makes gas
products, and basic chemistry suggests
the same will be true of any other plausible biochemistry,” they write.
All agree that the technology for
identifying life on exoplanets has not
yet been demonstrated. Perhaps with
TESS, the James Webb, and other telescopic surveys instruments, the coming decades will produce dramatic
atmospheric findings—if not discover
life itself.
In the meantime, astrobiology is
maturing, in large part because of the
interdisciplinary discussion and the
leadership of NASA. “Earlier in astrobiology, people were working on separate and related things but were not
talking to each other. We have really
benefited from all interacting together.
Because the search for life elsewhere is
a very hard problem,” says Seager, “I
have really benefited from talking with
biochemists and just understanding
the extremes, what life can do, how life
works, what gases we might consider
in the future.… From biologists, for
example, you learn the sheer range of
molecules produced by life on Earth,
and for a lot of these molecules, we
don’t even know why life is producing
them.”
Voyteck emphasizes the rigor of the
science that is encompassed within
astrobiology: “The science that NASA
funds is not pseudoscience; it is not
science fiction. It is rigorous, driven by
the scientific method, [and] it’s most
often hypothesis driven—we do do
some discovery when we go into a new
system on Earth. But it is solid basic
research to answer those questions to
serve one of the mission statements of
our agency, which is to understand if
life exists elsewhere.”
The maturing of the field also helps
respond to the challenge of the Fermi
paradox. “A lot of astrobiology has
so far been theoretical—our work on
habitable zones is theoretical—but
ultimately, it is an empirical science,”
explains Kasting. “So by building space
telescopes to look at exoplanets, by
sending planetary missions to Mars,
maybe Europa, we can answer these
questions empirically—and to a skeptical biologist or any other skeptical
scientist, what you really want is data.
So that is what the ultimate goal is.”
Richard Blaustein is a freelance science and
environmental journalist. He can be reached at
[email protected] and can be followed
at Twitter @richblaustein.
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