Glowing Planets and Chemical Fingerprints
By Jackie Swift
In 1995, astronomers found the first known planet around a sun-like star outside our solar system. The news caused a buzz of excitement around the world. Finally we had proof of a planet orbiting another star like our own — the stepping stone, perhaps, to finding extraterrestrial life in far-distant solar systems.
Now, as of 2022, close to 5,000 exoplanets have been found. “We’ve gotten to the point where missions like TESS [Transiting Exoplanet Survey Satellite], for instance, are monitoring so many stars that when we process the data, planets just fall out of it,” says Nikole K. Lewis, an assistant professor of Astronomy in the College of Arts and Sciences at Cornell University. “They just found 10 new planets today.”
The proliferation of new exoplanet discoveries is a gold mine for Lewis, a planetary scientist who studies the physics and chemistry that shape worlds beyond Earth. In particular, she looks for planets that exhibit relatively unique properties. “I like to find the unusual,” she says. “And then I think about how those unique processes may indicate something we scientists haven’t thought about before. I also explore why we don’t see those same processes happening on Earth, or maybe even on any of the other planets in our solar system.”
More Than a Single Point of Light
Lewis and her collaborators create multidimensional maps of exoplanets using data from the now-retired Spitzer Space Telescope and from the Hubble Space Telescope. Soon they will also have access to observations made by the James Webb Space Telescope (JWST), the largest space telescope ever built, which is now fully deployed and gearing up to begin science operations as it orbits the sun a million miles from Earth.
“We’ve gotten to the point where [telescope survey missions] are monitoring so many stars that when we process the data, planets just fall out of it.”
Lewis anticipates that the JWST’s ultrasensitive infrared observations will be game-changing as the telescope peers back in time to the formation of the earliest stars and galaxies, as well as turns its powerful sights on planets in our own Milky Way galaxy. “Infrared is perfect for studying planets because the planets glow brightly in those wavelengths of light,” she explains. “We can also study the planets’ chemistry by using the wavelengths to look at the chemical fingerprints for molecules like water, carbon dioxide, methane, and others. And we look for aerosols — or clouds and hazes — which give very specific signatures as a function of locations on the planet and also at different wavelengths.”
The Lewis group uses all this planetary information to explore primarily gas giants, those planets, like Jupiter, composed mostly of helium and hydrogen. “We want to understand exoplanets as more than just a single point,” Lewis says. “We want to map a three-dimensional structure, to be able to say, ‘It has this much water and this much carbon dioxide, and it’s this temperature.’ We’re trying to figure out how those things vary throughout the planet: on the day side and on the night side, high up in the atmosphere and down low.
“If you think about Jupiter, how would you know what it looks like if you couldn’t send a craft there to take pictures?” she continues. “How would you figure out it has all those stripes? How would you know it has a Great Red Spot? Those are the types of things we can discern using our observations.”
First Steps in Finding Earth 2.0
Once the JWST is fully deployed, Lewis will be among the first scientists to use it. As the primary investigator for 130 hours of time and a team member for 200 to 300 hours’ worth of observations, she will be able to carry out a number of projects focused on specific planets.
One of her favorites is WASP-17b, a gas giant that orbits a star 1,324 light-years from Earth. WASP-17b takes only 3.7 days to complete an orbit, which means it transits, or passes in front of its host star, on a short timetable. This allows more opportunities for scientists to observe it in front of its star and to measure the light coming directly from it when it goes behind the star, Lewis explains. She and her colleagues will use the full bevy of the JWST’s instrumentation to capture WASP-17b in more detail than ever before.
The researchers will study more than a dozen planets altogether, ranging in size from gas giants down to a small terrestrial system of approximately Earth-sized planets. “By looking at different types of planets in different environments we can start to tease out why they are the way they are,” Lewis says. “That can help build the context for maybe finding Earth 2.0 someday. For me finding another Earth-like planet is important because it answers fundamental questions: How did we get here? Are we alone? Are there other planets out there like ours?”
The Possibilities of Bayesian Inference
In another branch of research, Lewis and her colleagues focus on theoretical questions. In one project she has joined with postdoctoral researcher Ryan J. MacDonald to develop a new theoretical software infrastructure using the statistical method known as Bayesian inference. The software will allow researchers to test hypotheses about a planet’s environmental conditions by rapidly generating a broad range of possible models as more evidence becomes available, to try to zero-in on the most likely reality.
The Lewis group has multiple projects in the works that use this new application of Bayesian inference methodology. In one, Lewis; Jonathan I. Lunine, Astronomy; and Ishan Mishra, PhD ’22 Astronomy, are looking at the icy surface of Jupiter’s moon Europa, exploring the potential to detect organics in the chemical signatures. In another, Lewis and postdoctoral researcher Eileen Gonzalez, a 51 Pegasi b Fellow, are applying the technique to point-of-light photos of planets, seeking to glean details of the planets’ atmospheres.
Spanning Disciplinary Divides
Lewis has a background in aerospace engineering and physics, and she loves to solve puzzles. Her interest in interdisciplinary endeavors and problem solving drew her to the Carl Sagan Institute, where she is currently the deputy director. “The search for life in the universe requires biologists, chemists, planetary scientists, atmospheric scientists, engineers — you really need people across a lot of different disciplines to answer the questions,” she says. “The institute is all about providing some infrastructure that allows you to span the barriers between the disciplines so researchers can be productive. My role is to bring together people to try to answer these questions about the search for extraterrestrial life in ways that maybe they wouldn’t have thought of before.”
Before coming to Cornell in 2018, Lewis worked at the Space Telescope Science Institute, which manages both the Hubble and the JWST telescopes. Indeed, she was a project scientist on the JWST, supporting the development of the telescope. “Part of my blood, sweat, tears, and definitely my soul are now in the JWST,” she says.
Still, she chose to leave that immediate world of the Hubble and the JWST to join Cornell because of the university’s emphasis on interdisciplinary research, she says. “The Cornell Astronomy Department is a mixture of astrophysics and planetary science — which doesn’t really exist at any other institution,” she explains. “It’s a place where researchers like me can thrive.”