Worlds of Fire: What Molten Exoplanets Teach Us About Planet Formation

SETI Live recently hosted York University planetary scientist Dr. Charles-Édouard Boukaré, whose latest research examines molten rocky exoplanets that orbit so close to their stars that their surfaces remain at temperatures hot enough to keep rock in a liquid state. These so-called lava planets reach surface temperatures of 2,000 to 4,000 Kelvin (3140–6740°F), hot enough to melt or even vaporize rock and create an atmosphere of vaporized minerals.

Unlike early Earth, which cooled to form a solid crust, these exoplanets maintain a persistent magma ocean (a global layer of molten rock) because of their extreme proximity to their host stars. Dr. Boukaré explains that the term “magma ocean” originally referred to the early molten phases of young planets, but observations over the last two decades have shown that some exoplanets sustain molten surfaces for billions of years.

How Lava Planets Form and Persist

Lava planets orbit their stars at remarkable speeds, completing an orbit in as little as a few hours. This proximity generates intense tidal locking, a gravitational phenomenon where one side of the planet always faces the star. The day side is perpetually exposed to stellar radiation, while the night side remains in darkness and is much colder.

Because these orbits are too hot for planet formation, Dr. Boukaré notes that such planets must have migrated inward after forming farther from their stars. Some may even be the exposed rocky cores of former sub-Neptunes—planets that once held thick hydrogen-helium atmospheres that were later stripped away by intense stellar radiation. Their densities, measured through precise transit and radial velocity data, confirm that these worlds are primarily rocky rather than gaseous.

Interior Dynamics and Atmospheric Chemistry

Dr. Boukaré’s study models the interaction between the molten interior and the mineral-rich atmosphere of these planets. He and his colleagues demonstrate that the atmosphere's composition is in equilibrium with the magma ocean. Elements that remain in the liquid phase influence the gases that escape into the atmosphere, creating a direct chemical link between the surface magma and the atmospheric makeup.

The research combines fluid dynamics to measure the motion of molten rock with the chemistry of melting and crystallization. On the hot day side, rock remains liquid and vaporizes into the atmosphere. On the cooler night side, rock solidifies. Global circulation carries solidified material back into the molten region, creating a planetary-scale cycle that continuously separates elements based on their affinity for the solid or liquid phase.

This mechanism provides a potential way to infer the interior composition of a lava planet by measuring its atmospheric composition. Although the James Webb Space Telescope (JWST) cannot yet achieve the precision needed for detailed mineral analysis, upcoming facilities such as the Extremely Large Telescope (ELT) may be able to test these predictions. JWST, however, already detects infrared signals strong enough to measure the heat distribution between day and night sides, a key step toward probing interior temperatures.

Probing Interiors Through Temperature

Dr. Boukaré and collaborators have secured JWST observing time to measure the night-side temperature of five known lava planets. Determining whether an older lava planet remains hot inside could reveal details of its thermal evolution and migration history.

If the dark side of an old planet is warmer than expected, it probably means the planet is making its own heat inside. One way this can happen is through tidal heating, where the gravitational pull stretches the planet and converts that energy into warmth. Discovering this would reveal that the planet’s past is more complex than we thought. It would also provide clues about what the planet's interior is composed of and how it gradually cools over time.

Implications for Planetary Science

Studying these extreme exoplanets advances two major goals in planetary science:

• Determining interior composition: Linking atmospheric chemistry to mantle composition moves beyond the broad “rocky planet” category to quantify elements such as magnesium, iron, and calcium.
• Understanding planetary migration: Evidence of inward migration and tidal heating informs models of how planetary systems evolve, challenging assumptions drawn from our own solar system.

These results also extend to sub-Neptune planets, which may host deep, high-pressure magma oceans beneath thick atmospheres. Similar chemical interactions between magma and atmosphere could shape the observable signatures of those planets as well.

Watch the full SETI Live conversation with Dr. Charles-Édouard Boukaré here, read the York University press release here, and the published study in Nature Astronomy here.

Learn more about the SETI Institute’s exoplanet research.