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 universe?
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 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 https://astrobiology.nasa.gov
Written by Elizabeth Landau
NASA Headquarters
Source: https://www.nasa.gov/feature/are-we-alone-in-the-universe-nasa-calls-for-new-framework
