---
title: Lava world
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.
science_status: [observed, model, sim]
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
  name: Lava world
  code: "LWN (not tidally locked); LWTL (tidally locked)"
  legacy_code: "LW1-H; LW2-TL"
  level: type
  series: "Lava (LW)"
  science_status: [observed, model, sim]
  image: File:Lava_world_sim.avif
  literature_equivalent: "Lava planet, magma-ocean planet; most known examples are ultra-short-period (USP) rocky planets"
  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"
  mass_earth: {observed: "about 0.6 (GJ 367 b) to 8 (55 Cancri e)", sim: "0.5 to 5.0 (LWN); 0.3 to 3.0 (LWTL)"}
  radius_earth: {observed: "about 0.7 to 1.9", sim: "R = M^0.27, about 0.7 to 1.5"}
  density_g_cm3: {observed: "about 5 to 10"}
  equilibrium_temperature_k: {observed: "about 1,300 to 2,700"}
  surface_temperature_k: {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", sim: "dayside about 1,350 to 3,550 (5th to 95th percentile)"}
  bond_albedo: {observed: "about 0.1 or lower where measured"}
  surface_pressure_bar: {observed: "rock vapour atmospheres are thin, well below 1 bar; some planets may keep thicker volatile atmospheres"}
  dominant_gases: "Na, K, SiO, O, O2, Fe, Mg from the magma; or outgassed CO and CO2 (55 Cnc e, probable)"
  clouds_hazes: "Mineral condensates on the cooler limbs and nightside ('rock rain')"
  interior: "Rocky, often iron-rich; a dayside magma ocean over a solid or partly molten mantle"
  tidal_state: "Almost always tidally locked; unlocked lava worlds need a wider orbit around a very luminous star"
  typical_orbit: "Periods shorter than about one day; semi-major axes below about 0.02 AU for Sun-like stars"
  occurrence_observed: "Ultra-short-period planets orbit about 0.5% of G dwarfs and about 0.8% of K dwarfs"
  frequency_in_sim: "916 of 8,742 generated planets (LWTL 814, LWN 102), in the committed sheets as of 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)"]
  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"
  last_verified: 2026-09-27
sim:
  entity: [planet_class.LWN, planet_class.LWTL]
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  - id: nasa_55cnce
    type: webpage
    author: [{literal: "NASA Science"}]
    title: "NASA's Webb hints at possible atmosphere surrounding rocky exoplanet"
    container-title: "NASA Science: Webb"
    issued: 2024-05-08
    accessed: 2026-09-27
    URL: https://science.nasa.gov/missions/webb/nasas-webb-hints-at-possible-atmosphere-surrounding-rocky-exoplanet/
images_wanted:
  - file: File:Lava_world_sim.avif
    subject: "A tidally locked lava world from the sim (LWTL-HT): glowing magma dayside fading through a crusted terminator to a dark nightside"
    source: sim
    note: "Shot list: render an LWTL plate on the lit sphere with the star behind the camera; label 'Sim render'. Confirm the plate generator's licence."
  - file: File:55_Cancri_e_Webb_illustration.jpg
    subject: "Artist's concept of 55 Cancri e with a possible CO2 or CO atmosphere, based on JWST observations (May 2024)"
    source: nasa
    page_url: https://science.nasa.gov/missions/webb/nasas-webb-hints-at-possible-atmosphere-surrounding-rocky-exoplanet/
    credit: "Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)"
    licence: "Public domain (NASA/STScI release; follow the caption)"
    note: "Label 'Artist's concept'."
  - file: File:55_Cancri_e_Webb_spectrum.png
    subject: "JWST NIRCam and MIRI thermal emission spectrum of 55 Cancri e compared with bare-rock and atmosphere models"
    source: nasa
    page_url: https://science.nasa.gov/missions/webb/nasas-webb-hints-at-possible-atmosphere-surrounding-rocky-exoplanet/
    credit: "Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Renyu Hu (NASA-JPL), Aaron Bello-Arufe (NASA-JPL), Michael Zhang (University of Chicago), Mantas Zilinskas (SRON)"
    licence: "Public domain (NASA/STScI release; follow the caption)"
    note: "Label 'Diagram'."
---

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.

::figure{src="File:55_Cancri_e_Webb_illustration.jpg" size=wide alt="An illustration of a dark planet with glowing orange cracks, close to a large bright star." caption="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."}

## 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 :cite[hirschmann2000], 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 :cite[schaefer2009]. 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 :cite[ito2015]. Gas is continually exchanged between the magma and the atmosphere, which slowly changes the composition of both :cite[kite2016].

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" :cite[castan2011]. Models of K2-141 b, one of the hottest known, predict supersonic winds of more than 5,000 km/h :cite[nguyen2020].

### 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 :cite[zieba2022]. 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 :cite[demory2016].

## 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 :cite[winn2018]. Most are smaller than about twice Earth's radius :cite[sanchisojeda2014]. 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]].

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 :cite[hon2025].

## 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 :cite[leger2009] :cite[leger2011]. **Kepler-10 b**, Kepler's first confirmed rocky planet, followed in 2011 :cite[batalha2011]. 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 :cite[sanchisojeda2014].

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 :cite[hu2024] :cite[nasa_55cnce]. 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 :cite[patel2024].
- **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 :cite[teske2025].

## 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 | :cite[bourrier2018]; probable CO or CO₂ atmosphere (JWST) |
| CoRoT-7 b | 20.5 hours | about 1.6 R⊕ | 4.7 M⊕ | about 6.6 g/cm³ | :cite[haywood2014] |
| Kepler-10 b | 20.1 hours | 1.47 R⊕ | 3.3 M⊕ | about 5.8 g/cm³ | :cite[dumusque2014] |
| K2-141 b | 6.7 hours | 1.51 R⊕ | 5.1 M⊕ | about 8.2 g/cm³ | dayside about 2,050 K :cite[malavolta2018] :cite[zieba2022] |
| 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 :cite[sanchisojeda2013] :cite[howard2013] :cite[pepe2013] |

Kepler-78 b's two mass measurements, made independently by two teams with two telescopes and published together, agree within their uncertainties.

:::callout{type=sim title="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]]
- [[Volcanic world]]
- [[Super-Earth]]
- [[Radius valley]]
- [[Exotic worlds]]
- [[Atmospheric escape]]
- [[JWST and rocky exoplanet atmospheres]]
- [[Planet classification]]