Universe Today
Lava planets are some of Nature's most perplexing objects. Though they're rocky, they're locked in orbits so tight to their stars that they're molten. Scientists think that these planets are almost certainly tidally locked to their stars, meaning that their daysides always face their stars, while their nightsides never do. As a result, a lava planet's dayside may be molten while its nightside may not be.
Lava planets are difficult to study because there's nothing like them in our Solar System. Their bulk compositions might be the same as the bulk composition of our system's rocky planets, but that's where the similarities end. To try to understand these worlds, researchers from Canada, France, and the UK simulated their interiors.
Their research article is titled "The role of interior dynamics and differentiation on the surface and in the atmosphere of lava planets," and it's published in Nature Astronomy. The lead author is Charles-Édouard Boukaré, Assistant Professor in York University’s Department of Physics and Astronomy in the Faculty of Science.
Lava planets present new planetary physics regimes. On a world like Earth, the interactions between the planet's rocky mass and its atmosphere is limited. Important, but limited. For example, Earth has a carbon cycle that slowly and stately moves carbon around between the crust, mantle, atmosphere, and living things, moderating the world's climate. But on blistering hot lava worlds, conditions are much different and planetary cycles are turbocharged.
This artist's illustration shows the exoplanet Kepler-78b, which is very close to its star and orbits it in only 8.5 hours. It's roughly Earth-sized and has a similar density, but is classified as a lava planet. Image Credit: NASA
“Lava planets are in such extreme orbital configurations that our knowledge of rocky planets in the solar system does not directly apply, leaving scientists uncertain about what to expect when observing lava planets,” said first author Boukaré in a press release. "Our simulations propose a conceptual framework for interpreting their evolution and provide scenarios to probe their internal dynamics and chemical changes over time. These processes, though greatly amplified in lava planets, are fundamentally the same as those that shape rocky planets in our own solar system.”
Lava planets are roughly Earth-size or super-Earth size and complete an orbit in a single Earth day or less. Though they're unusual, they're not difficult to spot. Since they orbit so quickly, their transits are rapid and easily caught in dedicated observations. Understanding them, however, is a different matter.
Earth was once molten and cooled long ago. But understanding lava worlds can help scientists understand Earth's evolution. Lava worlds undergo a process similar to distillation, where different elements vaporize and then are partitioned between vapor, liquid, and solid phases. Earth underwent a similar phase early in its history, while exoplanets can remain molten or partially molten for billions of years.
This illustration shows another suspected lava world, Kepler-10b. It shows how the dayside region closest to the star can remain molten after the nightside has cooled. Image Credit: NASA/Kepler Mission/Dana Berry
"In this study, we are interested in the long-term evolution of lava planets over billions of years," the researchers explain. "We numerically modeled the interior dynamics of lava planets from the primordial fully molten state until its full solidification."
The simulations were based on mineralogy, geophysical fluid mechanics, and exoplanet atmospheres.
The researchers explored two different internal thermal states for lava worlds. In one state, the interior is fully molten; in the other, the interior is mostly solid, with a shallow magma ocean on the dayside. In each state, the planet's atmosphere is much different than the other. "Here we report the results of numerical simulations showing that solid-liquid fractionation has a major impact on the composition and evolution of lava planets," the authors write.
Lava planets, or magma ocean worlds, are characterized by turbulent convection. This convection keeps everything mixed, and as a result, the atmosphere is homogenized. However, the global magma phase is only expected to last hundreds of years. Terrestrial planets in our Solar System likely experienced these phases, but as they cooled they differentiated into layers like Earth's core, mantle, and crust.
While lava planets may not have long global magma phases, they never truly cool because of their proximity to their stars. So while the nightside can cool and solidify, the dayside never will. Eventually, as different components solidify at different temperatures, they experience a mushy magma ocean phase. The mushy phase eventually occupies the entire mantle, and the mushy phase could persist for hundreds of millions of years.
Lava planets have both vertical and horizontal convection, so material is transported from side to side preferentially. Horizontal convection transports some melted portions to the nightside, where they solidify. As a result, one side of the planet ends up with a different composition than the other.
Eventually, a lava planet enters its nearly-solid state, where only the region directly facing the star is molten. That magma ocean is shallow, only about 200 km thick, because the dayside cooling has penetrated to the core.
However in some rare cases, the lava planet may remain as a global magma ocean for billions of years due to tidal heating, extreme radiogenic heating, or a combination of both.
The simulations show that lava planets end up in one of two end states. "If the interior is fully molten, the atmosphere will reflect the planet’s bulk silicate composition, and the nightside solid surface is gravitationally unstable and constantly replenished," the authors explain.
The situation is different if the planet has mostly cooled. "If the interior is mostly solid with only a shallow magma ocean on the dayside, the outgassed atmosphere will lack in Na, K and FeO, and the nightside will have an entirely solid mantle with a cold surface," the researchers write.
This figure shows the two stages of lava planet magma ocean for a hot vs. cold interior. Panel (a) shows the hot end-member, which is characterized by an essentially liquid interior (orange). Vigorous convection in the liquid state favors compositional mixing and efficient cooling by heat transfer from the day-side to the night-side hemisphere. Panel (b) shows the cold end-member, characterized by a solid interior (green) with a shallow day-side magma ocean. Solid-liquid gravitational segregation has chemically differentiated the magma ocean (yellow). In this case, solid-state convection is too weak to generate an observable thermal signature on the night-side surface. Image Credit: Boukaré et al. 2025. NatAst.
"We investigated the multiphase fluid dynamics of lava planets over billion-year timescales, from their formation to the point where they achieve a thermal (pseudo) steady-state," the authors write. Immediately after formation, lava planets are mostly molten. "Despite being heated from the top on the day-side, they solidify almost as quickly as magma oceans in our solar system."
But even after billions of years of cooling, these worlds maintain a shallow yet long-lived magma ocean on their daysides, which leads to different atmospheric compositions.
"Given their short orbital periods, lava planets are particularly amenable to full orbit phase-resolved spectroscopy," the authors explain. "In the light of our results, we postulate two observable end-member stages for lava planets: either a hot, homogeneous global magma ocean stage, or a cold, chemically differentiated solid-state stage."
A planet's present-day thermal state is a direct consequence of its thermochemical history, from formation to the current time. If we can observe the planet's mantle temperature with the JWST, that should also reveal fundamental parts of the planetary evolution process. There are actually five different JWST observing programs aimed at magma ocean planets, and upcoming telescopes like the ELT may be able to observe their atmospheres in detail.
"We demonstrated that measuring the night-side surface temperature of lava planets is currently within the capabilities of telescopes such as JWST, offering promising constraints on their internal thermal state. Future ground-based observations, such as those from the ELT, which should be able to characterize the composition of lava planet silicate atmospheres, may allow us to test intriguing thermo-chemical couplings between the atmosphere, silicate melt, and silicate minerals in the planet’s interior, as suggested in this study," the authors conclude.
