The Search for Exoplanet Life Goes Broad and Deep

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The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist's view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA's Goddard Space Flight Center Conceptual Image Lab)
The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist’s view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA’s Goddard Space Flight Center Conceptual Image Lab)

I had the good fortune several years ago to spend many hours in meetings of the science teams for the Curiosity rover, listening in on discussions about what new results beamed back from Mars might mean about the planet’s formation, it’s early history, how it gained and lost an atmosphere, whether it was a place where live could begin and survive.  (A resounding ‘yes” to that last one.)

At the time, the lead of the science team was a geologist, Caltech’s John Grotzinger, and many people in the room had backgrounds in related fields like geochemistry and mineralogy, as well as climate modelers and specialists in atmospheres.  There were also planetary scientists, astrobiologists and space engineers, of course, but the geosciences loomed large, as they have for all Mars landing missions.

Until very recently, exoplanet research did not have much of that kind interdisciplinary reach, and certainly has not included many scientists who focus on the likes of vulcanism, plate tectonics and the effects of stars on planets.  Exoplanets has been largely the realm of astronomers and astrophysicists, with a sprinkling again of astrobiologists.

But as the field matures, as detecting exoplanets and inferring their orbits and size becomes an essential but by no means the sole focus of researchers, the range of scientific players in the room is starting to broaden.  It’s a process still in its early stages, but exoplanet breakthroughs already achieved, and the many more predicted for the future, are making it essential to bring in some new kinds of expertise.

A meeting reflecting and encouraging this reality was held last week at Arizona State University and brought together several dozen specialists in the geo-sciences with a similar number specializing in astronomy and exoplanet detection.  Sponsored by NASA’s Nexus for Exoplanet Systems Science (NExSS), NASA Astrobiology Institute (NAI) and the National Science Foundation,  it was a conscious effort to bring more scientists expert in the dynamics and evolution of our planet into the field of exoplanet study, while also introducing astronomers to the chemical and geological imperatives of the distant planets they are studying.

Twenty years after the detection of the first extra-solar planet around a star, the time seemed ripe for this coming together — especially if the organizing goal of the whole exoplanet endeavor is to search for signs of life beyond Earth.

 

Our vast body of knowledge about the formation, processes and evolution of Earth will become increasingly important in the exoplanet field as new generations of instruments make different and more precise kinds of measurements possible. Using Earth dynamics as a guide, those measurements will be made into models of what might be occurring on the exoplanets. The artist rendering of exoplanet Upsilon Andromedea g by Ron Howard.
Our vast body of knowledge about the formation, processes and evolution of Earth will become increasingly important in the exoplanet field as new generations of instruments make different and more precise kinds of measurements possible. Using Earth dynamics as a guide, those measurements will be made into models of what might be occurring on the exoplanets. The artist rendering of exoplanet Upsilon Andromedae g is by Ron Howard, Black Cat Studios.

Ariel Anbar, a biogeochemist at ASU, was one of the leaders of the meeting and the call for a broader exoplanet effort.

“The astronomical community has been pushing hard to make very difficult measurement, but they really haven’t been thinking much about the planetary context of what they’re finding.  And for geoscience, our people haven’t thought much about astronomical observations because they are so focused on Earth.”

“But this makes little sense because exoplanets open up a huge new field for geoscientists, and the astronomers absolutely need them to make the calls on what many of the measurements of the future actually mean.”

What’s more, the knowledge of researchers familiar with the dynamics of Earth will be essential when planet hunters and planet characterizers put together their wish lists for what kind of instruments are included in future telescopes and spectrographs.  For instance, a deep knowledge would be useful of the Earth’s carbon cycle, or what makes for a stable planetary climate, or what minerals and chemistry a habitable planet probably needs.

And then there are all the false positives and false negatives that could come with detections (or non-detections) of possible signatures of life.  The search for life beyond Earth has already had two highly-public and controversial seeming detections of extraterrestrial life — first by the Viking landers in the 1970s and the Mars meteorite ALH84001 in the mid 1990s.  The two are now considered inconclusive at best, and discredited at worst.

The risk of a similar, and even more complex, confusing and ultimately controversial, discovery of signs of life on an exoplanet are great.  The Arizona State workshop debated this issue at length.

President’s Professor at ASU’s School of Earth and Space Exploration and Department of Chemistry and Biochemistry.
Ariel Anbar, President’s Professor at ASU’s School of Earth and Space Exploration and School of Molecular Sciences. He hopes that the drive to understand exoplanets will push his field to develop a missing general theory for the evolution of Earth and Earth-like planets.

What they came away with was the understanding that while one or two measured biosignatures from a distant planet would be enormously exciting, a deeper understanding of the planet’s atmosphere, interior, chemical makeup and relationship to its host star are pretty much required to make a firm conclusion about biological vs non-biological origins.  (Here is a link to an introductory and cautionary tale to the workshop by another of its organizers, astrophysicist Steven Desch.)

And so the issues under debate were:  Does a planet need plate tectonics to be able to support life?  (Yes on Earth, perhaps elsewhere.) Would the detection of oxygen in an exoplanet atmosphere signify the presence of life? (Possibly, but not definitively.)  Does the chemical and mineral composition of a planet determine its ability to support life? (As far as we can tell, yes.)  Does photosynthesis inevitably lead to an oxygen atmosphere?  (It’s complicated.)

All these issues and many more serve to make the case that exoplanet science and Earth or planetary science need each other.

This is by no means an entirely new message — the Virtual Planetary Laboratory at the University of Washington has taken the approach for a decade from the standpoint of astronomy and the New Earths team of the NAI from a geological standing point.   But still, its urgency and proposed reach was  quite unusual.

It is also a reflection of both the success and direction of exoplanet science, because scientists have — or will have in the years ahead — the instruments and knowledge to learn more about an exoplanet than its location.  The James Webb Space Telescope is expected to provide much advanced ability to read the chemical compositions exoplanet atmospheres, as will a new generation of mammoth ground-based telescopes under construction and (scientists in the field fervently hope) a NASA flagship mission for the 2030s that would be able to directly image exoplanets with great precision.

But really, it’s when more and better measurements come in that the hard work begins.

Transmission spectrum of exoplanet MIT
Information about the make-up of exoplanets comes largely by studying the transmission spectra produced as the planet crosses in front of its star.  The spectra can identify some of the elements and compounds present around the exoplanet. Christine Naniloff/MIT, Julien De Wit.

 

Astrophysicist Steve Desch, for instance,  believes it is highly important to know what Earth-sized planets are like without life.  Starting with a biologically dead exoplanet in the Earth-sized ballpark, it would be possible to get a far better idea of the signatures of a similar planet with life.  But that’s a line of thinking that Earth scientists and geochemists are not, he said, used to addressing.  He felt the ASU workshop provided some consciousness-raising about the kinds of issues that are important to the exoplanet community, and to the Earth scientist, too.

Scientists from the geoscience side see similar limitations in the thinking of exoplanet astronomers.  Christy Till, a geologist and volcano specialist at ASU, said that at the close of the three-day workshop, she wasn’t at all sure that exoplanet scientists have been aware of just how complex the issue of “habitability” will be.

“Our field has learned over the decades that the solid interior of a planet is a big control on whether that planet can be habitable — along with the presence of volcanoes, the cycling elements like carbon and iron, and a relatively stable climate.  These issues were not widely discussed in terms of exoplanets, so I think we can help move the research further.”

Till is relatively new to thinking about exoplanets, brought into the field by the indisciplinary ASU (and NExSS/NAI) approach. But she said it has been most exciting to have the potential usefulness of her kind of knowledge expand on such a galactic scale.

Although the amount of detailed information about exoplanets is very limited, Till (and others) said what is and will be available can be used to create predictive models.  Absent the models that researchers can start building now, future information coming in could easily be misunderstood or simply missed.

ASU geologist and assistant professor Christy Till, a relatively new and enthusiastic member of the exoplanet community. (Abigail Wiebel)
ASU geologist and assistant professor Christy Till, a relatively new and enthusiastic member of the exoplanet community. (Abigail Wiebel)

While the usefulness of geosciences is being largely embraced in the exoplanet field, there are clear caveats.  If Earth becomes the model for what is needed for life in the cosmos, then is the field falling into a new version of the misguided Earth-centric view that long dominated astronomy and cosmology?

With that concern in mind, astronomer Drake Deming of the Harvard-Smithsonian Center for Astrophysics made the case for collecting potential biosignatures of all kinds.  Since we don’t know how potential life on another planet might have formed, we also may well be unaware of what kind of signatures it would put out.  ASU geochemist Everett Shock was similarly wary of relying too heavily on the Earth model when trying to understand planets that may seem similar but are inevitably different.

And Ariel Anbar felt challenged by his more complete realization post-workshop that the exoplanets available to study for the foreseeable feature will not be Earth-sized, but will be “Super-Earths” with radii up to 1.5 times as great as that of our planet.  A proponent of much greater exoplanet-geoscience collaboration, he said the Earth science community has a big job ahead figuring out how the processes and dynamics understood on Earth would actually apply on these significantly larger relatives.

One participant at the workshop pretty much personifies the interdisciplinary bridge under construction , and he was encouraged by the extensive back-and-forth between the space scientists and the Earth scientists.

Shawn Domogal-Goldman, a research space scientist at the Goddard Space Flight Center and a leader of the NExSS group, is an expert in ancient earth as well the astrophysics of exoplanet detection and characterizing.  His view is that the Earth provides 4.5 billion years of physical, chemical, climatic and biological dynamics  that need to be mined for useful insights about exoplanets.

“For me, and I think for others, we’ll look back at this meeting years from now and say to ourselves, ‘We were there at the beginning of something big.'”

 

 

 

 

 

 

 

 

 

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Marc Kaufman
Marc Kaufman is the author of two books about space: "Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.

To contact Marc, send an email to marc.kaufman@manyworlds.space.

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