Thermal evolution models for TOI-431 b and other ultra-short-period lava planets

Researchers have modeled the atmospheric and surface conditions of five rocky exoplanets that orbit their stars very closely. These ultra-short-period planets, including TOI-431 b, likely host permanent magma oceans due to extreme temperatures. By simulating their internal and atmospheric evolution, the study provides predictions for future observations of how these worlds maintain or lose their atmospheres.

Star system: TOI 431 | Distance from the Sun: 106 light years

Ultra-short-period rocky planets are worlds that orbit so close to their host stars that their dayside temperatures are hot enough to create permanent oceans of molten rock or magma. These planets represent a unique laboratory for studying the relationship between a planet's deep interior and its atmosphere. As the surface melts and gases are released into the atmosphere, these volatile gases can either build up to form a stable envelope or be stripped away by the intense radiation and stellar winds from the nearby star. Predicting the final state of these planets requires understanding how their interior geology interacts with their external environment over time.

To better understand these processes, new models were developed using a framework that links a planet's internal heat with its atmospheric composition. This simulation grid classifies the possible outcomes for five specific targets, including TOI-431 b, based on the final state of their interior and their ability to keep an atmosphere. By analyzing different scenarios for heat distribution and atmospheric escape, the models provide specific predictions on what to expect when observing these planets. These results help clarify which conditions allow a planet to retain a gas envelope capable of redistributing heat to the nightside versus those that become bare, rocky worlds.

Most of the studied targets gravitate toward a bare-rock state where the global amount of melted material is relatively low. For planets that do manage to hold onto a thin atmosphere, the chemical makeup of that gas changes significantly depending on whether the surface is molten or solid. Notably, the presence of sulfur gas serves as a potential indicator of whether the surface remains in a solid state or is covered in magma. These findings provide a clear guide for upcoming observations, helping to identify which specific chemical signatures can distinguish between different internal conditions on these irradiated rocky worlds.

Key findings
TOI-431 b and four other planets were analyzed to predict their atmospheric and geological evolution.
Most simulated scenarios for these planets point toward a bare-rock end state with low global melt fractions.
The presence of sulfur gas in an atmosphere may indicate that a planet has a solid rather than molten surface.
Atmospheric survival on these hot planets depends on the efficiency with which the planet loses gas to space.
The models will guide the interpretation of upcoming data from the JWST telescope to determine the true geophysical state of these worlds.

Scientific study

Thermal Evolution of Lava Planets Across System Ages: Predictions for Hell of a Survey (external link)