LHS 3844b is an exoplanet slightly larger than Earth orbiting the red dwarf star LHS 3884, 48.5 light-years from our solar system. Unlike Earth, it has fixed tidal forces and rotates around its axis in exactly the same amount of time it takes to orbit a star. As a result, one hemisphere is constantly exposed to scorching sunlight, while the other remains in eternal darkness, cold enough to approach absolute zero (zero Kelvin).
At first glance, these extreme environments seem completely inhospitable. Daytime temperatures can reach around 1,000 to 2,000 Kelvin, but the night side is so cold that particle movement virtually stops. But new research suggests these worlds may not be as hostile to life as they seem.
“Just by looking at the extreme temperatures on the dayside and nightside (such as 1,000 to 2,000 Kelvin on the dayside and absolute zero on the nightside), one might conclude that these exoplanets are too harsh for life,” says Daisuke Noto, a postdoctoral fellow in Hugo Ulloa’s Penn-Geffrow Laboratory at the University of Pennsylvania.
In a study published in nature communicationsNoto and his collaborators from the Japan Agency for Marine-Earth Science and Technology and Hokkaido University found that “such exoplanets may be more tolerant of supporting life because ‘tidal locks’ can help maintain a moderate thermal environment locally by dispersing heat flux laterally.”
Why are tidally locked exoplanets so common?
This discovery challenges the common assumption that planets always show the same face to their stars. Noto said worlds with constant day and night are far more common than planets like Earth, which experience a regular day-night cycle.
“Many objects such as moons and planets that are very close to their parent stars are so-called tidally locked,” he explains. “So as they rotate on their own axes and orbit around their parents, their speeds and frequencies match, causing a phenomenon where we only see one side of the moon.”
This constant orientation creates dramatic temperature differences across the globe. Rather than focusing only on surface conditions, the researchers wanted to understand what was going on deep within the planet, specifically the mantle, the thick layer of rock between the crust and core.
Recreating an alien planet in the laboratory
Rather than relying solely on computer simulations, the research team built a physical laboratory model that mimics the interior of a tidally locked planet.
“Building an actual exoplanet in a lab was not in the budget,” Noto jokes.
Instead, the researchers used a tabletop rectangular tank filled with viscous glycerol and tiny thermochromic liquid crystals that change color in response to changes in temperature. Similar experimental systems have long been used to study how heat moves through slowly moving materials, and can serve as a stand-in for planets’ rocky interiors.
Unlike weather and ocean currents, which are strongly influenced by the Earth’s rotation and gravity, convection within the rocky mantle is mainly caused by differences in temperature and density. To recreate these conditions, the team installed four thermostats around the tank to heat and cool different areas, creating temperature gradients similar to those expected between the permanently illuminated side, permanently dark side, surface, and deep interior of a tidally locked exoplanet.
planetary heat engine
Experiments revealed a very stable pattern. The hot material consistently rose beneath the dayside, flowed across the upper region, cooled as it reached the nightside, and sank before returning through the lower mantle. The result is one continuous circulation loop that acts like the planet’s steady heartbeat.
“It’s not chaotic like Earth’s mantle,” Noto says. “It’s slow and steady. It’s predictable. It’s a little boring, but in a good way.”
Researchers also observed occasional mushroom-shaped plumes rising from the bottom of the heated tank. Unlike volcanic hotspots on Earth, such as those underground in Hawaii or Iceland, these plumes were fixed in one place rather than drifting over time.
Measurements of heat transport, known as the Nusselt number, were comparable to those observed in Earth’s mantle. The discovery suggests that some tidally-locked exoplanets may maintain local geothermal environments that provide favorable conditions for life, especially in warmer mid-latitudes.
What does this mean for alien life?
Stable circulation patterns can affect more than just surface temperature. Noto believes it could also affect the movement of the planet’s liquid core, creating a magnetic field different from Earth’s familiar dipole field.
“That’s something we weren’t able to test in this experiment, but it’s an interesting direction for future research,” he says.
See beyond the rest of the world
Noto and Ulloa continue to develop similar laboratory models to investigate a wide range of geophysical processes. Previous research from the Penn GEFLOW Institute investigated how heat and mass move through confined spaces, providing new insights into the role of fluids in hydrothermal systems.
“We plan to further expand our experimental methods to probe deeper into different systems on Earth in different contexts. The possibilities are literally out of this world,” Noto says.
Daisuke Noto is a postdoctoral fellow in the College of Arts and Sciences at the University of Pennsylvania.
Hugo Ulloa is an assistant professor in the Department of Earth and Environmental Sciences in Penn State College of Arts and Sciences.
Other authors include Takehiro Miyakoshi and Takatoshi Yanagisawa of the Japan Agency for Marine-Earth Science and Technology. Tomomi Terada and Yuji Tasaka of Hokkaido University.
