Could exoplanets locked in eternal day and endless night support life?

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 and Hokkaido University recently reported in Nature Communications that “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.

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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.