Looking for life in all the wrong places: How one project is flipping the script on the search for habitable worlds

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Thursday, November 18, 2021 


By Katy Cain

In the 1990s, scientists discovered the first planet orbiting another star. Just thirty years later, we know of a staggering 4000+ worlds outside of our own Solar System. With a solid spread of planets to choose from, some scientists have shifted focus from the discovery of planets to something even rarer than the planets themselves—they’re looking for life.

But it turns out that to find life from light-years away, we first have to know what isn’t life.  This is a challenging task, considering the planet we know best, Earth, is also abundant with things like trees and humans—and humans learn about unknown things by first comparing them to the known.

This tendency meant we assumed our planetary neighbors were just like us.


The search for life on other planets

It wasn’t that long ago that scientists thought we might find sophisticated life on our next-door neighbor Mars. As we took our first steps into space, sent out exploratory missions, and learned more about the development of our Solar System, we realized that we are probably not going to get to meet and greet with Martians. 

In fact, right now Earth is the only place we know that can host life in our Solar System—give or take some organic meteorite molecules. While some scientists hold out hope for finding life in places like our Solar System's water worlds—Saturn’s moon Enceladus, for example—others began looking further afield. 

Astronomers today use giant telescopes, both on Earth and orbiting in space, to search the skies looking for potentially habitable worlds orbiting other stars. But what makes a planet habitable?

Panorama of the Magellan Telescopes at Carnegie’s Las Campanas Observatory in Chile as the Milky Way stretches overhead. Courtesy Carnegie Institution for Science/Yuri Beletsky.

In December 2021, the James Webb Space Telescope will join NASA's Hubble Space Telescope in space. It will be the most powerful space telescope ever built and will play an essential role in understanding the development of planets, planetary systems, and galaxies. Image credit courtesy NASA/Desiree Stover.

“The question of planetary habitability has likely inhabited human thought for quite some time,” says Richard Carlson, Director of Carnegie’s Earth and Planets Laboratory, “and has taken on additional significance with the discovery that essentially every star you see in the night sky has planets orbiting it.”

While you might think we’d be on the hunt for microbes and little green men, Anat Shahar, a geochemist at the Earth and Planets Laboratory, explains: “Planetary habitability begins as more of a chemical and physical process than a biological one.”

Generally speaking, potentially habitable exoplanets have three key ingredients: 
 
  1. 1. Liquid water: They orbit their stars at a distance where water can remain a liquid at the surface of the planet—neither frozen from being too far away, nor so close that water boils off into a vapor. These Goldilocks planets are in what we call the “Habitable Zone.”


  2. 2. Magnetic field: Habitable worlds must have a magnetic field to protect the surface from dangerous ionizing particles and solar wind. This most likely requires a churning and convecting iron core. Scientists think Earth’s magnetic field also played an essential role in keeping our atmosphere from being stripped away by the solar wind.


  3. 3. Tectonic activity: For the full trifecta, researchers suspect that a habitable world needs tectonic activity, which drags surface materials into the planet and back up to the surface again over billion-year timescales—recycling nutrients as well as other volatile materials like water and carbon dioxide.

 
“If you ask an Earth scientist like me what habitability means, I would tell you that I’m not 100% sure,” Shahar explains. “But I know that on Earth, these three key ingredients definitely make our planet a more habitable environment. So, if an exoplanet was found that had water, a magnetic field, and plate tectonics, it would be a great bet that it would have life as well.” 


Examining exoplanets

This visualization shows all of the Kepler multi-planet systems (1815 planets in 726 systems). The size of the orbits is to scale but the planets are not. Overall, the Kepler mission discovered 2,662 exoplanets. Courtesy Ethan Kruse (http://www.ethankruse.com/dataviz.php).

The statistics are extremely pro-alien. Thanks to the sheer number of exoplanets that have been discovered, the ease with which models tell us planets organize themselves into orbits around stars, and recent leaps forward in our knowledge of the Milky Way’s chemical evolution, today’s scientists are fairly certain we will find life (or a planet that could host life) out there somewhere—if we know where to look.

We can’t look at a planet light-years away to physically see if it has tectonic activity. So what can we learn about a planet by studying them through modern telescopes and instruments?
 

This commonly used graph shows the habitable zones for different star types in addition to known planets that reside in habitable zones. Image courtesy Chester Harman.
For one, we can generally tell if a planet is in the habitable zone—where liquid surface water can exist—by measuring the distance to its host star and figuring out its surface temperature from there. But, being in the habitable zone is far from the whole story. 
 
We can also determine a planet’s mass by seeing how it interacts with its host star. Stars and their planets are locked in a gravitational dance in which they are both pushing and pulling on each other.  This means even a small planet causes a slight wobble in its host star that we can measure and use to work out its mass. Once we know a planet's mass and radius, we can figure out its density and determine if it’s rocky, gaseous, or metallic. Because Earth is our only source of comparison, scientists assume that habitable planets should be rocky.

Then there’s the atmosphere. 
 
When we find a planet we want to study, we look at the starlight that permeates and interacts with the planet’s atmosphere. Each chemical in the atmosphere gives off or absorbs a signature wavelength of light. As the light makes its way to our telescopes, we use an instrument called a spectrograph to break it apart—creating a colorful spectral fingerprint that astronomers decipher to tell us what makes up the atmosphere.

In the example above, we see the absorption spectra of hydrogen. If an astronomer saw this fingerprint in their data, they would know that there is hydrogen in the atmosphere of the planet they are stuyding.  Image courtesy. Credit: NASA, ESA, and L. Hustak (https://webbtelescope.org/resource-gallery/articles/pagecontent/filter-articles/spectroscopy-101--how-absorption-and-emission-spectra-work)
In these spectra, scientists can detect things like water, carbon dioxide, and methane that could be signs of life. They can also use the atmospheric composition to get a glimpse into the planet’s internal processes—since the planet is what’s building and sustaining the atmosphere.

Shahar explains, “In our lifetime, the most likely place we will be able to see signs of life outside of our Solar System is in the atmosphere of another planet.”

That means scientists need to get extremely good at deciphering spectra. 


Cutting through the noise

It’s not easy to detect gases in the atmospheres of distant planets.

If you were following astronomical news in 2020, you might remember how a controversial discovery of phosphine in Venus’  atmosphere led people around the world to think there might be alien microbes as close as our neighboring planet. You’ll also remember how the finding was almost immediately disproven as the phosphine signal was determined to be sulfur dioxide. The two materials have very similar wavelengths.

The phosphine story is a great example of how deciphering planetary atmospheres from afar can be a major challenge. As astronomers probe the atmospheres of planets looking for anomalies in wavelengths that could indicate some key component that we know is associated with life, they are also sifting through a sea of noise that can lead to false positives.


That’s where the AEThER project comes in (AEThER: Atmospheric Empirical, Theoretical, and Experimental Research). The project recently received a 1.5 million dollar grant from the Alfred P. Sloan Foundation.

Shahar, who is the principal investigator on the project, wants to flip the script on exoplanet habitability.
Instead of looking for signs of life directly, the project aims to first define the non-life signals. To do this, she is leading an interdisciplinary team of scientists to study the most common size planet in the galaxy—sub-Neptunes.

Sub-Neptunes: the missing planet

NASA’s Kepler Space Telescope was launched in 2009 with the express mission of finding Earth-like exoplanets orbiting other stars. The Kepler mission officially ended in 2018, having observed half a million stars and discovered 2,662 exoplanets. 

The Kepler mission also revealed a surprising truth about the galaxy. About 75 percent of the discovered planets have radii somewhere between that of Earth and Neptune—a planetary size that is curiously missing from our own Solar System. These are called sub-Neptunes. 
 
This graph shows the surprising abundance of sub-Neptune planets in the galaxy and the two populations of sub-Neptunes: super-Earths and mini-Neptunes. Image courtesy of Peter Gao, based on the graph by Fulton and Petigura.
“There is a big gap in our Solar System,” says Peter Gao, another staff scientist at the Earth and Planets Laboratory who studies exoplanet atmospheres. “But for exoplanets, this sub-Neptune space is where most exoplanets actually are.”

There appear to be two main categories of sub-Neptunes: super-Earths and mini-Neptunes. Super-Earths are planets up to around 1.8 Earth radii. They are rocky and may have similar atmospheres to our own planet, so they get all the hype. It’s easy to fantasize about visiting them one day and turning them into sci-fi utopias full of space ships and algae farms. 
 
For the AEThER project, the real star of the show is the group of slightly larger planets called mini-Neptunes—which are up to around 3.5 Earth radii. After about 1.5 Earth radii, planets start retaining more atmosphere, until they are swathed in a thick blanket of gas. However, they often stop growing before they reach the size of Neptune-like gas or ice-giant planets. Mini-Neptunes are more abundant than super-Earths and their thick gaseous blankets give scientists a lot to work with, since the main way we can study a planet’s composition is to observe how light passes through its atmosphere.

“We have to lay the groundwork,” says Shahar. “When we started this project, we asked ourselves, ‘What is the most common type of planet in the galaxy?’ and, ‘What is the most common atmosphere a planet could have?’”

Sub-Neptunes make up most of the exoplanets that Kepler observed, but because we don’t have them in our Solar System they are still a bit of a mystery. Studying this population of planets, Shahar’s team hopes to build a more thorough understanding of what the majority of planets in our galaxy look like.

“Once we have the baseline, we can start getting more complicated,” Shahar explains.

With this information, she hopes to cut through the non-living noise, rule out “false positives,” and give future scientists a reliable framework to detect life from light-years away.

 
“The goal,” says Shahar, “​​is to find what the abiotic atmosphere of an exoplanet should be. We want to figure out what no-life looks like in a whole bunch of different scenarios.”

Overall, the AEThER team is trying to come to an understanding of planetary habitability that answers the following questions:

1) What controls a planet’s atmosphere and evolution?
2) What is the abiotic composition of the atmosphere of the most common type of planet?
3) How sensitive is atmospheric chemistry to interior chemistry?
4) How does the composition of a planet influence its capacity to harbor life? 
5) What percentage of small planets have atmospheres? 
6) What is the range of planetary and atmospheric conditions that exist? 
 

A holistic approach to habitability

Anat Shahar put together this flowchart to illustrate how planetary habitability is an extremely complicated field of science. Courtesy Anat Shahar.
Despite the seemingly astronomical nature of the project, Shahar is not an astronomer—she’s a geochemist. That is by design. Members of AEThER think a diversity of thought will be key to the success of the expansive project.

The research questions cross what seems like the entire breadth of planetary sciences —from modeling inner core dynamics to directly observing atmospheres. To get at them, scientists from a wide variety of fields will have to work together.

Right now, geochemists in Shahar’s group have started conducting experiments looking at chemical interactions occurring at the interface between a planet’s surface and its atmosphere. A group of geophysicists is building computer models of exoplanetary interiors to see how they impact their atmospheres. Another is looking at the structure of the atmosphere itself and modeling clouds and hazes.

Astronomers will take all of the parameters set by the experimental and modeling efforts and use them to build a hypothetical spectra of what the average abiotic atmosphere should look like. The group will then test their hypothesis by comparing the theoretical spectra to actual spectra of key sub-Neptunes to see if they match up.

“It’s really varied,” says Shahar, “and that’s what makes it both exciting but also complicated. We’re doing everything from high-pressure experiments to modeling not only a solid planet but also its atmosphere, and then we get to bring the astronomy online.”

“It’s a full-spectrum approach. It’s holistic."



The gang’s all here! 

So far, around 40 scientists are involved in the AEThER project. Image courtesy Anat Shahar.
Our campus is home to an academic community of researchers who study a whole suite of planetary and physical sciences. Geophysicists sit next to astronomers at lunch; atmospheric scientists share stories with seismologists. It’s a community designed for academic collaboration—precisely what the field of exoplanet study needs.

Shahar explains that she, astronomer Alycia Weinberger, and geophysicist Peter Driscoll had plans to create a multidisciplinary project to generally explore habitability on exoplanets from a variety of research perspectives—but organizing such large projects is often challenging, especially when it comes to finding funding. The call for applications from the Alfred P. Sloan Foundation provided a rare opportunity for them to fund such a multi-faceted and multi-disciplinary project.
 
“We sat down and thought about all the different disciplines we needed in order to get this done,” explains Shahar. “Within those disciplines, we found people that were experts, and—this is very key—also had reputations of being very nice people to work with.”

Shahar and her colleagues brought together an intentionally diverse team of scientists from different backgrounds, disciplines, institutions, and stages in their careers. Among the AEThER team are geochemists, astronomers, atmospheric scientists, and geophysicists. There are grad students and very senior tenured professors—all coming together to answer one of the biggest questions available to humanity.

Typically, scientists from different fields may not ever incorporate such multi-disciplinary thinking into their work. And even if they do, that type of collaboration is probably done after publication. This project depends on scientists bringing different and sometimes competing perspectives to the table before the work is done. This collaboration encourages scientists to dig deep and come to an understanding even before the first spectrum is analyzed.

“One of the most unique things about our meetings is that everyone is coming together from such different perspectives that they might not even know a word someone is using,” says Shahar. “We’re trying to cultivate a culture where we ask questions and explore the assumptions we make in every field.”

Including postdocs and graduate researchers, the team had grown to include more than 40 people.



The start of something bigger

The AEThER project officially started in September and will continue for three years. As things get rolling, the team will dedicate the first year to laying out the questions and starting the research. Shahar is hopeful that by the second year, the group will have some results to publish.

“I’m not suggesting we’ll be able to pin it all down in three years. But hopefully, we can get the ball rolling and then secure more funding,” says Shahar, who is looking at the Sloan grant as a seed for a larger project.  

And like planting the seed of a tree so your children can sit in the shade, the AEThER project looks to the future. By setting the stage and establishing a baseline for what an abiotic atmosphere looks like, Shahar and the AEThER team will give the habitability research community a vital tool for filtering through the signals to pinpoint signs of life. 

In the future, this work could set the stage for exoplanet scientists to identify not just habitable planets, but inhabited planets. Or, perhaps, find a new planetary home for humanity, though that is currently in the realm of sci-fi.

While these big findings may be years off, Shahar and all of the scientists on the AEThER team must continue to do science for the sake of knowledge itself—but the idea of finding life on other worlds still sits in the back of their minds.

“I mean, who doesn’t want to be one of the first people to find life on an exoplanet?” says Shahar with a laugh. 


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