Could exoplanets locked in eternal day and endless night support life?
Penn scientist Daisuke Noto and his collaborators explore the possibility of life on exoplanets with conditions seemingly too inhospitable.
7 min. read
The exoplanet LHS 3844b (left) is 1.3 times the mass of Earth and orbits the star LHS 3844 (right). Discovered in 2018 by NASA’s Transiting Exoplanet Satellite Survey, LHS 3844b is located 48.6 light-years from Earth and makes one full revolution around its parent star in just 11 hours. Because it has such a tight orbit, LHS 3844b is most likely tidally locked, meaning one side of the planet always faces the star while another side always faces away.
(Image: Courtesy of NASA)
Ever so slightly bigger than Earth, the exoplanet LHS 3844b orbits its parent star, LHS 3884, a red dwarf 48.5 light-years away from our solar system, in such a way that the speed of its axial spin mirrors the speed of its orbit. The result? One side of LHS 3844b is perpetually bathed in scorching sunlight, locked into a never-ending, blistering hot day, while the other is forever shrouded in darkness so cold that particles are incapable of movement, a state known as absolute zero (zero Kelvin).
Daisuke Noto, a postdoctoral researcher in Hugo Ulloa’s Penn GEFLOW Lab at the University of Pennsylvania, wondered if such a severe environment could support life. “Just looking at the extreme temperatures on the day and night sides—like 1,000-2,000 Kelvin on the day side and absolute zero on the night side—might lead one to conclude these exoplanets are too harsh for life. But,” says Noto, “life might find a way.”
Following that line of exploration, Noto and his collaborators at the Japan Agency for Marine-Earth Science and Technology andHokkaido University recently reported in Nature Communicationsthat “such exoplanets may be more tolerant of sustaining life as ‘tidal locking’ can contribute to maintaining moderate thermal environments locally by distributing heat flux laterally.”
In discussing the study and technique used, Noto explains why he thinks life may exist where conventional wisdom would say otherwise, and how he and Ulloa apply similar methods to unravel the hidden rhythms of Earth itself.
Noto says exoplanets—planets that orbit stars outside of our solar system—with permanent day-night conditions like LHS 3844b are far more common than ones with day-night cycles like Earth.
“Many celestial bodies like moons and planets that are very close to their parent stars are what we call tidally locked,” he explains. “Meaning, as they spin around on their axes and orbit around their parents, those rates/frequencies match, leading to the phenomena like us only seeing one side of our moon.”
This “locking” subjects the planet to an extreme and permanent temperature dichotomy. Noto’s work sought to understand what happens beneath the surface—more specifically, it looks at how this heat imbalance affects the mantle, the vast rocky layer sandwiched between a planet’s crust and its core.
“Building an actual exoplanet in the lab wasn’t in the budget,” quips Noto, so he and his team use a tried and tested workaround: a tabletop-sized rectangular tank filled with viscous fluid (glycerol) and seeded with thermochromic liquid crystals, colorful particles that shift hue with temperature.
Daisuke Noto’s tabletop tank experiment (left) mimics the extreme day-night temperature contrast on the tidally locked exoplanet LHS 3844 b, using colored liquid to visualize mantle motion. Hot fluid rises on the day side, cools, and sinks on the night side, creating a stable loop. When mapped onto a real planetary interior (right), the same pattern predicts focused volcanic activity on the sunlit side and a potentially habitable twilight zone in between.
(Image: Courtesy of Daisuke Noto)
This deceptively simple setup echoes a lineage of elegant analog models used to probe how temperature gradients and geometry influence convection in sluggish, stratified systems, from Earth's crust to imagined alien mantles.
Unlike atmospheric or oceanic systems, where Coriolis forces and gravity’s vertical pull dominate, explains Noto, mantle convection is driven primarily by temperature and density differences, especially in the sluggish interiors of rocky planets. To that end, the team placed four thermostats controlling heat and cooling at the tank’s edges, thereby creating gradients that sort of mimicked an exoplanet’s substellar and antistellar regions, surface, and deep interior.
They found that the mantle flow settles into a consistent loop, with hot material rising on the day side, sweeping across the top, cooling, and sinking back down on the night side before returning along the bottom—a single, persistent circulation, looping like a planetary heartbeat.
“It’s not chaotic like Earth’s mantle,” Noto says. “It’s slow and steady. Predictable. Kind of boring—but in a good way.”
Occasionally, that rhythm was punctuated by mushroom-shaped upwellings, plumes, rising from the tank’s heated base. But unlike Earth’s restless hotspots like those in Hawaii or Iceland, which drift across tectonic plates, these plumes remained anchored, always forming in the same place.
Noto's model revealed Nusselt numbers, or metrics for heat transport, similar to Earth’s, suggesting that some exoplanets might support localized geothermal conditions ripe for the ingredients that life needs—especially at the more temperate mid-latitudes.
The implications go deeper, Noto speculates, as this persistent hemispheric mantle flow could affect the planet’s liquid core, possibly giving rise to magnetic fields different from Earth’s dipole.
“That’s something we couldn’t test in this experiment,” he says, “but it’s an exciting direction for future work.”
Noto and Ulloa are working on a spate of new topics that mimic diverse geophysical systems, through similar laboratory analogs they’ve developed at GEFLOW. Previous work highlight mass and heat transport within a tight space, enlightening fluids’ role in hydrothermal systems.
“We are planning on further extending the experimental methods to delve deeper into different systems on our planet in different contexts, the possibilities are, quite literally, out of this world,” says Noto.
Daisuke Noto is a postdoctoral researcher in the School of Arts & Sciences at the University of Pennsylvania.
Hugo Ulloa is an assistant professor in the Department of Earth and Environmental Science in Penn Arts & Sciences.
Other authors include Takehiro Miyagoshi and Takatoshi Yanagisawa of the Japan Agency for Marine-Earth Science and Technology; and Tomomi Terada and Yuji Tasaka of Hokkaido University.
Penn engineers and collaborators have developed a transparent, micro-engineered device that houses a living, vascularized model of human lung cancer—a “tumor on a chip”—and show that the diabetes drug vildagliptin helps more CAR T cells break through the tumor’s defenses and attack it effectively.
Tumor-on-a-chip offers insight into cancer-fighting cells in immunotherapy
Penn engineers and collaborators have built a living tumor on a chip to expose how cancers block immune attacks, and how one existing drug could make immunotherapy like CAR T more effective against solid tumors.
Professor of city and regional planning Erick Guerra recently published a book exploring the economic and societal impacts of American highways. He explains some of the pitfalls associated with an ever-expansive highway system, arguing that spending more on highways might not be the solution to the country’s transportation issues.
Penn urban planner Erick Guerra’s new book, “Overbuilt,” argues that additional spending on building more highways might not be the solution to the country’s transportation issues. In a Q&A, Guerra shares his insights.
Xin Sun prepares samples collected from the Eastern Tropical North Pacific aboard a research vessel. By adding stable isotope tracers to these vials, Sun and her team can track how different microbial groups convert nitrogen compounds into nitrous oxide, revealing how subtle shifts in oxygen and organic matter change the ocean’s chemistry.
Can tiny ocean organisms offer the key to better climate modeling?
In the shadowy layers of the Pacific, microbes decide how much nitrous oxide—a potent greenhouse gas—rises skyward. New research from Penn’s Xin Sun offers an improved understanding of microbial ecology and geochemistry—key to forecasting global emissions in response to natural and man-made climate change.
Two X-ray plates from Arthur Goodspeed, believed to have created the world’s first X-ray image, were donated by his family to Penn’s University Archives.
