Orbital and atmospheric evolution of the mini-Neptune TOI-2257 b

The exoplanet TOI-2257 b orbits its host star every 35 days with a highly eccentric path. New simulations show the planet sits near a critical boundary where tidal forces and atmospheric escape processes transition. Depending on whether the planet is a gaseous mini-Neptune or a rocky super-Earth, it may retain a significant hydrogen envelope or maintain conditions potentially suitable for liquid water.

Star system: TOI-2257 | Distance from the Sun: 189 light years

TOI-2257 b represents a unique case in planetary science due to its 35-day orbital period and high eccentricity, which describes how much an orbit deviates from a perfect circle. Located in a sparsely populated region of planetary parameter space, it sits at a critical threshold where the combined effects of stellar radiation and tidal forces - gravitational stresses caused by the host star - begin to lose their influence over the planet. Simulations show that the planet remains near this transition boundary, whether it evolved with a thick, primordial hydrogen envelope or as a rocky world.

Researchers tested these evolutionary scenarios by modeling the interaction between the host star and the planet over billions of years. In a mini-Neptune scenario, where the planet possesses a gas-rich atmosphere, it maintains at least a thin layer of hydrogen for 8 billion years. Tidal evolution, which gradually circularizes an orbit, only affects the planet if its initial atmosphere was massive enough to create sufficient gravitational torque. Conversely, as a rocky super-Earth, the planet would lose significant surface water early in its life but could still remain potentially habitable if it started with a large enough reservoir of water.

If the planet is composed primarily of rock, its current orbital eccentricity cannot exceed 0.62. This limit provides a testable prediction for future observations. If researchers measure an eccentricity higher than this value, it would confirm that TOI-2257 b must possess a significant gaseous envelope rather than a rocky surface. The findings help clarify how different types of planets evolve in the region between gaseous worlds and terrestrial-like planets.

Key findings
TOI-2257 b sits at a critical boundary for tidal and atmospheric evolution processes.
Mini-Neptune models predict the retention of a hydrogen envelope for at least 8 billion years.
A rocky super-Earth version of the planet would require significant initial water content to remain potentially habitable today.
Tidal heating levels are comparable to the moon Io and insufficient to trigger a runaway greenhouse effect.
Orbital eccentricities above 0.62 would strongly suggest a gaseous composition for the planet.

Scientific study

The Limits of Tidal Circularization and Atmospheric Escape for Mini-Neptunes and Super-Earths: The Case of TOI-2257 b (external link)