Getting Real About the Oxygen Biosignature

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Oxygen, which makes up about 21 percent of the Earth atmosphere, has been embraced as the best biosignature for life on faraway exoplanets. New research shows that detecting distant life via the oxygen biosignature is not so straight-forward, though it probably remains the best show we have. (NASA)

I remember the first time I heard about the atmospheres of distant exoplanets and how could and would let us know whether life was present below.

The key was oxygen or its light-modified form, ozone.  Because both oxygen and ozone molecules bond so quickly with other molecules — think rust or iron oxide on Mars, silicon dioxide in the Earth’s crust — it was said that oxygen could only be present in large and detectable quantities if there was a steady and massive source of free oxygen on the planet.

On Earth, this of course is the work of photosynthesizers such as planets, algae and cyanobacteria, which produce oxygen as a byproduct.  No other abiotic, or non-biological, ways were known at the time to produce substantial amounts of atmospheric oxygen, so it seemed that an oxygen signal from afar would be a pretty sure sign of life.

But with the fast growth of the field of exoplanet atmospheres and the very real possibility of having technology available in the years ahead that could measure the components of those atmospheres, scientists have been busy modelling exoplanet formations, chemistry and their atmospheres.

One important goal has been to search for non-biological ways to produce large enough amounts of atmospheric oxygen that might fool us into thinking that life has been found below.

And in recent years, scientists have succeeded in poking holes in the atmospheric oxygen-means-life scenario.

Oxygen bonds quickly with many other molecules. That means has to be resupplied regularly to be present as O2 in an atmosphere . On Earth, O is mostly a product of biology, but elsewhere it might be result of non-biological processes. Here is an image of oxygen bubbles in water.

Especially researchers at the University of Washington’s Virtual Planetary Laboratory (VPL) have come up with numerous ways that exoplanets atmospheres can be filled (and constantly refilled) with oxygen that was never part of plant or algal or bacteria photo-chemistry.

In other words, they found potential false positives for atmospheric oxygen as a biosignature, to the dismay of many exoplanet scientists.

In part because she and her own team were involved in some of these oxygen false-positive papers, VPL director Victoria Meadows set out to review, analyze and come to some conclusions about what had become the oxygen-biosignature problem.

The lengthy paper (originally planned for 6 pages but ultimately 34 pages because research from so many disciplines was coming in) was published last month in the journal Astrobiology.  It seeks to both warn researchers about the possibilities of biosignature false-positives based on oxygen detection, and then it assures them that there are ways around the obstacles.

“There was this view in the community that oxygen could only be formed by photosynthesis, and that no other process could make O2,”  Meadows told me.  “It was a little simplistic.  We now see the rich complexity of what we are looking at, and are thinking about the evolutionary paths of these planets.

 

Artist’s impression of the exoplanet GJ 1132 b, which orbits the red dwarf star GJ 1132.  Earlier this year, astronomers managed to detect the atmosphere of this Earth-sized planet and have determined that water and methane are likely prevalent in the atmosphere.  (Max Planck Institute for Astronomy)

“What I see is a maturing of the field.  We have models that show plausible ways for oxygen to be produced without biology, but that doesn’t mean that oxygen is no longer an important biosignature.

“It is very important.  But it has to be seen and understood in the larger context of what else is happening on the planet and its host star.”

Before moving forward, perhaps we should look back a bit at the history of oxygen on Earth.

For substantial parts of our planet’s history there was only minimal oxygen in the atmosphere, and life survived in an anaerobic environment.  When exactly oxygen went from a small percentage of the atmosphere to 21 percent of the atmosphere is contested, but there is broader agreement about the source of the O2 in the atmosphere.  The source was photosynthesis, most importantly coming from cyanobacteria in the oceans.

As far back as four billion years ago, photosynthesis occurred on Earth based on the capturing of the energy of near infrared light by sulfur-rich organisms, but it did not involve the release of oxygen as a byproduct.

A chart showing the percentage rise in oxygen in Earth’s atmosphere over the past 3.8 billion years. The great oxidation event occurred some 2.3 billion years ago, but it took more than a billion additional years for the build-up to have much effect on the composition of the planet’s atmosphere.

Then came the the rise of cyanobacteria in the ocean and their production of oxygen.  With their significantly expanded ability to use photosynthesis, this bacterium was able to generate up to 16 times more energy than its counterparts, which allowed it to out-compete and explode in reproduction.

It took hundreds of millions of years more, but that steady increase in the cyanobacteria population led to what is called the “Great Oxidation Event” of some 2.3 billion years ago, when oxygen levels began to really climb in Earth’s atmosphere.  They did level off and remained well below current levels for another billion years, but then shot up in the past billion years.

As Meadows (and others) point out, this means that life existed on Earth for at least two billion years years without producing a detectable oxygen biosignature.  It’s perhaps the ultimate false negative.

But as biosignatures go, oxygen offers a lot.  Because it bonds so readily with other elements and compounds, it remains unbonded or “free” O2 only if it is being constantly produced.  On Earth, the mode of production is overwhelmingly photosynthesis and biology.  What’s more, phototrophs — organism that manufacture their own food from inorganic substances using light for energy — often produce reflections and seasonally dependent biosignatures that can serve as secondary confirmations of biology as the source for abundant Oin an atmosphere.

So in a general way, it makes perfect sense to think that O in the atmosphere of an exoplanet would signify the presence of photosynthesis and life.

Victoria “Vikki” Meadows is the director of the Virtual Planetary Institute at the University of Washington, which has been an important engine for NASA’s Astrobiology Institute (NAI) since 2001.  Among its many lines of research, her group focuses on the Earth as a template for understanding exoplanets, and so Meadows is holding up a rock here as a whimsical nod to that approach.  (University of Washington.)

The problem arises because other worlds out there orbiting stars very different than our own can have quite different chemical and physical dynamics and evolutionary histories, with results at odds with our world.

For instance, when it comes to the non-biological production of substantial amounts of oxygen that could collect in the atmosphere, the dynamics involved could include the following:

Perhaps the trickiest false positive involves the possible non-biological release of O2 via the photolysis of water — the breaking apart of H2O molecules by light.  On Earth, the water vapor in the atmosphere condenses into liquids after reaching a certain height and related temperature, and ultimately falls back down to the surface.  How and why that happens is related to the presence of large amounts of nitrogen in our atmosphere.

But what if an exoplanet atmosphere doesn’t have a lot of an element like nitrogen that allows the water to condense?  Then the water would rise into the stratosphere, where it would be subject to intense UV light,. The molecule would be split, and an H atom would fly off into space — leaving behind large amounts of oxygen that had nothing to do with life.  This conclusion was reached by Robin Wordsworth and Raymond Pierrehumbert of the University of Chicago and was published by the The Astrophysical Journal.

Another recently proposed mechanism to generate high levels of abiotic oxygen, first described by Rodrigo Luger and Rory Barnes of Meadow’s VPL team, focuses on the effects of the super-luminous phase of young stars on any rocky planets that might be orbiting them.

Small-mass M dwarfs in particular can burn much brighter when they are young, exposing potential planets around those stars to very high levels of radiation for as long as one billion years.

Modeling suggests that during this super-luminous phase a terrestrial planet that forms within what will become the main sequence habitable zone around an M dwarf star may lose up to several Earth ocean equivalents of water due to evaporation and hydrodynamic escape, and this can lead to generation of large amounts of abiotic O via the same H2O photolysis process.

Red dwarf, or M stars, are the most common in the cosmos.  They start off with a long period of extremely high luminosity and radiation before evolving into low-energy cool (and red) stars.  While a mature red dwarf star might have habitable zone planets that appear today to have characteristics conducive to life,  exoplanet modelers have determined that many of those red dwarf stars may well have lost their oceans during their early  long exposure to intense radiation. This is an artist rendering of three exoplanets around a red dwarf star. (ESO/M. Kornmesser)

Non-biological oxygen can also build up on an exoplanet, according to a number of researchers, if the host star sends out a higher proportion of far ultraviolet light than near ultraviolet.  The dynamics of photo-chemistry are such, they argue, that the excess far ultraviolet radiation would split CO2 to an extent that O2 would build up in the atmosphere.

There are other potential scenarios that would produce an oxygen false positive, and almost all of them involve radiation from the host star driving chemistry in the planet’s atmosphere, with the planetary environment then allowing O2 to build up.  While some of these false positive mechanisms can produce enough oxygen to make a big impact on their planets, some may not produce enough to even be seen by telescopes currently being planned.

As Meadows tells it, it was Shawn Domagal-Goldman of NASA Goddard and VPL who first brought the issue of oxygen false-positives to her attention. It was back in 2010 after he found an anomaly in his photo-chemical code results regarding atmospheric oxygen and exoplanets, and followed it. Since that initial finding, several other VPL researchers discovered new ways to produce O2 without life, and often while undertaking research focused on a different scientific goal.

Six years later, when she was writing up a VPL annual report, it jumped out that the group (and others) had found quite a few potential oxygen false positives — a significant development in the field of biosignature detection and interpretation.  That’s when she decided that an analysis and summary of the findings would be useful and important for the exoplanet community.  “Never let it be said that administrative tasks can’t lead to inspiration!” she wrote to me.

While Meadows does not downplay the new challenges to defining oxygen and ozone as credible biosignatures, she does say that these new understandings can be worked around.

Some of that involves targeting planets and stars for observation that don’t have the characteristics known to produce abiotic oxygen.  Some involves finding signatures of this abiotic oxygen that can be identified and then used to discard potential false positives.  And perhaps most telling, the detection of methane alongside free oxygen in an exoplanet atmosphere would be considered a powerful signature of life.

The Virtual Planetary Laboratory investigates the potential habitability of extrasolar planets. The research will help in predicting the habitability of discovered bodies like the Earth-size planets orbiting TRAPPIST-1 and the planet orbiting our closest neighbor, Proxima Centauri. (NASA)

The official goal of Meadows’ VPL is to wrestle with this question: “How would we determine if an extrasolar planet were able to support life or had life on it already?”

This has led her to a highly interdisciplinary approach, bringing together fifty researchers from twenty institutions.  In addition to its leading role in the NASA Astrobiology Institute, the VPL is also part of a broad NASA initiative to bring together scientists from different locales and disciplines to work on issues and problems of exoplanet research — the Nexus for Exoplanet System Science, or NExSS.

Given this background and these approaches, it is hardly surprising that Meadows would be among the first to see the oxygen-false positive issue in both scientific and collective terms.

“I wanted the community to have some place to go to when thinking about Ofalse positives,” she said. “We’re learning now about the complexity and richness of exoplanets, and this is essential for preparing to do the best job possible {in terms of looking for signs of life on exoplanets} when we get better and better observations to work with.”

“This story needed to be told now. Forewarned is forearmed.”

 

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Coming to Terms With Biosignatures

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Exoplanets are much too far away for missions to visit and explore, so scientists are learning about them remotely.  That includes the question of whether they might support life — an aspect of exoplanet science that is getting  new attention. This is artist Ron Miller’s impression of an exoplanet.
Exoplanets are much too far away for missions to visit and explore, so scientists are learning about them remotely. That includes the question of whether they might support life — an aspect of exoplanet science that is getting new attention. This is artist Ron Miller’s impression of an exoplanet.

The search for life beyond our solar system has focused largely on the detection of an ever-increasing number of exoplanets, determinations of whether the planets are in a habitable zone, and what the atmospheres of those planets might look like.  It is a sign of how far the field has progressed that scientists are now turning with renewed energy to the question of what might, and what might not, constitute a sign that a planet actually harbors life.

The field of “remote biosignatures” is still in its early stages, but a NASA-sponsored workshop underway in Seattle has brought together dozens of researchers from diverse fields to dig aggressively into the science and ultimately convey its conclusions back to the exoplanet community and then to the agency.

While a similar NASA-sponsored biosignatures workshop put together a report in 2002, much has changed since then in terms of understanding the substantial complexities and possibilities of the endeavor.  There is also a new sense of urgency based on the observing capabilities of some of the space and ground telescopes scheduled to begin operations in the next decade, and the related need to know with greater specificity what to look for.

“The astrobiology community has been thinking a lot more about what it means to be a biosignature,” said Shawn Domogal-Goldman of the Goddard Space Flight Center, one of the conveners of the meeting.  Some of the reason why is to give advice to those scientists and engineers putting together space telescope missions, but some is the pressing need to maintain scientific rigor for the good of one of humankind’s greatest challenges.

“We don’t want to spend 20 years of our lives and billions in taxpayer money working for a mission to find evidence of life, and learn too late that our colleagues don’t accept our conclusions,” he told me.  “So we’re bringing them all together now so we can all learn from each other about what would be, and what would not be, a real biosignature.”

 

How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. Credits: NASA/JPL-Caltech.
How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. ( NASA/JPL-Caltech)

The three-day workshop is bringing together some 50 scientists ranging from astronomers, astrobiologists and planetary scientists to microbiologists and specialists in photosynthesis.  Organized by NASA’s Nexus for Exoplanet System Science (NExSS) — an initiative created to encourage interdisciplinary collaboration — it has been tasked with putting together a report for the larger exoplanet community and ultimately for NASA.

The first day of the workshop featured a review of previous work on biosignatures, which initially put forward the presence of oxygen in an exoplanet atmosphere as a strong and almost certain sign that biology was at work below. This is because oxygen, which is a byproduct of much life, bonds quickly with other molecules and so would be undetectable unless it was continuously replenished.

But as outlined by Victoria Meadows, director of the Virtual Planet Laboratory at the University of Washington, more recent research has shown large amounts of oxygen can be produced without biology under a number of (usually extreme)  conditions.  There has been a resulting focus on potential false positive signals regarding oxygen and other molecules.

From another perspective, Tim Lyons, a biogeochemist from the University of California, Riverside, used the early and middle Earth as an example how easy it is to arrive at a false negative result.

He said that current thinking is that for as long as two billion years, Earth was inhabited but the lifeforms produced little oxygen.  If analyzed from afar for all those years, the result would be a complete misreading of life on Earth.

With these kinds of false positives and negatives in mind, Meadows said that the current approach to understanding biosignatures is to look beyond a single molecule to the broader planetary and solar environment.

“We have to look not just at single biosignatures, but at their their context on the planet. How might life have modified an environment in a potentially detectable way?  And having stepped back a bit, does the biosignature make sense?”

As one example, while oxygen alone is no longer considered a sure biosignature, oxygen in an atmosphere in the presence of methane would be convincing because of the known results of the chemical interactions of the two.

 

Schematic for the concept of considering all small molecules in the search for biosignature gases. The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. Further investigation relates to the detectability: the sources and sinks that ultimately control the molecules’ accumulation in a planetary atmosphere of specific conditions as well as its spectral line characteristics. Geophysically or otherwise generated false positives must also be considered. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. Figure credit: S. Seager and D. Beckner.
Schematic for the concept of considering all small molecules in the search for biosignature gases.
The goal is to start with chemistry and generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. (S. Seager and D. Beckner)

 

In part because of the false positive/false negative issues involving oxygen, some have begun a concerted effort to produce a list of additional possible biosignatures.  William Bains, a member of Sara Seager’s team at the Massachusetts Institute of Technology, described the blunderbuss approach they have adopted:  examining some 14,000 compounds simple (fewer than six non-hydrogen atoms) and stable enough to exist in the atmosphere of an exoplanet.

In their Astrobiology Journal article, Seager, Bains and colleagues wrote that “To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases.”

Elaborating during the workshop, Bains asked:  “Why does life produce the gases that it does? We really don’t know, so we’re bringing in everything as a possibility.”   Not surprisingly, he said, “The more you search, the more you find.”

And as for the possibility of life existing in extreme environments, Bains referred to the microbes known to live in radioactive environments, in plastic, and virtually everywhere else on Earth.

Because the science of remote biosignatures is still in its early stages, the unknowns can seem to overwhelm the knowns, making the whole endeavor seem near impossible.  After all, it’s proven extremely difficult to determine whether there was ever life on “nearby” Mars, and scientists have Martian meteorites to study and rovers sending back information about the geology, the geochemistry, the weather, the atmospheric conditions and the composition of the planet.

By comparison, learning how to probe the atmospheres of faraway exoplanets and assess what might or might not be a biosignature will have to be done entirely with next generation space telescopes and the massive ground telescopes in development.  The information in the photons they collect will tell scientists what compounds are present, whether liquid water is present on the surface, and potentially whether the surface is changing with seasons.  And then the interpretation begins.

That’s why Mary Voytek, the originator of NExSS and the head of the NASA astrobiology program, said at the workshop that the goal was to test and ultimately provide as many biosignatures as possible.  She wants many molecules potentially associated with life to be identified and then studied and restudied in the same critical way that oxygen has been — embraced for the biosignature possibilities it offers, and understood for the false positives and false negatives that might mislead.

“What we need is an arsenal,” she said, as many ways to sniff out the byproducts of exoplanet life as that daunting task demands.

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Exoplanet Biosignatures: Crucial and Confounding

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Curiosity rover and evidnce of managnese oxide on rock at xxx
The Curiosity rover at the Windjana outcrop on Mars, where it found evidence of mangnese oxide on rocks and in rock fissures.  The mineral is formed only in the presence of water and plentiful oxygen. (NASA)

Early in the Curiosity rover’s trek across Gale Crater on Mars, team member and Los Almos National Laboratory  planetary scientist Nina Lanza reported finding surprisingly high concentrations of the mineral manganese oxide.  It was showing up as a blackish-purple fill to cracks in rocks, and possibly as a surface covering to others.

Lanza, who had some experience with the common and much-debated mineral– found in the American Southwest and other arid climes — initially proposed that it just might be related to terrestrial rock varnishes.  This was a bold proposal because manganese-oxide rock varnishes on Earth are almost always associated with microbes, which are known to concentrate the mineral.  So was this a biosignature coming from Mars?

Two years later, Lanza and others on the Curiosity team have published a paper describing in detail the regular detection of Martian manganese oxide, sometimes in concentrations higher than what is found on Earth.  Based on the surrounding geology and geochemistry, the team then concluded that when the mineral was formed, the Mars atmosphere had levels of oxygen much higher than previously imagined.

This conclusion flows from the fact that the mineral is only formed, on Earth at least, when plentiful oxygen and plentiful water are present.  Indeed, manganese oxides (and many other minerals) began forming in earnest here only after the so-called “great oxygenation event” that, through bacterial photosynthesis, delivered vastly more oxygen to Earth’s atmosphere.

On Mars, the manganese oxide was found largely in sedimentary rock cracks, and to geologists that means it was distributed via flowing water after the rocks had solidified.

Finding substantial water and oxygen together on a planet — in our solar system or beyond — has often been described as providing a strong case for a habitable, and perhaps inhabited, planet.

 

A plankton bloom off the coast of Washington state, US, June 2002, taken by astronauts from the International Space Station.
The oxygen in the Earth’s atmosphere increased dramatically around 2.3 billion years ago with the fast spread of cyanobacteria, or blue-green algae, and other photosynthesizing micro organisms in the oceans.  The image is of a plankton bloom (dominated by blue-green algae) off the coast of Washington state, taken by astronauts from the International Space Station. (NASA)

This all sounds suggestive of life on what Lanza calls “middle-aged” Mars.  But here’s where things get tricky.

The paper also pretty much dismisses the possibility that the manganese oxide was a rock varnish formed by microbes.  Instead, it argues that the oxygen almost certainly was formed, in massive amounts, by non-biological processes — possibly including the widespread breakdown of H2O after the protective Martian magnetic field began to fall apart, letting in destructive radiation.  The lighter hydrogen would, in this scenario, sail away from the planet, leaving high oxygen concentrations for a time.

“Based on some of the manganese oxide abundances, we think there had to be a lot of oxygen in the air when they formed,” she said.  “We can only hypothesize how the oxygen got there, but it does suggest a Martian atmosphere different than what we expected, at least for a period of time.”

Nina Lanza, research scientist at the Los Alamos National Laboratory and a member of the Chem-cam team for the Mars rover Curiosity.
Nina Lanza, research scientist at the Los Alamos National Laboratory and a member of the Chem-cam team for the Mars rover Curiosity.

I read the Lanza paper, in the journal Geophysical Research Letters,  while thinking about an upcoming gathering about exoplanet biosignatures — a NASA-sponsored workshop to take place in late July in Seattle.

The field of “remote biosignatures” is just now becoming a hot discipline as the hunt for possible life on exoplanets speeds up.

It will still be years before the launch of a space mission advanced enough to analyze an exoplanet with the power and range needed to detect signs of life.  But it is nonetheless considered time now to dive deep into the the question of what might constitute a biosignature — something identified in an exoplanet atmosphere and even on the surface that can only be produced by life.  Clearly, it’s essential for NASA to know what are the most compelling extrasolar signatures of possible life before it begins looking in earnest.  And so a new scientific community is being formed.

“We’re at a juncture now where we’ve become a science of biosignatures, remotely observed,” said Nancy Kiang of NASA Goddard Institute for Space Studies in New York City.

She is one of the science leads of the Seattle workshop, which is organized by the NASA Astrobiology Program and NASA’s Nexus for Exoplanet System science (NExSS) program. The workshop will be a review of what has been learned about remote biosignatures over the past decade or so, and how the field should go forward.

 

Chemical signatures of the contents of planet atmospheres is central to the emerging science of biosignatures remotely observed. NASA
The chemical signatures of the contents of planet atmospheres, collected through spectroscopy, is central to the emerging science of biosignatures, remotely observed. (NASA)

It’s findings and recommendations will be forwarded on to two NASA science and technology definition teams studying what an exoplanet life-detection space mission for the 2030s could and should be able to do.

From the beginning of scientific thinking about exoplanet biosignatures, oxygen and ozone were the prime targets because they are abundant in our atmosphere only because of photosynthesis.  Oxygen bonds quickly with other molecules, and so to keep an atmosphere filled with oxygen requires constant replenishment.  Especially early on, scientists described the detection of oxygen or ozone in an exoplanetary atmosphere has a very promising signal that the planet was home to life.

That is still the case, but with a substantial and growing number of caveats.

Nancy Kiang, a co-science lead of the upcoming "Biosignatures" workshop and a planetary scientist with NASA's Goddard Institute of Space Studies.
Nancy Kiang, a co-science lead of the upcoming “Biosignatures” workshop and a planetary scientist with NASA’s Goddard Institute of Space Studies.

While oxygen, and its byproduct ozone, remain central to the biosignature search, their exoplanetary context has become increasingly important as well.

That’s because scientists have found or theorized various pathways for putting abundant oxygen into an atmosphere without any biology or photosynthesis whatsoever.  The seemingly once oxygen-rich Martian atmosphere is a potential example relatively close to home.

The result of this has been the growing awareness of a “false positive” problem when it comes to exoplanets, oxygen and life.  How will scientists know whether an oxygen or ozone rich atmosphere is being sustained through biology or non-biological means?

This is an issue that has many researchers working in labs and with their computer models to get a firmer handle on how they can distinguish the false positives from the real positives.  And it will be a much discussed subject in the workshop ahead.

With the false positive issue in mind, Kiang said that the remote biosignature effort nonetheless remain focused on oxygen.  But the effort and understanding,  she said “it’s becoming a lot more nuanced.”

“We have to look at the full environmental context of the planetary system– many different factors.  We can find oxygen and water on a planet and that still does not mean life. ”

Biosignature science is definitely science at the very edge.  It is not only essential to understand exoplanets, but Kiang said new ways of thinking are making researchers ask questions about why things are as they are on Earth, too.

Her own specialty involves photosynthesis and plant pigments, and so she looks at how different kinds of light from different kinds of stars could effect the search for extrasolar biosignatures.  We reflectively think of plants here as green, but Kiang says that exoplanet science raises the complicated question of ‘Why?”

Green, yellow or even red-dominant plants may live on extra-solar planets, according to scientists whose two scientific papers appear in the March issue of the journal, Astrobiology. The scientists studied light absorbed and reflected by organisms on Earth, and determined that if astronomers were to look at the light given off by planets circling distant stars, they might predict that some planets have mostly non-green plants. Green, yellow or even red-dominant plants may live on extra-solar planets, according to scientists whose two scientific papers appear in the March issue of the journal, Astrobiology. The scientists studied light absorbed and reflected by organisms on Earth, and determined that if astronomers were to look at the light given off by planets circling distant stars, they might predict that some planets have mostly non-green plants. Green, yellow or even red-dominant plants may live on extra-solar planets, according to scientists whose two scientific papers appear in the March issue of the journal, Astrobiology. The scientists studied light absorbed and reflected by organisms on Earth, and determined that if astronomers were to look at the light given off by planets circling distant stars, they might predict that some planets have mostly non-green plants. Green, yellow or even red-dominant plants may live on extra-solar planets, according to scientists whose two scientific papers appear in the March issue of the journal, Astrobiology. The scientists studied light absorbed and reflected by organisms on Earth, and determined that if astronomers were to look at the light given off by planets circling distant stars, they might predict that some planets have mostly non-green plants. (Doug Cummings, NASA/Goddard/Caltech
An iconic illustration that accompanied a paper by Nancy Kiang, where she argued that if plant life existed on planets orbiting stars quite different from our own, their pigments and therefore colors might be very different from on Earth.   (Doug Cummings, NASA/Goddard/Caltech)

David Catling, a planetary scientist and astrobiologist at the University of Washington, has studied the origins and dynamics of atmospheres of the early Earth, Mars and now exoplanets, with a focus on oxygen as the key element.

But he sees the same problems with oxygen as a singular biosignature for exoplanet life, and says that field should be looking for a “cocktail of biogenic gases” in their atmospheres.  The presence of oxygen together with methane in an atmosphere, for instance, would be “hard to explain without a biosphere.”

“There is no single silver bullet,” he said, “but oxygen comes closest.”

He also said that other compounds that are produced by life on Earth — such as nitrous oxide and hydrogen sulfide — are potential exoplanet biosignatures, but they exist in only small amounts in Earth’s atmosphere and at similar levels would be impossible to detect on distant exoplanets.

(This abundance issue has not stopped MIT professor Sara Seager, a pioneer in studying exoplanet atmospheres, from collecting spectroscopic samples of scores of other gases produced by living things.  Working with colleague William Bains, Seager has been cataloguing the chemical signatures of specialized gases from the likes of plankton.  These often sulfur and chlorine based gases exist in tiny amounts on Earth, but Seager argues they could be much more widespread on other planets.)

David Catling, a professor of Earth and space science at the University of Washington. He is also a researcher at the school's Virtual Planetary Laboratory.
David Catling, a professor of Earth and space science at the University of Washington. He is also a researcher at the school’s Virtual Planetary Laboratory.

Catling said that there is one sure way to know that an exoplanet is home to life:  if compounds made by intelligent beings, rather than nature, are detected.

Short of that, he says, it is doubtful that we will ever make a 100 percent convincing detection of life on an exoplanet from afar.  Or even if we are on another exoplanet.  There are just too many conflating issues regarding biosignatures.

Take, for example, the manganese oxide finding on Mars.  Catling says that while the oxygen needed to form the mineral could come from the atmosphere, it could also come from compounds on the surface of Mars known to contain large amounts of bound, but potentially unbindable, oxygen. Gases, he said, can be made available for further chemistry in many ways.

While an exoplanet biosignature that comes with a 100 percent may be unattainable, the process of learning how to get to a daunting 90 or 95 percent certainty is indeed pushing ahead at the edges of space and biological science.  Expect difficulties, and expect surprises.

 

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