Exoplanet hunters: What it takes to find needles in a cosmic haystack

Exoplanet hunters Jessie Christiansen and Eric Ford describe the methods used to discover planets outside our solar system.
By | Published: December 7, 2023

A purely hypothetical field a few decades ago, exoplanet hunters inhabit one of the most dynamic areas in astronomy today. Each discovery brings new insights. From giant gas planets to so-called super Earths and everything in between, each detection expands our understanding of the universe’s cosmic landscape.

For the exoplanet hunters themselves, these discoveries are not just scientific data points; they are windows into the universe’s vast and varied architecture. The process of finding an exoplanet can be tedious. However, planet hunters are drawn by the thrill of the hunt.

What makes an exoplanet hunter tick?

First, a quick rundown of exoplanet identification methods is in order.

Radio astronomers Aleksander Wolszczan and Dale Frail discovered the first exoplanet in 1992, a planet orbiting a pulsar. Three years later a breakthrough find happened when Michel Mayor detected the first planet, 51 Pegasi b, orbiting a sunlike star. Since then, thousands have been found using various detection methods. By now, astronomers have discovered greater than 5,500 planets orbiting more than 4,000 stars.

The most prolific hunting method is the transit method, which has uncovered 74.6 percent of exoplanets thus far. This technique is akin to observing a miniature eclipse. Astronomers monitor the brightness of a star, looking for periodic dimming which indicates a planet passing in front of the star. This not only reveals an exoplanet’s existence, but also provides insights into its mass. The Kepler Space Telescope, which totaled the most finds to date, confirmed 2,778 planets, with an additional 1,984 candidates, before its lifetime ended in 2018. More recently, the Transiting Exoplanet Survey Satellite (TESS) is hard at work making discoveries, as is the James Webb Space Telescope.

Another crucial method is the radial velocity (RV) method. This method detects the subtle wobble of a star caused by an orbiting planet’s gravitational pull. This shift in the star’s light spectrum reveals the presence of a planet, its mass, and its orbit. Thus far, the radial velocity method has discovered some 19 percent of exoplanets.

Gravitational microlensing is based on Einstein’s Theory of General Relativity and detects planets through a star’s gravitational lensing effect passing in front of another. Almost 4 percent of exoplanets have been discovered via this method. The direct imaging method, though challenging, attempts to capture images of exoplanets directly. Think of spotting a firefly near a bright light. Just over 1 percent of the known exoplanets have been found this way.

Meet the exoplanet hunters

California Institute of Technology astrophysicist Jessie Christiansen found her calling growing up in rural Australia, where the night sky offered a clear, dark sky. As a child, she would lie on her trampoline, eyes fixed on the stars, captivated by the constellations and an allure of the unknown. Her early fascination with the sky led to a career that would put Christiansen at the forefront of exoplanet research. Now she is one of the leading exoplanet discoverers and is the lead scientist for NASA’s Exoplanet Archive.

“The idea that I can apply a love of visible planets in our solar system to the search for new worlds in the vastness of space is amazing,” she says.

Christiansen’s contribution to astronomy has been significant, particularly in the use of the transit method.

Christiansen and her colleagues begin by observing large swaths of the sky to maximize the likelihood of finding exoplanets. They utilize data from automated surveys conducted by space- and ground-based telescopes, which diligently record the star’s brightness. Each fluctuation in a star’s brightness, each minute variation, could be a clue pointing to a previously unknown planet.

“The process of identifying potential exoplanets from this data is complex and demanding,” says Christiansen. “Meticulous analysis is required, sifting through vast datasets to find patterns indicative of planetary transits. Once a potential planet is identified, our work is far from over. The detection of exoplanets using the transit method necessitates a vast amount of data due to the low probability of a planet transiting directly in front of its star from our viewpoint.”

Peering at the cosmos from Penn State

Eric Ford is an astronomer and distinguished professor at Penn State University. He was spurred to become an exoplanet hunter by the serendipitous discovery of planets like Mayor’s discovery of 51 Pegasi b during his freshman year. This discovery shifted his focus from cosmology to the burgeoning field of exoplanet hunting. Ford’s interest now lies not just in the hunt for Earth-like planets, but the complexity of planetary systems.

“Other planetary systems offer glimpses and clues to the formation and evolution of our own solar system,” he says. “It’s not just potential habitual worlds, it’s the universe as a whole I find fascinating.”

Ford’s hunting passion is driven by a blend of scientific intrigue, the challenge of deciphering cosmic signals, and the desire to understand our place in the universe. While he likes the radial velocity method, Ford emphasizes the importance of using multiple methods to obtain a comprehensive understanding of an exoplanet’s nature.

“I think it’s important to use a combination of methods for a more complete understanding,” he says. “By combining the radial velocity method to measure the planet’s mass and the transit method to measure its radius, we can infer the planet’s density and make educated guesses about its composition and atmosphere.”

Ford’s typical day involves extensive data analysis and collaboration. He works with data from the NASA archives, which might include observations from major telescopes or space missions. Collaborating with students, postdocs, and other faculty members, Ford then examines the data to identify potential exoplanets.

The details are essential to exoplanet hunters

“In our search for exoplanets, we rely heavily on precise measurements and data analysis,” Ford explains. “Planet hunting is a meticulous process, requiring both patience and a deep understanding of the data we’re working with. Essentially, we’re not just spotting these distant worlds; we’re trying to unravel the complex details of their existence.”

Ford continues that “The quest for exoplanets is not just about unearthing new worlds; it’s the scientific discovery and technological ingenuity. Every tiny shift in spectral lines we observe, every transit and minute Doppler shift we measure brings us closer to understanding other worlds.”

Christiansen agrees. “Exoplanet hunting is like piecing together a puzzle where each discovery adds to a grander picture of our universe,” she says. “Exhilaration lies not just in the finding, but in the continuous learning and the challenges we overcome. As we investigate the mysteries of these distant worlds, we are constantly reminded of space’s vastness and just how extraordinary it all is. This journey, with its blend of precision and unpredictability, is what makes exoplanet hunting an endlessly fascinating endeavor.”