Conceptual image of water-bearing (left) and dry
(right) exoplanets with oxygen-rich atmospheres. Crescents are other
planets in the system, and the red sphere is the M-dwarf star around
which the exoplanets orbit. The dry exoplanet is closer to the star, so
the star appears larger. Credits: NASA/GSFC/Friedlander-Griswold
Researchers may have found a way that NASA's James Webb Space Telescope
can quickly identify nearby planets that could be promising for our
search for life, as well as worlds that are uninhabitable because their
oceans have vaporized.
Since planets around other stars (exoplanets) are so far away, scientists cannot look for signs of life by visiting these distant worlds. Instead, they must use a cutting-edge telescope like Webb
to see what's inside the atmospheres of exoplanets. One possible
indication of life, or biosignature, is the presence of oxygen in an
exoplanet’s atmosphere. Oxygen is generated by life on Earth when
organisms such as plants, algae and cyanobacteria use photosynthesis to
convert sunlight into chemical energy.
But what should Webb look for to determine if a planet has a lot of
oxygen? In a new study, researchers identified a strong signal that
oxygen molecules produce when they collide. Scientists say Webb has the
potential to detect this signal in the atmospheres of exoplanets.
“Before our work, oxygen at similar levels as on Earth was thought to
be undetectable with Webb, but we identify a promising way to detect it
in nearby planetary systems,” said Thomas Fauchez of the Universities
Space Research Association at NASA’s Goddard Space Flight Center in
Greenbelt, Maryland. “This oxygen signal is known since the early 80’s
from Earth’s atmospheric studies, but has never been studied for
exoplanet research.” Fauchez is the lead author of the study, appearing in the journal Nature Astronomy January 6.
The researchers used a computer model to simulate this oxygen
signature by modeling the atmospheric conditions of an exoplanet around
an M dwarf, the most common type of star in the universe. M dwarf stars
are much smaller, cooler, and fainter than our Sun, yet much more
active, with explosive activity that generates intense ultraviolet
light. The team modelled the impact of this enhanced radiation on
atmospheric chemistry, and used this to simulate how the component
colors of the star's light would change when the planet would pass in
front of it.
As starlight passes through the exoplanet’s atmosphere, the oxygen
absorbs certain colors (wavelengths) of light— in this case, infrared
light with a wavelength of 6.4 micrometers. When oxygen molecules
collide with each other or with other molecules in the exoplanet’s
atmosphere, energy from the collision puts the oxygen molecule in a
special state that temporarily allows it to absorb the infrared light.
Infrared light is invisible to the human eye, but detectable using
instruments attached to telescopes.
“Similar oxygen signals exist at 1.06 and 1.27 micrometers and have
been discussed in previous studies but these are less strong and much
more mitigated by the presence of clouds than the 6.4 micrometer
signal,” said Geronimo Villanueva, a co-author of the paper at Goddard.
Intriguingly, oxygen can also make an exoplanet appear to host life
when it does not, because it can accumulate in a planet’s atmosphere
without any life activity at all. For example, if the exoplanet is too
close to its host star or receives too much star light, the atmosphere
becomes very warm and saturated with water vapor from evaporating
oceans. This water could be then broken down by the strong ultraviolet
radiation into atomic hydrogen and oxygen. Hydrogen, which is a light
atom, escapes to space very easily, leaving the oxygen behind.
Over time, this process can cause entire oceans to be lost while
building up a thick oxygen atmosphere. So, abundant oxygen in an
exoplanet’s atmosphere does not necessarily mean abundant life, but may
instead indicate a rich water history.
“Depending upon how easily Webb detects this 6.4 micrometer signal,
we can get an idea about how likely it is that the planet is habitable,”
said Ravi Kopparapu, a co-author of the paper at Goddard. “If Webb
points to a planet and detects this 6.4 micrometer signal with relative
ease, this would mean that the planet has a very dense oxygen atmosphere
and may be uninhabitable.”
The oxygen signal is so strong that it also can tell astronomers
whether M dwarf planets have atmospheres at all, using just a few Webb
transit observations.
“This is important because M dwarf stars are highly active, and it
has been postulated that stellar activity might ‘blow away’ entire
planetary atmospheres,” said Fauchez. “Knowing simply whether a planet
orbiting an M dwarf can have an atmosphere at all is important for
understanding star-planet interactions around these abundant but active
stars.”
Although the oxygen signal is strong, cosmic distances are vast and M
dwarfs are dim, so these stars will have to be relatively nearby for
Webb to detect the signal in exoplanet atmospheres within a reasonable
amount of time. An exoplanet with a modern Earth-like atmosphere will
have to be orbiting an M dwarf that is within approximately 16
light-years of Earth. For a desiccated exoplanet with an oxygen
atmosphere 22 times the pressure of Earth’s, the signal could be
detected up to about 82 light-years away. One light-year, the distance
light travels in a year, is almost six trillion miles. For comparison,
the closest stars to our Sun are found in the Alpha Centauri system a
little over 4 light-years away, and our galaxy is about 100,000
light-years across.
The research was funded in part by Goddard’s Sellers Exoplanet
Environments Collaboration (SEEC), which is funded in part by the NASA
Planetary Science Division's Internal Scientist Funding Model. This
project has also received funding from the European Union’s Horizon 2020
research and innovation program under the Marie Sklodowska-Curie Grant,
the NASA Astrobiology Institute Alternative Earths team, and the NExSS
Virtual Planetary Laboratory.
Webb will be the world's premier space science observatory, when it
launches in 2021. It will solve mysteries in our solar system, look
beyond to distant worlds around other stars, and probe the mysterious
structures and origins of our universe and our place in it. Webb is an
international project led by NASA with its partners, ESA (European Space
Agency) and the Canadian Space Agency.
Bill Steigerwald / Nancy Jones
NASA Goddard Space Flight Center, Greenbelt, Maryland
301-286-8955 / 301-286-0039
william.a.steigerwald@nasa.gov / nancy.n.jones@nasa.gov
