Are We Alone in the Universe? NASA Calls for New Framework

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Oct 30, 2021, 7:33:12 PM10/30/21

Oct 27, 2021
Are We Alone in the Universe? NASA Calls for New Framework

How do we understand the significance of new scientific results related
to the search for life? When would we be able to say, “yes,
extraterrestrial life has been found?”

NASA scientists are encouraging the scientific community to establish a
new framework that provides context for findings related to the search
for life. Writing in the journal Nature, they propose creating a scale
for evaluating and combining different lines of evidence that would
ultimately lead to answering the ultimate question: Are we alone in the

In the new article led by Jim Green, the agency's chief scientist, a
NASA group offers a sample scale to use as a starting point for
discussions among anyone who would use it, such as scientists and
communicators. They envision a scale informed by decades of experience
in astrobiology, a field that probes the origins of life on Earth and
possibilities of life elsewhere.

“Having a scale like this will help us understand where we are in terms
of the search for life in particular locations, and in terms of the
capabilities of missions and technologies that help us in that quest,”
Green said.

The scale contains seven levels, reflective of the winding, complicated
staircase of steps that would lead to scientists declaring they’ve found
life beyond Earth. As an analogy, Green and colleagues point to the
Technology Readiness Level scale, a system used inside NASA to rate how
ready a spacecraft or technology is to fly. Along this spectrum,
cutting-edge technologies such as the Mars helicopter Ingenuity begin as
ideas and develop into rigorously tested components of history-making
space missions.

The authors hope that in the future, scientists will note in published
studies how their new astrobiology results fit into such a scale.
Journalists could also refer to this kind of framework to set
expectations for the public in stories about new scientific results, so
that small steps don’t appear to be giant leaps.

“Until now, we have set the public up to think there are only two
options: it’s life or it’s not life,” said Mary Voytek, head of NASA’s
Astrobiology Program in at NASA Headquarters in Washington and study
co-author. “We need a better way to share the excitement of our
discoveries, and demonstrate how each discovery builds on the next, so
that we can bring the public and other scientists along on the journey.”

It’s exciting each time a rover or orbiter finds proof that water once
flowed on Mars. Each new finding shows us that Mars’ past climate was
similar to Earth’s, and the red planet could have once supported life.
But that doesn’t necessarily mean any sort of life ever lived there, or
that anything lives there now. Discoveries of rocky planets orbiting
stars beyond our Sun, especially those that could harbor liquid water on
their surfaces, are similarly tantalizing, but not proof by themselves
of life beyond Earth. So how do we understand these observations in context?

Scientists building a staircase that represents the search for life
beyond Earth
Scientists worldwide collaborate, using different tools and methods, to
search for life beyond Earth. NASA scientists propose having a scale to
contextualize the significance of new results related to this search.
Credits: NASA/Aaron Gronstal
All of science is a process of asking questions, coming up with
hypotheses, developing new methods to look for clues, and ruling out all
alternative explanations. Any individual detection may not be completely
explained by a biological process, and must be confirmed through
follow-up measurements and independent investigations. Sometimes, there
are problems with the instruments themselves. Other times, experiments
don’t turn up anything at all, but still deliver valuable information
about what doesn’t work or where not to look.

Astrobiology is no different. The field pursues some of the most
profound questions that anyone could ask, regarding our origins and
place in the universe. As scientists learn more and more about what
kinds of signals are associated with life in diverse environments on
Earth, they can create and improve upon technologies needed to find
similar signs elsewhere.

While the exact details of the scale will evolve as scientists,
communicators, and others weigh in, the Nature article offers a starting
point for discussion.

At the first step of the scale, “level 1,” scientists would report hints
of a signature of life, such as a biologically relevant molecule. An
example would be a future measurement of some molecule on Mars
potentially related to life. Moving up to “level 2,” scientists would
ensure that the detection was not influenced by the instruments having
been contaminated on Earth. At “level 3” they would show how this
biological signal is found in an analog environment, such as an ancient
lakebed on Earth similar to the Perseverance rover’s landing site,
Jezero Crater.

To add evidence to the middle of the scale, scientists would supplement
those initial detections with information about whether the environment
could support life, and rule out non-biological sources. For Mars in
particular, samples returned from Mars could help make this kind of
progress. Perseverance will soon be collecting and storing samples with
the goal of a future mission returning them one day. Since different
teams on Earth would have the opportunity to independently verify hints
of life in Mars samples with a variety of instruments, the combination
of their evidence could achieve “level 6,” the second highest step on
the scale. But in this example, to reach level 7, the standard by which
scientists would be most sure they had detected life on Mars, an
additional mission to a different part of Mars may be required.

“Achieving the highest levels of confidence requires the active
participation of the broader scientific community,” the authors write.

This scale would apply to discoveries from beyond the solar system, too.
Exoplanets, planets outside our solar system, are believed to outnumber
the 300 billion stars in the Milky Way. But small, rocky planets are
harder to study from afar than gas giants. Future missions and
technologies would be necessary to analyze the atmospheres of Earth-size
planets with Earth-like temperatures receiving adequate amounts of
starlight for life as we know it. The James Webb Space Telescope,
launching later this year, is the next big advance in this area. But it
will likely take an even more sensitive telescope to detect the
combination of molecules that would indicate life.

Detecting oxygen in the atmosphere of an exoplanet, a planet outside our
solar system, would be a significant step in the process of searching
for life. We associate oxygen with life because it is made by plants and
we breathe it, but there are also geological processes that generate
oxygen, so it is not proof by itself of life. To move upward on the
scale, a mission team could demonstrate that the oxygen signal was not
being contaminated by light reflected from Earth and study the chemistry
of the planet’s atmosphere to rule out the geological explanation.
Additional evidence of an environment that supports life, such as an
ocean, would bolster the case that this hypothetical planet is inhabited.

Scientists who study exoplanets are eager to find both oxygen and
methane, a combination of gases in Earth’s atmosphere indicative of
life. Because these gases would lead to reactions that cancel each other
out unless there are biological sources of both present, finding both
would be a key “level 4” milestone.

To reach level 5, astronomers would need a second, independent detection
of some hint of life, such as global images of the planet with colors
suggestive of forests or algae. Scientists would need additional
telescopes or longer-term observations to be sure they had found life on
an exoplanet.

Study authors emphasize that the scale should not be seen as a race to
the top. The scale emphasizes the importance of the groundwork that many
NASA missions lay without directly detecting possible biological
signals, such as in characterizing environments on other planetary bodies.

Upcoming missions such as Europa Clipper, an orbiter headed for
Jupiter’s icy moon Europa later this decade, and Dragonfly, an
octocopter that will explore Saturn’s moon Titan, will provide vital
information about the environments in which some form of life may one
day be found.

“With each measurement, we learn more about both biological and
nonbiological planetary processes,” Voytek said. “The search for life
beyond Earth requires broad participation from the scientific community
and many kinds of observations and experiments. Together, we can be
stronger in our efforts to look for hints that we are not alone.”

Learn more about the NASA Astrobiology Program at

Written by Elizabeth Landau
NASA Headquarters

Last Updated: Oct 28, 2021
Editor: Elizabeth Landau
Tags: Ames Research Center, Astrobiology, Europa (Moon), Exoplanets,
Goddard Space Flight Center, Mars, NASA Headquarters, Planets, Solar System
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