Scientists examine climate of exoplanets, inspired by Milankovitch cycle

The team modeled seven planets in the TRAPPIST-1 system for this study. They examined the planets' spins in this system. 

Their preliminary findings suggest that planets close together in multiplanet systems can influence each other's spin rate and that the spin rate can change significantly over time, as per the statement.

"This means that the star shines on a planet unequally at different times. It's not the constant or fixed or equal case anymore, which is what the usual assumption is for these 'tidally-locked' planets. Instead, it's distributed. The sunlight's distributed unevenly across the planet. And that has major implications for a subtype of a planet which are planets at the outer edge of the habitable zone,” explained Chen in a statement.

The study notes that it is important for a system to be compact with planets of a certain size and mass. Less massive planets may not affect the spin rates of other planets.

In our solar system, for example, Mars has a minor influence on Earth's spin, whereas Jupiter has a much larger influence on Earth. From a geological standpoint, Earth's climate has changed over time, which is also due to the influence of the moon, other planets, and the sun on the planet's orbit.

"What we can verify is the climate predictions, the surface chemistry of the planet. We can look at the thermal emission, and then we can see the temperature and surface features of these planets. Is it what we found? If it is, then our model is correct," said Chen.

The results have been published in the journal Astrophysical Journal Letters.

Study abstract:

Climate modeling has shown that tidally influenced terrestrial exoplanets, particularly those orbiting M-dwarfs, have unique atmospheric dynamics and surface conditions that may enhance their likelihood to host viable habitats. However, sporadic libration and rotation induced by planetary interactions, such as those due to mean motion resonances (MMR) in compact planetary systems, may destabilize attendant exoplanets away from synchronized states (1:1 spin-orbit ratios). Here, we use a three-dimensional N-rigid-body integrator and an intermediately complex general circulation model to simulate the evolving climates of TRAPPIST-1 e and f with different orbital- and spin-evolution pathways. Planet f scenarios perturbed by MMR effects with chaotic spin variations are colder and dryer compared to their synchronized counterparts due to the zonal drift of the substellar point away from open ocean basins of their initial eyeball states. On the other hand, the differences between perturbed and synchronized planet e are minor due to higher instellation, warmer surfaces, and reduced climate hysteresis. This is the first study to incorporate the time-dependent outcomes of direct gravitational N-rigid-body simulations into 3D climate modeling of extrasolar planets, and our results show that planets at the outer edge of the habitable zones in compact multiplanet systems are vulnerable to rapid global glaciations. In the absence of external mechanisms such as orbital forcing or tidal heating, these planets could be trapped in permanent snowball states.