The Virtual Planetary Lab and Its Search for What Makes an Exoplanet Habitable, or Even Inhabited

For more than two decades now, the Virtual Planetary Laboratory (VPL) at the University of Washington in Seattle has been at the forefront of the crucial and ever-challenging effort to model how scientists can determine whether a particular exoplanet is capable of supporting life or perhaps even had life on it already.

To do this, VPL scientists have developed or combined models from many disciplines that characterize and predict a wide range of planetary, solar system and stellar attributes that could identify habitability, or could pretty conclusively say that a planet is not habitable.

These include the well known questions of whether water might be present and if so whether temperatures would allow it to be sometimes in a liquid state, but on to questions involving whether an atmosphere is present, what elements and compounds might be in the atmospheres, the possible orbital evolution of the planet, the composition of the host star and how it interacts with a particular orbiting planet and much, much more, as shown in the graphic above.

This is work that has played a significant role in advancing astrobiology — the search for life beyond Earth.

More specifically, the VPL approach played a considerable part in building a body of science that ultimately led the Astro2020 Decadal Study of the National Academy of Sciences to recommend last year that the NASA develop its  first Flagship astrobiology project — a mission that will feature a huge space telescope able to study exoplanets for signs of biology in entirely new detail.  That mission, approved but not really defined yet, is not expected to launch until the 2040s.

With that plan actually beginning to move forward, the 132 VPL affiliated researchers at 28 institutions find themselves at another more current-day inflection point:  The long-awaited James Webb Space Telescope has begun to collect and send back what will be a massive and unprecedented set of spectra  of chemicals from the atmospheres of distant planets.

Rather than using the bulk characteristics of exoplanets collected by earlier telescopes to power their models– size, mass, location, orbital characteristics and some basic chemical composition — VPL scientists (and many others) will have actual opportunities to observe exoplanet atmospheres, collect data and have the community’s broad range of new data available to lead them forward.

As described by VPL’s founder and longtime Principal Investigator Victoria Meadows, everything learned over the last twenty years at VPL will now be used to understand and interpret the treasures that JWST (another Flagship mission) will be sending back.

“For years, our motto has been ‘Planets are Hard,'” she said, a play on both the difficulty of the work and the search for terrestrial (potentially Earth-like) planets.

“But five years ago it became ‘photons are coming’, and now it’s ‘photons are here’. And we’ll be using everything we’ve learned through our previous modeling work to interpret what’s coming in and to put it into the broadest possible context — with input from scientists coming from many different fields.”

“You have to understand,  we’ve been working toward this for 20 years.”

Before getting into what lies ahead at VPL — both the near and distant future —  a little about its past and logic.

While VPL is well known in the astrobiology and exoplanet communities, some of you perhaps have not been made their acquaintance.  Their work has been tireless but they have generally not landed headline-grabbing findings.  They have not (so far) claimed to have found signs of life elsewhere, or found a sure-fire way to someday identify distant exoplanet life, or found a particular habitable zone exoplanet that is especially alluring.

That is not what they do.  They are modellers,  using the exoplanet data available to make informed predictions about possible (likely?) conditions on a planet, or atmospheric chemical combinations that could, or could not, indicate life on the surfaces.  In other words, they focus on what could make a planet habitable and how life might impact a planet’s environment, and what are the possible ways to know that.

This has led them to a number of deep-dive skeptical responses to some high-profile claims. Meadows and colleagues wrote that the chemical phosphine found (or maybe not found) in the atmosphere of Venus and claimed to be a potential sign of biology below is very likely no such thing ; that the chances that the exoplanet closest to us, Proxima Centuri b is habitable are pretty low; and that oxygen in an exoplanet atmosphere is not an unambiguous sign of life on its own.

Started by Meadows when she worked at NASA’s Jet Propulsion Lab, VPL’s overriding goal has always been to create a robust science of habitability and signs of life for exoplanets.

This is a daunting task because what is known about the more than 5,000 exoplanets identified so far is limited while the diversity of planetary, solar system and stellar inputs is almost infinite.  With so many variables, how can a robust science be created? But it has been.

The tools are many and often ingenious:  VPL pioneered the use of Earth-over-time as a way to gain insights into how other distant planets might be similarly evolving — using what we know about early Earth and how it changed to model what researchers might predict about the current-day characteristics and evolution of exoplanets.

VPL scientists search broadly for novel atmospheric biosignatures — chemical signs of potential life or of chemicals out of predicted balance below — and then work doubly hard to learn whether observers are being fooled and whether those chemical signatures can also be produced without life.

The group has studied the predictable consequences of the eccentricities and tilts (obliquities) of planets as they orbit their stars.  VPL scientists have created microbial environments to test what extreme conditions the lifeforms can survive, including the hazes that seem to surround many exoplanets.    They have even modeled the glint of an Earth ocean and the presence of plants and forests and seasons to better know how to identify potential exoplanet versions.

Underlying their science is the view that an exoplanet must be viewed as enmeshed in webs of systems, cycles and connections rather than as a singular entity.

Their approach — which has been supported by the NASA Astrobiology Program for all of its 20 years with $33 million in grants — has spread and prospered on campuses and at institutes around the country and the world.

“VPL science, this idea of bringing a broad, interdisciplinary and systems approach to habitability research … has become a major theme in the community,” Meadows said.

“Astrobiology addresses questions so big that they can’t be answered by a single researcher, or even a single field. It takes a community with a staggering breadth of expertise and techniques, and the willingness to work with and learn from each other” — the kind of organization she has created and led.

The result is that she does have good reason to believe that VPL science was important in convincing the National Academy’s Decadal Study members to recommend such a largescale exoplanet and astrobiology mission because VPL work was regularly cited in the report.   “I don’t think it’s an overreach to say that we played a big role in helping the community to be able to justify this next telescope,” she said, referring to the 2040s NASA Flagship astrobiology project.

Mary Voytek, the longtime Director of the NASA Astrobiology Program, agreed that VPL has played that pioneering role.

“They embraced the study of exoplanets before it was trendy,” she said. “And for last two decades they have set the bar for how we understand habitability and how we propose to search for life” beyond Earth.

Voytek said that VPL was an inspiration for the NASA NExSS initiative, which has brought VPL’s  interdisciplinary and systems science approach to habitability to a much larger number of scientists, and with funds to help support them.  NExSS also supports this column.

The scientists at VPL will continue modelling characteristics of exoplanets that can be studied by a variety of future telescopes — including the next generation of massive ground-based telescopes and the large space-based telescope that was recently prioritized by astronomers as the flagship for the the 2040s.

But JWST, and the data collected by observing exoplanet atmospheres, has definitely become a lodestar.

“For twenty years we worked in a largely theoretical space with our models, ” she said.  “We hoped for observations and now they are coming and we’ll increasingly become observers.”  She described VPL as being  “positioned to be a tip of the arrow” in terms of putting  JWST observing time to good use and interpreting the data when it comes down and work it into systems that tell a larger story.

Meadows proposed JWST observing projects for herself and colleagues in Cycle 1 and for the global JWST T-1 Community Initative, but they weren’t selected in the first round.  Six VPL members are on JWST observing teams and Meadows expects the VPL team to be be well represented observers on JWST in the future.

Meadows is, however, a member of a JWST Cycle 1 observing team, led by Laura Kreidberg of Max Planck Institute for Astronomy in Germany.  It will study the very intriguing TRAPPIST-1c, a terrestrial planet in a solar system with seven rocky planets, the only one of its kind discovered so far. The overriding goal of the team effort is to determine whether the planet has an atmosphere — something that is essential for life.

Meadows said she and colleagues will be developing photo-chemical climate models of different plausible evolved planetary atmospheres for TRAPPIST-1 c — that is, atmospheres that may have undergone prior ocean and atmospheric loss.  In addition, she will produce simulations of thermal emission from a model of TRAPPIST-1 c, with the results becoming part of the team interpretation about whether the planet does, or did once,  have an atmosphere.

If it does have an atmosphere, then that alleviates broad and deep concerns that red dwarf systems such as TRAPPIST-1 burn off planetary atmospheres in their early phases due to a brighter young star and massive stellar flaring. (Red dwarf stars almost always have very active flaring at their inceptions, and their orbiting planets are closer than with a star like ours.)  So detecting an atmosphere around TRAPPIST 1-c, or any of the seven TRAPPIST-1 planets, would be huge.

But if there is no atmosphere to be found on any TRAPPIST-1 planet, then the likelihood of finding habitable planets around red dwarf stars certainly does decline.  And since red dwarf stars are by far the most common type of stars in the galaxy, that has substantial significance for the overall search for life beyond Earth.

That so much new in astrobiology will be coming from JWST explains why scientists in the field worked so long and hard to make that 2040s NASA Flagship astrobiology mission a reality, or at least the outlines of a reality.

Innovative and ever-more powerful observatories are essential to push forward the goals of astrobiology and the search for potential extraterrestrial life and habitable worlds.   It’s a long-term project, though with a flood of ground-breaking and important science being produced along the way.

While VPL is a science engine, it is also affiliated of the University of Washington astrobiology program and has produced a stream of scientists who have gone on to NASA centers, universities and institutes.

It is a virtual lab and always has been by design, with VPL team members from around the country joining as part of the periodic VPL funding proposal to NASA.  Those team members may then bring in students and postdocs involved in related work.

The result has been the creation of a long-lasting, quite cohesive collection of scientists who are sometimes part of VPL proposals and sometimes not — depending on the focus of the proposal and interests of the researchers.  But the connection always seems to remain, especially for early career scientists.

As Meadows described it, “We have a long standing joke that VPL is the ‘Hotel California of teams’ in that you can check out anytime you like, but you can never leave.”  She said that might sound jokingly sinister, ” but I think it is a happy thing, in that it enhances team cohesion.”

Three current students came together to talk about VPL and what they do there.  As it turned out, their work traces a progression of exoplanet topics — from stellar research to atmospheric studies to work on what can be learned about exoplanet interiors, all in the context of modelling.

Evan Davis is a post baccalaureate research scientist at UW and he focuses on the many ways that a star (or our sun) will influence an orbiting planet.   In particular, he is simulating how solar flares, UV radiation and other high-energy particles coming from sun-like and red dwarf stars change the atmospheric chemistry of exoplanets that orbit them.

For Megan Gialluca, a grad student the UW’s Astronomy and Astrobiology Program, the focus is on the atmospheres of terrestrial planets, especially those in the TRAPPIST-1 system.  Her focus now is on thermal atmospheric escape — modeling what happens to a planet with an atmosphere and ocean when it’s bombarded by the radiation of an early red dwarf star.

And Rodolfo Garcia , a graduate student in the school’s Astrobiology Program, is interested in what can be learned about the interiors of exoplanets by learning about the components of their atmospheres.  This involves the dynamics of volcanoes and what they spew out, and trying to learn (using Earth data) what the presence of a particular compound in the atmosphere means about the makeup rocky exoplanet interiors.

Garcia said, and the others agreed, that they especially look forward to working with the in-coming  JWST data because it will allow them to modify the focus of their modelling.  Rather than producing “forward” models that predict what will happen given certain bulk measurements of an exoplanet, they will be able to begin with the presence of certain observed chemicals and model “backwards” to see how and why those chemicals might be present.

Gialluca brought up another issue that she — and others — are concerned about at the dawning of the JWST era.  With so much new data coming in,  she said there will likely be many new possible biosignature chemicals and atmospheres detected and proposed.

While that is an exciting development, it is also potentially worrisome in terms of how that information will be shared with the public.  As astronomy and astrobiology icon Carl Sagan said long ago, extraordinary claims require extraordinary evidence.  But will the claims of newfound biosignatures via JWST come with that level of “extraordinary evidence?”

This is an issue very much at the heart of VPL’s goal of creating a robust and trusted science of exoplanet habitability and potential life.  And Meadows was co-chair of a concerted effort to produce a “standards of evidence” protocol” that the community would be encouraged to follow when scientifically assessing “extraordinary” claims regarding signs of extraterrestrial life.

While top NASA officials initially called for this standards of evidence effort,  the astrobiology community itself has been leading the endeavor.