--- title: Lava world canonical_url: https://paxabyssi.com/wiki/Lava_world markdown_url: https://paxabyssi.com/wiki/Lava_world.md type: wiki-page revision_id: 196 revision_view: stable last_updated: 2026-09-27 license: CC BY-SA 4.0 license_url: https://creativecommons.org/licenses/by-sa/4.0/ science_status: - observed - model - sim summary: A rocky planet so close to its star that its dayside rock is molten, often a magma ocean under a thin atmosphere of vaporised rock. Dozens are known, most on orbits shorter than a day. categories: - Planets - Planet classes - Lava worlds - Exoplanets aliases: - LWN - LWTL - LW1-H - LW2-TL - Lava planet - Magma ocean planet - Molten planet - Hot lava world - Tidally locked lava world - Ultra-short-period planet - USP planet infobox: type: planet_class code: LWN (not tidally locked); LWTL (tidally locked) name: Lava world image: File:Lava_world_sim.avif level: type series: Lava (LW) interior: Rocky, often iron-rich; a dayside magma ocean over a solid or partly molten mantle subtypes: - LWTL-UH Ultra-hot - LWTL-HT Hot - LWTL-MD Moderate - LWTL-VR Volatile-rich - LWN-UM Ultra-hot magma - LWN-HC Hot crusted - LWN-HH Hot hazy - LWN-HV Hot volatile - LWN-MC Moderate crusted mass_earth: sim: 0.5 to 5.0 (LWN); 0.3 to 3.0 (LWTL) observed: about 0.6 (GJ 367 b) to 8 (55 Cancri e) sim_source: Lava world physics engine and its integration, temperature and magma-ocean modules; lava world science references bond_albedo: observed: about 0.1 or lower where measured legacy_code: LW1-H; LW2-TL tidal_state: Almost always tidally locked; unlocked lava worlds need a wider orbit around a very luminous star clouds_hazes: Mineral condensates on the cooler limbs and nightside ('rock rain') radius_earth: sim: R = M^0.27, about 0.7 to 1.5 observed: about 0.7 to 1.9 density_g_cm3: observed: about 5 to 10 last_verified: 2026-09-27 real_examples: - 55 Cancri e - CoRoT-7 b - Kepler-10 b - K2-141 b - Kepler-78 b - TOI-561 b - BD+05 4868Ab (disintegrating) typical_orbit: Periods shorter than about one day; semi-major axes below about 0.02 AU for Sun-like stars dominant_gases: Na, K, SiO, O, O2, Fe, Mg from the magma; or outgassed CO and CO2 (55 Cnc e, probable) science_status: - observed - model - sim frequency_in_sim: 916 of 8,742 generated planets (LWTL 814, LWN 102), in the committed sheets as of 2026-09-27 occurrence_observed: Ultra-short-period planets orbit about 0.5% of G dwarfs and about 0.8% of K dwarfs surface_pressure_bar: observed: rock vapour atmospheres are thin, well below 1 bar; some planets may keep thicker volatile atmospheres defining_criteria_sim: Rocky planet whose surface rock is partly molten (melt fraction of 2.5% or more), or whose equilibrium temperature passes a type's limit when the system is built literature_equivalent: Lava planet, magma-ocean planet; most known examples are ultra-short-period (USP) rocky planets surface_temperature_k: sim: dayside about 1,350 to 3,550 (5th to 95th percentile) observed: dayside about 1,800 (55 Cnc e, JWST) to about 2,050 (K2-141 b, Spitzer); nightsides from near zero to about 1,400 equilibrium_temperature_k: observed: about 1,300 to 2,700 related: - https://paxabyssi.com/wiki/Super-Earth.md - https://paxabyssi.com/wiki/Mini-Neptune.md - https://paxabyssi.com/wiki/Barren_rock_world.md - https://paxabyssi.com/wiki/Planet_classification.md - https://paxabyssi.com/wiki/Radius_valley.md - https://paxabyssi.com/wiki/Volcanic_world.md --- # Lava world > Source: https://paxabyssi.com/wiki/Lava_world > > Licence: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/). Text by Pax Abyssi Wiki contributors; history at https://paxabyssi.com/wiki/Lava_world/history > > Revision 196, 27 September 2026 A **lava world** is a rocky planet so close to its star that the rock on its dayside is molten. Most known examples circle their stars in less than a day, a few times their star's radius away, and are tidally locked, so one hemisphere faces the star permanently. The dayside can hold a sea of magma hundreds or thousands of kilometres across, under a thin atmosphere of vaporised rock, while the night side may be frozen solid. Lava worlds are the easiest small planets to study, because short orbits mean frequent transits and hot daysides shine brightly in infrared light, and their magma oceans and rock-vapour atmospheres test ideas about how planets gain and lose their lighter elements. ![An illustration of a dark planet with glowing orange cracks, close to a large bright star.](https://media.paxabyssi.com/public/9a4eacfdbae6158965ec089877d4fdfe1a493a6d3c75cbd5b27fd11dfb7d9369/2560.webp "Artist's concept: 55 Cancri e, a lava world on an 18-hour orbit. JWST's measurements suggest it has an atmosphere of carbon dioxide or carbon monoxide rather than bare rock.") *Figure 1.* Artist's concept: 55 Cancri e, a lava world on an 18-hour orbit. JWST's measurements suggest it has an atmosphere of carbon dioxide or carbon monoxide rather than bare rock. Credit: Illustration: NASA, ESA, CSA, Ralf Crawford (STScI). Licence: Public domain (NASA). ## Characteristics ### How hot is hot enough On a tidally locked planet with no atmosphere to move heat, the hottest point is directly beneath the star, where the ground reaches the substellar temperature $$ T_\mathrm{ss} = T_\star \sqrt{\frac{R_\star}{a}}\,(1 - A_B)^{1/4} = \sqrt{2}\,(1 - A_B)^{1/4}\,T_\star\sqrt{\frac{R_\star}{2a}}, $$ about 1.4 times the planet's equilibrium temperature for a dark surface. Dry mantle rock starts to melt at about 1,390 K at low pressure [1], so a planet whose substellar point is hotter than that holds at least a pool of magma. Around a Sun-like star that happens inside about 0.08 AU, an orbit of about eight days, although only a small pool beneath the star melts at that distance. Most known lava worlds are much closer, with equilibrium temperatures of 1,300 to 2,700 K and molten daysides. ### Magma oceans and rock-vapour atmospheres At these temperatures the melt itself evaporates. Chemical models show that a magma ocean at 2,000 to 3,000 K is covered by a thin atmosphere made from the rock: sodium, potassium, silicon monoxide, atomic and molecular oxygen, iron and magnesium, with sodium dominant at the cool end and silicon monoxide growing at the hot end [2]. Such an atmosphere absorbs starlight high up and should be warmer at altitude than below, a temperature inversion that would show up in its spectrum [3]. Gas is continually exchanged between the magma and the atmosphere, which slowly changes the composition of both [4]. Because the atmosphere exists only where the rock is hot enough to evaporate, it is thickest beneath the star and thins towards the terminator. The pressure difference drives winds from day to night, which carry rock vapour to cooler regions where it condenses and falls as mineral grains: "rock rain" [5]. Models of K2-141 b, one of the hottest known, predict supersonic winds of more than 5,000 km/h [6]. ### Dayside and nightside With only a thin atmosphere, a lava world's night side receives little heat, and on K2-141 b the night side is too faint to detect [7]. 55 Cancri e is different. Spitzer found its hottest point shifted away from the substellar point and a night side at about 1,400 K, too warm for bare rock, which implies that something, an atmosphere or flowing lava, carries heat around the planet [8]. ## Formation and evolution Planets probably do not form this close to their stars: there is too little material and it is too hot. Ultra-short-period planets are more likely to have formed a little farther out and been pulled inward, their orbits shrinking through tidal interaction with the star and gravitational nudges from sibling planets [9]. Most are smaller than about twice Earth's radius [10]. At this distance any hydrogen envelope a planet started with is stripped away by the star's radiation, leaving a bare rocky core, which is one explanation for the super-Earths below the [Radius valley](https://paxabyssi.com/wiki/Radius_valley.md). What happens next depends on the planet's size. A large lava world can outgas carbon, hydrogen and other volatiles from its magma and hold them as a secondary atmosphere. A small one can lose its rock outright. BD+05 4868Ab, a planet thought to be no more massive than Mercury on a 30.5-hour orbit, is disintegrating: the star's heat vaporises its surface and the vapour condenses into dust that streams away in tails ahead of and behind the planet [11]. ## How we know Lava worlds are found by transits and weighed by radial velocity. **CoRoT-7 b**, found by the French-led CoRoT satellite and announced in 2009, was the first rocky planet with a measured radius and one of the first recognised as a probable lava world [12] [13]. **Kepler-10 b**, Kepler's first confirmed rocky planet, followed in 2011 [14]. The Kepler survey showed that planets with periods shorter than a day orbit about 0.5 per cent of G-type stars and about 0.8 per cent of K-type stars [10]. Their atmospheres are tested by their heat. As a hot planet passes behind its star, the drop in infrared light measures the dayside's temperature; comparing it with the bare-rock prediction reveals whether an atmosphere is carrying heat away. JWST has now done this in detail: - **55 Cancri e.** The dayside measures about 1,800 K, well below the roughly 2,500 K expected for bare rock. The spectrum rules out a thin rock-vapour atmosphere and points to a genuine volatile atmosphere, likely rich in carbon dioxide or carbon monoxide, outgassed from and sustained by a magma ocean [15] [16]. It is strong evidence rather than a confirmation. The planet's brightness also changes markedly from one observation to the next, perhaps with volcanic activity or clouds [17]. - **TOI-561 b.** An ultra-hot super-Earth around an old, metal-poor star, its dayside is far cooler than the roughly 3,000 K expected for bare rock, which the authors read as a thick volatile envelope on a planet that should have lost one [18]. ## Notable examples | Planet | Orbital period | Radius | Mass | Density | Notes | | ----------- | -------------- | ------------ | ------------- | ---------------------- | --------------------------------------------------------------------- | | 55 Cancri e | 17.7 hours | 1.88 R⊕ | 8.0 M⊕ | about 6.4 g/cm³ | [19]; probable CO or CO₂ atmosphere (JWST) | | CoRoT-7 b | 20.5 hours | about 1.6 R⊕ | 4.7 M⊕ | about 6.6 g/cm³ | [20] | | Kepler-10 b | 20.1 hours | 1.47 R⊕ | 3.3 M⊕ | about 5.8 g/cm³ | [21] | | K2-141 b | 6.7 hours | 1.51 R⊕ | 5.1 M⊕ | about 8.2 g/cm³ | dayside about 2,050 K [22] [7] | | Kepler-78 b | 8.5 hours | about 1.2 R⊕ | 1.7 to 1.9 M⊕ | about 5.3 to 5.6 g/cm³ | first Earth-sized planet with an Earth-like density [23] [24] [25] | Kepler-78 b's two mass measurements, made independently by two teams with two telescopes and published together, agree within their uncertainties. > **In Pax Abyssi** > > The sim treats any rocky world whose surface rock is partly molten as a lava world. A planet can become one in two ways: when its system is built, a rocky or icy type placed too close to its star is converted directly, and afterwards any physics engine that finds a melt fraction of 2.5 per cent or more hands the planet over. Lava worlds split by rotation. **Tidally locked lava worlds** (LWTL) have a permanent magma dayside and a dark night side, with subtypes set by dayside temperature: **ultra-hot** (LWTL-UH, above 2,500 K), **hot** (LWTL-HT, 2,000 to 2,500 K) and **moderate** (LWTL-MD, below 2,000 K), plus **volatile-rich** worlds (LWTL-VR) whose atmospheres are thick enough to hide the surface. **Lava worlds that still rotate** (LWN) glow more evenly and come as ultra-hot magma, hot crusted, hot hazy, hot volatile and moderate crusted variants. In the game each is drawn as a prebaked texture plate chosen by its subtype code. ## See also - [Barren rock world](https://paxabyssi.com/wiki/Barren_rock_world.md) - [Volcanic world](https://paxabyssi.com/wiki/Volcanic_world.md) - [Super-Earth](https://paxabyssi.com/wiki/Super-Earth.md) - [Radius valley](https://paxabyssi.com/wiki/Radius_valley.md) - [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md) - [Atmospheric escape](https://paxabyssi.com/wiki/Atmospheric_escape.md) - [JWST and rocky exoplanet atmospheres](https://paxabyssi.com/wiki/JWST_and_rocky_exoplanet_atmospheres.md) - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md) ## References 1. Hirschmann, M. M. (2000). Mantle solidus: Experimental constraints and the effects of peridotite composition. Geochemistry, Geophysics, Geosystems 1, 2000GC000070. 2. Schaefer, L. and Fegley, B. (2009). Chemistry of silicate atmospheres of evaporating super-Earths. The Astrophysical Journal 703, L113-L117. 3. Ito, Y. et al. (2015). Theoretical emission spectra of atmospheres of hot rocky super-Earths. The Astrophysical Journal 801, 144. 4. Kite, E. S. et al. (2016). Atmosphere-interior exchange on hot, rocky exoplanets. The Astrophysical Journal 828, 80. 5. Castan, T. and Menou, K. (2011). Atmospheres of hot super-Earths. The Astrophysical Journal 743, L36. 6. Nguyen, T. G. et al. (2020). Modelling the atmosphere of lava planet K2-141b: implications for low- and high-resolution spectroscopy. Monthly Notices of the Royal Astronomical Society 499, 4605-4612. 7. Zieba, S. et al. (2022). K2 and Spitzer phase curves of the rocky ultra-short-period planet K2-141 b hint at a tenuous rock vapor atmosphere. Astronomy & Astrophysics 664, A79. 8. Demory, B. O. et al. (2016). A map of the large day-night temperature gradient of a super-Earth exoplanet. Nature 532, 207-209. 9. Winn, J. N., Sanchis-Ojeda, R. and Rappaport, S. (2018). Kepler-78 and the Ultra-Short-Period planets. New Astronomy Reviews 83, 37-48. 10. Sanchis-Ojeda, R. et al. (2014). A study of the shortest-period planets found with Kepler. The Astrophysical Journal 787, 47. 11. Hon, M. et al. (2025). A Disintegrating Rocky Planet with Prominent Comet-like Tails around a Bright Star. The Astrophysical Journal Letters 984, L3. 12. Léger, A. et al. (2009). Transiting exoplanets from the CoRoT space mission. VIII. CoRoT-7b: the first super-Earth with measured radius. Astronomy & Astrophysics 506, 287-302. 13. Léger, A. et al. (2011). The extreme physical properties of the CoRoT-7b super-Earth. Icarus 213, 1-11. 14. Batalha, N. M. et al. (2011). Kepler's first rocky planet: Kepler-10b. The Astrophysical Journal 729, 27. 15. Hu, R. et al. (2024). A secondary atmosphere on the rocky exoplanet 55 Cancri e. Nature 630, 609-612. 16. NASA Science (2024). NASA's Webb hints at possible atmosphere surrounding rocky exoplanet. NASA Science: Webb. 17. Patel, J. A. et al. (2024). JWST reveals the rapid and strong day-side variability of 55 Cancri e. Astronomy & Astrophysics 690, A159. 18. Teske, J. K. et al. (2025). A Thick Volatile Atmosphere on the Ultrahot Super-Earth TOI-561 b. The Astrophysical Journal Letters 995, L39. 19. Bourrier, V. et al. (2018). The 55 Cancri system reassessed. Astronomy & Astrophysics 619, A1. 20. Haywood, R. D. et al. (2014). Planets and stellar activity: hide and seek in the CoRoT-7 system. Monthly Notices of the Royal Astronomical Society 443, 2517-2531. 21. Dumusque, X. et al. (2014). The Kepler-10 planetary system revisited by HARPS-N: a hot rocky world and a solid Neptune-mass planet. The Astrophysical Journal 789, 154. 22. Malavolta, L. et al. (2018). An Ultra-short Period Rocky Super-Earth with a Secondary Eclipse and a Neptune-like Companion around K2-141. The Astronomical Journal 155, 107. 23. Sanchis-Ojeda, R. et al. (2013). Transits and occultations of an Earth-sized planet in an 8.5 hr orbit. The Astrophysical Journal 774, 54. 24. Howard, A. W. et al. (2013). A rocky composition for an Earth-sized exoplanet. Nature 503, 381-384. 25. Pepe, F. et al. (2013). An Earth-sized planet with an Earth-like density. Nature 503, 377-380. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | LWN (not tidally locked); LWTL (tidally locked) | | Name | Lava world | | Image | File:Lava_world_sim.avif | | Level | type | | Series | Lava (LW) | | Interior | Rocky, often iron-rich; a dayside magma ocean over a solid or partly molten mantle | | Subtypes | LWTL-UH Ultra-hot, LWTL-HT Hot, LWTL-MD Moderate, LWTL-VR Volatile-rich, LWN-UM Ultra-hot magma, LWN-HC Hot crusted, LWN-HH Hot hazy, LWN-HV Hot volatile, LWN-MC Moderate crusted | | Sim source | Lava world physics engine and its integration, temperature and magma-ocean modules; lava world science references | | Legacy code | LW1-H; LW2-TL | | Tidal state | Almost always tidally locked; unlocked lava worlds need a wider orbit around a very luminous star | | Clouds hazes | Mineral condensates on the cooler limbs and nightside ('rock rain') | | Last verified | 2026-09-27 | | Real examples | 55 Cancri e, CoRoT-7 b, Kepler-10 b, K2-141 b, Kepler-78 b, TOI-561 b, BD+05 4868Ab (disintegrating) | | Typical orbit | Periods shorter than about one day; semi-major axes below about 0.02 AU for Sun-like stars | | Dominant gases | Na, K, SiO, O, O2, Fe, Mg from the magma; or outgassed CO and CO2 (55 Cnc e, probable) | | Science status | observed, model, sim | | Frequency in sim | 916 of 8,742 generated planets (LWTL 814, LWN 102), in the committed sheets as of 2026-09-27 | | Occurrence observed | Ultra-short-period planets orbit about 0.5% of G dwarfs and about 0.8% of K dwarfs | | Defining criteria sim | Rocky planet whose surface rock is partly molten (melt fraction of 2.5% or more), or whose equilibrium temperature passes a type's limit when the system is built | | Literature equivalent | Lava planet, magma-ocean planet; most known examples are ultra-short-period (USP) rocky planets | ## Related pages - [Super-Earth](https://paxabyssi.com/wiki/Super-Earth.md): A planet more massive than Earth but lighter than Neptune, usually rocky, with no counterpart in the Solar System. Super-Earths are among the most common planets found around other stars. - [Mini-Neptune](https://paxabyssi.com/wiki/Mini-Neptune.md): A planet between about 1.7 and 4 times Earth's radius with a rocky or icy core wrapped in a thin envelope of hydrogen and helium. The most common kind of planet Kepler found, and one the Solar System lacks. - [Barren rock world](https://paxabyssi.com/wiki/Barren_rock_world.md): A rocky planet with no real atmosphere, whose surface lies bare to starlight, cosmic rays and meteorites. Mercury is the Solar System's example, and JWST has found several around other stars. - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md): How astronomers sort planets by size, mass, temperature and composition, and how Pax Abyssi files every world it generates under one of 37 coded types. - [Radius valley](https://paxabyssi.com/wiki/Radius_valley.md): A shortage of planets between about 1.5 and 2 Earth radii that splits small close-in planets into rocky super-Earths and gas-wrapped sub-Neptunes. - [Volcanic world](https://paxabyssi.com/wiki/Volcanic_world.md): A rocky world whose surface is continually remade by eruptions, powered by internal heat far above Earth's. Jupiter's moon Io, heated by tides, is the most volcanic body known. Categories: [Planets](https://paxabyssi.com/wiki/Category:Planets.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md), [Lava worlds](https://paxabyssi.com/wiki/Category:Lava_worlds.md), [Exoplanets](https://paxabyssi.com/wiki/Category:Exoplanets.md)