Zifan Lin, a McDonnell Center Postdoctoral Fellow at Washington University in St. Louis, is part of a broader effort to understand exoplanets in some of the most extreme environments in the universe. He is affiliated with the McDonnell Center for the Space Sciences and the Department of Physics, where he works with Tansu Daylan to identify planets orbiting white dwarfs. White dwarfs are the dense, cooling remnants left behind after stars like our Sun exhaust their nuclear fuel.

Lin’s path to St. Louis spans continents and includes some of the world’s top academic institutions. He came to the United States from China in 2016 to attend Cornell University as an undergraduate, later earning his PhD at Massachusetts Institute of Technology. Since then, his work has focused on understanding where and how planets exist beyond our solar system.

His current research is tied to simulations of the Galactic Bulge Time-Domain Survey, a major observational campaign planned for NASA’s upcoming Nancy Grace Roman Space Telescope, scheduled to launch as early as fall 2026. The survey will focus on the galactic bulge, a dense, central region of the Milky Way. While most stars lie in a relatively thin disk, the bulge contains a tightly crowded population, making it a promising place to search for new planetary systems. However, observing that region directly remains a challenge.

“We currently don’t have the ability to observe very far into the galactic center in detail,” Lin explains. “So we rely on simulations to understand what we expect to find.”

Those simulations are grounded in light, specifically in the number of photons, or particles of light, that reach a telescope. By modeling how brightness changes when a planet passes in front of a star, Lin can predict what those signals should look like in real observations. This technique, known as the transit method, becomes especially powerful when applied to white dwarfs.

A white dwarf can contain the mass of our Sun compressed into a body roughly the size of Earth. That extreme density gives astronomers a unique observational advantage. If a planet passes in front of it, the drop in brightness can be dramatic, sometimes as much as 50 percent, compared to less than 1 percent for planets transiting larger stars.

“It’s a huge signal,” Lin says. “And because the star is so small, the transit can happen very quickly, sometimes in just a few minutes.”

Even with such a strong signal, these systems remain elusive. To date, only one transiting exoplanet around a white dwarf has been confirmed. Lin’s work aims to change that by predicting how many such systems might exist and how future instruments can detect them.

The Roman Space Telescope will play a pivotal role. Over five years, it will dedicate 438 days to the Galactic Bulge Time-Domain Survey, observing more than 100 million stars across six high-cadence observing seasons, or periods of very frequent observations. In what scientists describe as a cosmic treasure hunt, the mission will use techniques like microlensing, a phenomenon where the gravity of a star or planet bends light from a more distant star, briefly making it appear brighter, almost like a natural magnifying glass. It is expected to uncover thousands of new exoplanets, including worlds similar to those in our own solar system, while also revealing new insights into how stars and planetary systems behave.

For Lin, the excitement lies in what hasn’t yet been seen.

“We’re exploring a region where stars are most densely packed, and planetary systems may behave very differently,” he says. “There’s a lot we still don’t know.”

Lin’s work also includes theoretical studies of planetary systems. A recent paper co-authored with Daylan, “Persistent Thermal Anomalies in Rocky Worlds,” has been accepted for publication in The Astrophysical Journal and examines the internal heating mechanisms that can shape planetary evolution and infrared emission. 

Outside the lab, Lin has found a home in St. Louis. He enjoys cooking and experimenting with cuisines from around the world, blending his love of traditional Chinese dishes with new culinary influences. He has also taken up home bartending, crafting both classic cocktails and original creations.

It’s a fitting parallel to his scientific work, combining precision, creativity, and a willingness to explore the unknown.

As the Roman Space Telescope prepares for launch, Lin’s simulations are helping lay the groundwork for what could be a transformative era in exoplanet discovery. In the dense glow of the galactic center, he is searching for fleeting shadows, brief dips in light that may reveal entirely new worlds.

Header image: An artist's concept of an exoplanet and debris disk orbiting a polluted white dwarf. (NASA/JPL-Caltech)