Looking for life in all the wrong places: How one project is flipping the script on the search for habitable worlds
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.
“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.”
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. 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. 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.
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?
Then there’s the atmosphere.
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
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.
Sub-Neptunes: the missing planet
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.
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.
“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.
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?
4) How does the composition of a planet influence its capacity to harbor life?
A holistic approach to habitability
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!
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.
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
“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.