Researchers with the ����Ӱ�Ӵ�ý-led are central to a group of published by NASA researchers in the journal Astrobiology outlining the history — and suggesting the future — of the search for life on exoplanets, or those orbiting stars other than the sun.

The research effort is coordinated by NASA’s Nexus for Exoplanet Systems Science, or NExSS, a worldwide network dedicated to finding new ways to study the age-old question: “Are we alone?”

A theme through the research and the discussions behind it is the need to consider planets in an integrated way, involving multiple disciplines and perspectives.

“For life to be detectable on a distant world it needs to strongly modify its planet in a way that we can detect,” said UW astronomy professor , lead author of one of the papers and principle investigator of the Virtual Planetary Laboratory, or VPL for short. “But for us to correctly recognize life’s impact, we also need to understand the planet and star — that environmental context is key.”

Work done by NExSS researchers will help identify the measurements and instruments needed to search for life using future NASA flagship missions. The detection of atmospheric signatures of a few potentially habitable planets may possibly come before 2030, although whether the planets are truly habitable or have life will require more in-depth study.

The papers result from two years of effort by some of the world’s leading researchers in astrobiology, planetary science, Earth science, , astrophysics, chemistry and biology, including several from the UW and the Virtual Planetary Laboratory, or VPL. The coordinated work was born of online meetings and an in-person workshop held in Seattle in July of 2016.

The pace of exoplanet discoveries has been rapid, with over 3,700 detected since 1992. NASA formed the international NExSS network to focus a variety of disciplines on understanding how we can characterize and eventually search for signs of life, called biosignatures, on exoplanets.

The NExSS network has furthered the field of exoplanet biosignatures and “fostered communication between researchers searching for signs of life on solar system bodies with those searching for signs of life on exoplanets,” said Niki Parenteau, an astrobiologist and microbiologist at NASA’s Ames Research Center, Moffett Field, California, and a VPL team member. “This has allowed for sharing of ‘lessons learned’ by both communities.”

The first of the papers reviews types of signatures astrobiologists have proposed as ways to identify life on an exoplanet. Scientists plan to look for two major types of signals: One is in the form of gases that life produces, such as oxygen made by plants or photosynthetic microbes. The other could come from the light reflected by life itself, such as the color of leaves or pigments.

Such signatures can be seen on Earth from orbit, and astronomers are studying designs of telescope concepts that may be able to detect them on planets around nearby stars. Meadows is a co-author, and lead author is , a VPL team member who earned his doctorate in astronomy and astrobiology from the UW and is now a post-doctoral researcher at the University of California, Riverside.

Meadows is lead author of the second review paper, which discusses recent research on “false positives” and “false negatives” for biosignatures, or ways nature could “trick” scientists into thinking a planet without life was alive, or vice versa.

In this paper, Meadows and co-authors review ways that a planet could make oxygen abiotically, or without the presence of life, and how planets with life may not have the signature of oxygen that is abundant on modern-day Earth.

The paper’s purpose, Meadows said, was to discuss these changes in our understanding of biosignatures and suggest “a more comprehensive” treatment.  She said: “There are lots of things in the universe that could potentially put two oxygen atoms together, not just photosynthesis — let’s try to figure out what they are. Under what conditions are they are more likely to happen, and how can we avoid getting fooled?”

Schwieterman is a co-author on this paper, as well as UW doctoral students , and .

With such advance thinking, scientists are now better prepared to distinguish false positives from planets that truly do host life.

Two more papers show how scientists try to formalize the lessons we have learned from Earth, and expand them to the wide diversity of worlds we have yet to discover.

, UW professor of Earth and space sciences, is lead author on a paper that proposes a framework for assessing exoplanet biosignatures, considering such variables as the chemicals in the planet’s atmosphere, the presence of oceans and continents and the world’s overall climate. Doctoral student is a co-author.

By combining all this information in systematic ways, scientists can analyze whether data from a planet can be better explained statistically by the presence of life, or its absence.

“If future data from an exoplanet perhaps suggest life, what approach can distinguish whether the existence of life is a near-certainty or whether the planet is really as dead as a doornail?” said Catling. “Basically, NASA asked us to work out how to assign a probability to the presence of exoplanet life, such as a 10, 50 or 90 percent chance. Our paper presents a general method to do this.”

The data that astronomers collect on exoplanets will be sparse. They will not have samples from these distant worlds, and in many cases will study the planet as a single point of light. By analyzing these fingerprints of atmospheric gases and surfaces embedded in that light, they will discern as much as possible about the properties of that exoplanet.

“Because life, planet, and parent star change with time together, a biosignature is no longer a single target but a suite of system traits,” said , a biometeorologist at NASA’s Goddard Institute for Space Studies in New York and a VPL team member. She said more biologists and geologists will be needed to interpret observations “where life processes will be adapted to the particular environmental context.”

The final article discusses the ground-based and space-based telescopes that astronomers will use to search for life beyond the solar system. This includes a variety of observatories, from those in operation today to ones that will be built decades in the future.

Taken together, this cluster of papers explains how the exoplanet community will evolve from their current assessments of the sizes and orbits of these faraway worlds, to thorough analysis of their chemical composition and eventually whether they harbor life.

“I’m excited to see how this research progresses over the coming decades,” said , an astrobiologist at NASA’s Goddard Space Flight Center, Greenbelt, Maryland, and a VPL team member. He is also a co-author on four of the five papers.

“NExSS has created a diverse network of scientists. That network will allow the community to more rigorously assess planets for biosignatures than would have otherwise been possible.”

NExSS is an interdisciplinary, cross-divisional NASA research coordination network.

###

Based on a . For more information, contact Meadows at vsm@astro.washington.edu or Catling at dcatling@uw.edu.

Is it life, or merely the illusion of life?

Research from the ����Ӱ�Ӵ�ý-based published Feb. 26 in Astrophysical Journal Letters will help astronomers better identify — and thus rule out — “false positives” in the search for life beyond Earth.

Powerful devices such as the , set for launch in 2018, may help astronomers look for life on a handful of faraway worlds by searching for, among other things, evidence of oxygen — a “biosignature” — in their atmospheres. This is done by transit spectroscopy, or studying the spectral features of light visible through a planet’s atmosphere when it transits or passes in front of its host star.

“We wanted to determine if there was something we could observe that gave away these ‘false positive’ cases among exoplanets,” said lead author , a doctoral student in astronomy. “We call them ‘biosignature impostors’ in the .

“The potential discovery of life beyond our solar system is of such a huge magnitude and consequence, we really need to be sure we’ve got it right — that when we interpret the light from these exoplanets we know exactly what we’re looking for, and what could fool us.”

Here on Earth, oxygen is produced almost exclusively by photosynthesis — plants and algae converting the sun’s rays into energy to sustain life. And so Earth’s oxygen biosignature is indeed evidence of life. But that may not be universally true.

from the Virtual Planetary Laboratory has found that some worlds can create oxygen “abiotically,” or by nonliving means. This is more likely in the case of planets orbiting low-mass stars, which are smaller and dimmer than our sun and the most common in the universe.

The first abiotic method they identified results when the star’s ultraviolet light splits apart carbon dioxide (CO₂) molecules, freeing some of the oxygen atoms to form into O_2, the kind of oxygen present in Earth’s atmosphere.

The giveaway that this particular oxygen biosignature might not indicate life came when the researchers, through computer modeling, found that the process produces not only oxygen but also significant and potentially detectable amounts of carbon monoxide. “So if we saw carbon dioxide and carbon monoxide together in the atmosphere of a rocky planet, we would know to be very suspicious that future oxygen detections would mean life,” Schwieterman said.

The team also found an indicator for abiotic oxygen resulting from starlight similarly breaking down atmospheric water, H₂O, allowing hydrogen to escape and leaving vast quantities of oxygen — far more than the Earth has ever had in its atmosphere.

In such cases, Schwieterman said, oxygen molecules collide with each other frequently, producing short-lived pairs of oxygen molecules that become O₄ molecules, with their own unique signature.

“Certain O₄ features are potentially detectable in transit spectroscopy, and many more could be seen in reflected light,” Schwieterman said. “Seeing a large O₄ signature could tip you off that this atmosphere has far too much oxygen to be biologically produced.”

“With these strategies in hand, we can more quickly move on to more promising targets that may have true oxygen biosignatures,” he said.

“It’s one thing to detect a biosignature gas, but another thing to be able to interpret what you are looking at, said , UW professor of astronomy and principal investigator of the Virtual Planetary Laboratory. “This research is important because biosignature impostors may be more common for planets orbiting low-mass stars, which will be the first places we look for life outside our solar system in the coming decade.”

Schwieterman’s other UW co-authors are astronomy professor and doctoral students and .

Other co-authors are Shawn Domagal-Goldman of the NASA Goddard Space Flight Center in Greenbelt, Maryland; Drake Deming of the University of Maryland; and Chester Harman of Pennsylvania State’s Center for Exoplanets and Habitable Worlds.

The research was funded by the NASA Astrobiology Institute.

###

For more information, contact Schwieterman at 321-505-1605 or eschwiet@uw.edu. Follow him on Twitter at @nogreenstars.

Cooperative agreement # NNA13AA93A.

An atmospheric haze around a faraway planet — like the one which probably shrouded and cooled the young Earth — could show that the world is potentially habitable, or even be a sign of life itself.

Astronomers often use the Earth as a proxy for hypothetical exoplanets in computer modeling to simulate what such worlds might be like and under what circumstances they might be hospitable to life.

In new research from the ����Ӱ�Ӵ�ý-based , UW doctoral student and co-authors chose to study Earth in its era, about 2 ½ billion years back, because it is, as Arney said, “the most alien planet we have geochemical data for.”

The work builds on geological data from other researchers that suggests the early Earth was intermittently shrouded by an organic pale orange haze that came from light breaking down methane molecules in the atmosphere into more complex , organic compounds of hydrogen and carbon.

“Hazy worlds seem common both in our solar system and in the population of exoplanets we’ve characterized so far,” Arney said. “Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself.”

Arney and co-authors will present their findings Nov. 11 at the American Astronomical Society’s Division of Planetary Sciences in National Harbor, Maryland.

The researchers used photochemical, climate and radiation simulations to examine the early Earth shrouded by a “fractal” hydrocarbon haze, meaning that the imagined haze particles are not spherical, as used in many such simulations, but agglomerates of spherical particles, bunched together not unlike grapes, but smaller than a raindrop. A fractal haze, they found, would have significantly lowered the planetary surface temperature.

However, they also found the cooling would be partly countered by concentrations of greenhouse gases that tend to warm a planet. They saw that this combination would result in a moderate, possibly habitable average global temperature.

Such a haze, the researchers found, also would have absorbed ultraviolet light so well as to effectively shield the Archean Earth from deadly radiation before the rise of oxygen and the ozone layer, which now provides that protection. The haze was a benefit to just-evolving surface biospheres on Earth, as it could be to similar exoplanets.

The researchers also found that, based on the early Earth data, it’s unlikely such a haze would be formed by abiotic, or nonliving means. So for exoplanets with Earthlike amounts of carbon dioxide in their atmospheres, Arney said, “organic haze might be a novel type of biosignature. However, we know these hazes can also form without life on worlds like Saturn’s moon Titan, so we are working to come up with more ways to distinguish biological hazes from abiotic ones.”

Co-author Shawn Domagal-Goldman of the NASA Goddard Space Flight Center in Greenbelt, Maryland, said, “Giada’s work shows that the haze could have intertwined with life in more ways than we previously suspected.”

Arney added that astronomers often think of Earthlike exoplanets as “pale blue dots” — after a famous of Earth taken by the Voyager spacecraft — “but with this haze, Earth would have been a ‘pale orange dot.'”

The research was funded through the NASA Astrobiology Institute.

Arney’s UW co-authors are , professor of astronomy and director of the Virtual Planetary Laboratory, and doctoral student and postdoctoral researcher . Other co-authors are Domagal-Goldman, Eric Wolf of the University of Colorado at Boulder and Mark Claire of the University of St. Andrews in the UK and Seattle’s Blue Marble Space Institute of Science.

###

For more information, contact Arney at giada@uw.edu or 206-685-0403.

Observations of nitrogen in Earth’s atmosphere by a NASA spacecraft 17 million miles away are giving astronomers fresh clues to how that gas might reveal itself on faraway planets, thus aiding in the search for life.

Finding and measuring nitrogen in the atmosphere of an exoplanet — one outside our solar system — can be crucial to determining if that world might be habitable. That’s because nitrogen can provide clues to surface pressure. If nitrogen is found to be abundant in a planet’s atmosphere, that world almost certainly has the right pressure to keep liquid water stable on its surface. Liquid water is one of the prerequisites for life.

Should life truly exist on an exoplanet, detecting nitrogen as well as oxygen could help astronomers verify the oxygen’s biological origin by ruling out certain ways oxygen can be produced abiotically, or through means other than life.

The trouble is, is hard to spot from afar. It’s often called an “invisible gas” because it has few light-altering features in visible or infrared light that would make it easy to detect. The best way to detect nitrogen in a distant atmosphere is to measure nitrogen molecules colliding with each other. The resulting, instantaneously brief “collisional pairs” create a unique and discernable spectroscopic signature.

A published Aug. 28 in The Astrophysical Journal by ����Ӱ�Ӵ�ý astronomy doctoral student and lead author , together with astronomy professor and co-authors, shows that a future large telescope could detect this unusual signature in the atmospheres of terrestrial, or rocky planets, given the right instrumentation.

The researchers used three-dimensional planet-modeling data from the UW-based — of which Meadows is principal investigator — to simulate how the signature of nitrogen molecule collisions might appear in the Earth’s atmosphere, and compared this simulated data to real observations of the Earth by NASA’s unmanned spacecraft, launched in 2005.

The craft undertook a revised mission, called , which included observation and characterization of the Earth as if it were an exoplanet. By comparing the real data from the EPOXI mission and the simulated data from Virtual Planetary Laboratory models, the authors were able to confirm the signatures of nitrogen collisions in our own atmosphere, and that they would be visible to a distant observer.

“One of the main messages of the Virtual Planetary Laboratory is that you always need validation of an idea — a proof of concept — before you can extrapolate your knowledge to studying a potentially Earth-like exoplanet,” Schwieterman said. “That’s why studying the Earth as an exoplanet is so important — we were able to validate that nitrogen produces an impact on the spectrum of our own planet as seen by a distant spacecraft. This tells us it’s something worth looking for elsewhere.”

This confirmation in hand, the researchers used a suite of Virtual Planetary Laboratory models that simulated the appearance of planets beyond the solar system bearing varying amounts of nitrogen in their atmospheres.

The detection of nitrogen will help astronomers characterize the atmospheres of potentially habitable planets and determine the likelihood of oxygen production by nonliving processes, the researchers write.

“One of the interesting results from our study is that, basically, if there’s enough nitrogen to detect at all, you’ve confirmed that the surface pressure is sufficient for liquid water, for a very wide range of surface temperatures,” Schwieterman said.

Schwieterman and Meadows’ UW co-author is , who recently completed his doctorate at the UW in astronomy. Other co-authors are of the NASA Ames Research Center in Moffet Field, California, who earned his doctorate at the UW; and Shawn Domagal-Goldman of the NASA Goddard Space Flight Center, who completed a postdoctoral appointment at the UW.

The research was funded by the NASA Astrobiology Institute.

###

For more information, contact Schwieterman at eschwiet@uw.edu, or 321-505-1605.

Cooperative agreement number NNA13AA93A.