“The first paper is about how the day-night cycles of the planets vary,” said Gongjie Li, assistant professor in the School of Physics and a co-author of both studies. “The second paper is about the effects of the day-night cycles on climate. It is the first (work) to combine rigid-body simulations with 3D global climate models.” In this instance, “rigid body” refers to extended planetary bodies, and this is different from traditional dynamical approaches that assume the planets are point masses. 

“We found that the chaotic variations in the day-night cycles led to fast snowball transitions for TRAPPIST-1f,” the fifth planet from the red dwarf star that is the system’s sun. “This can render the planet trapped in a permanent snowball state, and makes it less favorable to life that we know of,” Li said.

The team for the first study, published in The Astrophysical Journal in December 2022, “GRIT: A Package for Structure-Preserving Simulations of Gravitationally Interacting Rigid Bodies,” includes Li and two researchers from the Georgia Tech School of Mathematics: Associate Professor Molei Tao and graduate student Renyi Chen.

Li and Howard Chen, assistant professor at the Florida Institute of Technology, collaborated with Ravi Kumar Kopparapu, a planetary scientist for NASA; and Adiv Paradise, who developed a 3D climate model used for the second study, “Sporadic Spin-Orbit Variations in Compact Multi-planet Systems and their Influence on Exoplanet Climate.” That study was published this month in Astrophysical Journal Letters. 

Rotating into ‘crazy’ day-night cycles

The TRAPPIST-1 system, discovered in 2017 is often called “the miniature solar system with seven rocky planets.”  The system hosts the most Earth-sized planets found in the habitable zone of a single star to date. It is also, according to NASA, the most studied planetary system, second to our own. 

Three of the planets, TRAPPIST-e, f, and g, are considered to be in their sun’s habitable or “Goldilocks” zone, because their distance from their sun means temperatures aren’t too hot or cold. Liquid water may also flow near or on the surface of these planets. The research done by Li and her colleagues for the first study included formulas for computer simulations that take into account gravitational pulls from these planets and their sun, along with their tidal forces. 

The tidal forces in the TRAPPIST-1 system bring up a similarity to Earth’s moon. “The planets reside very close to their host star and experience strong tidal interactions with that star, and thus were expected to be tidally locked, similar to the case of our own Moon, with one face always towards Earth, and permanent dayside and nightside,” Li said. 

The formulas produced results that suggested the planets’ rotation could become asynchronous, or chaotic, in 10 years, as those dynamics significantly affect their rotation. Those factors “can kick the outermost three planets” — TRAPPIST-1f, 1g, and 1h — “out of a tidally locked stage, and into having crazy day-night cycles,” Li said.

A potentially habitable planet could become an uninhabitable snowball

The second research study combined those algorithms with a 3D climate model. “We include clouds, rainfall, and solar radiation in 3D in these models, and we coupled the spin-dynamics of the planets with 3D global climate models, the first time in the literature to do so.” said Chen, the lead author of the second paper.

The researchers knew that existing climate modeling had shown that tidally-influenced terrestrial exoplanets — particularly those orbiting M-dwarf stars like TRAPPIST-1 — have unique atmospheric dynamics and surface conditions that may boost their likelihood to host livable habitats. 

Yet imagine a planet showing a different “face” to its host star than it usually does. Different cooling and warming cycles would take over, especially for those exoplanets farthest away from TRAPPIST-1.

“TRAPPIST-1e is very warm, and the crazy day-night cycles don’t affect the climate much. However, TRAPPIST-1f is a lot colder, and a change in day-night cycles can make it a snowball.” The reason: when the planet rotates, the new hot dayside doesn’t have enough time for the existing ice to melt, and when the new nightside forms ice, it leaves the planet covered in snow.

The James Webb Space Telescope (JWST), launched in December 2021, recently began focusing on the outermost TRAPPIST-1 planets. The Telescope’s instrument package includes ways to detect carbon dioxide, methane, and oxygen molecules in an exoplanet’s atmosphere, which may yet yield clues to life.

 “We do not know what the climate is like on those planets yet,” Li said. “However, future studies by JWST on the atmosphere compositions of the planets will help us know more about its climate, and test our results.”

Funding for the two TRAPPIST-1 studies is provided by NASA. 

Citations: Citation Renyi Chen et al 2021 ApJ 919 50
DOI 10.3847/1538-4357/ac0e97

Howard Chen et al 2023 ApJL 946 L32
DOI 10.3847/2041-8213/acbd33

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