--- title: Barren rock world canonical_url: https://paxabyssi.com/wiki/Barren_rock_world markdown_url: https://paxabyssi.com/wiki/Barren_rock_world.md type: wiki-page revision_id: 73 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 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. categories: - Planets - Planet classes - Terrestrial planets - Barren worlds aliases: - TBR - T0-B1 - Mercury-like world - Airless rocky planet - Bare rock planet - Barren world - Barren rock - Airless world - Mercury analogue infobox: type: planet_class code: TBR name: Barren rock world image: File:Barren_rock_world_sim.avif level: type series: Barren (T0) interior: "Mercury: iron core about 2,020 km in radius, about 83% of the planet's radius, under a thin silicate mantle" subtypes: - TBR-HC Heavily cratered - TBR-SC Scarped - TBR-VP Volcanic plains - TBR-DK Dark mass_earth: sim: 0.3 to 1.5 observed: 0.055 (Mercury) sim_source: Barren rock physics engine and surface module; science set TBR_00 to TBR_19 bond_albedo: sim: 0.04 to 0.14 observed: 0.068 (Mercury) legacy_code: T0-B1 tidal_state: Close-in exoplanets are tidally locked; Mercury is in a 3:2 spin-orbit resonance radius_earth: sim: about 0.75 to 1.3 observed: 0.383 (Mercury); 1.30 (LHS 3844 b) density_g_cm3: observed: 5.43 (Mercury) last_verified: 2026-09-27 real_examples: - Mercury - LHS 3844 b - TRAPPIST-1 b (bare rock or thin CO2 atmosphere) - GJ 367 b typical_orbit: Close to the star, or small enough to lose any atmosphere; see the cosmic shoreline dominant_gases: "Exosphere of atoms knocked or boiled off the surface: Na, K, Ca, Mg, O, H, He" magnetic_field: "Mercury: weak global dynamo, about 1% of Earth's surface field" science_status: - observed - model - sim frequency_in_sim: 407 of 8,742 generated planets (4.7%); all five barren types together 1,700 (19%), in the committed sheets as of 2026-09-27 escape_velocity_km_s: observed: 4.3 (Mercury) surface_gravity_m_s2: observed: 3.70 (Mercury) surface_pressure_bar: observed: below 5e-15 (Mercury's exosphere) defining_criteria_sim: Rocky world with no significant atmosphere (an exosphere at most); cratered, volcanic or scarped silicate surface literature_equivalent: Airless rocky planet or bare-rock exoplanet surface_temperature_k: observed: Mercury about 100 at night to 700 at noon equilibrium_temperature_k: sim: about 250 to 1,050 (5th to 95th percentile of mean surface temperature) observed: 440 (Mercury); 805 (LHS 3844 b) related: - https://paxabyssi.com/wiki/Arid_world.md - https://paxabyssi.com/wiki/Greenhouse_world.md - https://paxabyssi.com/wiki/Dry_habitable_world.md - https://paxabyssi.com/wiki/Ice_world.md - https://paxabyssi.com/wiki/Ocean_world.md - https://paxabyssi.com/wiki/Mixed_world.md --- # Barren rock world > Source: https://paxabyssi.com/wiki/Barren_rock_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/Barren_rock_world/history > > Revision 73, 27 September 2026 A **barren rock world** is a rocky planet with no real atmosphere. Its surface lies open to space: starlight heats it directly, the night side cools freely, and meteorites, cosmic rays and the stellar wind strike the ground unimpeded. What gas it has is an **exosphere**, a scattering of atoms so thin that they bounce between surface rocks without ever colliding with one another. Mercury is the Solar System's example, and the James Webb Space Telescope has now found several bare rocks around other stars. They matter as the baseline against which any rocky planet's atmosphere has to be measured. ![The grey globe of Mercury, densely cratered, with a bright rayed crater near the bottom.](https://media.paxabyssi.com/public/fb9d8228f0804c4e1313c68bc978a166e9c764e32a4b46efd690198bdc4975e5/2147.webp "Observation: Mercury from NASA's MESSENGER spacecraft. The bright rays near the bottom come from the young crater Debussy.") *Figure 1.* Observation: Mercury from NASA's MESSENGER spacecraft. The bright rays near the bottom come from the young crater Debussy. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Licence: Public domain (NASA). ## Characteristics ### Why some rocky planets have no air A planet loses gas when molecules in its upper atmosphere move fast enough to escape its gravity, and when stellar radiation and wind strip gas away. Whether an atmosphere survives therefore depends on two numbers: how strongly the planet holds on, measured by its escape velocity $v_\mathrm{esc} = \sqrt{2GM/R}$, and how much energy it receives from its star. Zahnle and Catling (2017) plotted every body in the Solar System on those two axes and found a sharp dividing line, which they called the **cosmic shoreline**. Bodies with atmospheres lie on one side and airless bodies on the other, with the boundary running roughly as $$ I \propto v_\mathrm{esc}^4, $$ where $I$ is the insolation [1]. Mercury, with an escape velocity of 4.3 km/s at 0.39 AU from the Sun, falls on the airless side; Titan, with a lower escape velocity but far from the Sun, keeps a thick atmosphere. Planets close to red dwarfs sit near the line, because those stars emit strongly in X-rays and ultraviolet and flare often, and they are also the easiest rocky planets to observe, so that is where the line is being tested. ### Temperature without an atmosphere With no air to move heat around, a barren world's temperature is set point by point by the sunlight it absorbs. Mercury's surface reaches about 700 K at noon and falls to about 100 K before dawn, the widest swing of any planet [2]. Its equilibrium temperature, the average from the formula on [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md), is 440 K, a figure no place on its surface actually holds for long. For a tidally locked exoplanet the simplest prediction is for the dayside. If the ground re-radiates heat where it absorbs it and none reaches the night side, the dayside's average temperature is $$ T_\mathrm{day} = \left(\tfrac{2}{3}\right)^{1/4} T_\star \sqrt{\frac{R_\star}{a}}\,(1 - A_B)^{1/4}, $$ about 1.28 times the planet's equilibrium temperature. An atmosphere carries heat to the night side and lowers the dayside temperature, so measuring the dayside's glow is a way to test for one [3]. ### Surfaces Without wind or water, the surface records its history. Craters accumulate, and the soil, called regolith, is a layer of rock ground up by impacts. **Space weathering**, the bombardment of the soil by micrometeorites and charged particles, darkens it over time by coating grains with tiny particles of metallic iron. Mercury's surface is also darkened by graphite, possibly the remains of its original crust [4], and its Bond albedo, the fraction of all sunlight reflected, is only 0.068 [2]. The planet's interior still shapes the surface. As Mercury's large iron core cooled over four billion years, the planet shrank, and its crust buckled into thrust faults, cliffs hundreds of kilometres long called lobate scarps. More than 5,900 such landforms have been mapped, recording a shrinkage in radius of up to about 7 km [5]. MESSENGER also found **hollows**: shallow, bright, flat-floored pits that look fresh, apparently formed where a volatile component of the rock is escaping to space today [6]. ### Interior and magnetic field Mercury is dense for its size, 5.43 g/cm³, almost as dense as Earth, which is far larger and more compressed. Its iron core is about 2,020 km in radius, some 83 per cent of the planet's radius, under a silicate shell only about 400 km thick [7], and part of the core is solid [8]. That core still drives a weak magnetic dynamo, about 1 per cent of the strength of Earth's field at the surface, with its centre offset about 480 km north of the planet's centre [9]. Magnetised rocks in the crust show the dynamo has run for at least 3.7 billion years [10]. ## Formation A barren rock world can start barren or become barren. A small planet may never gather much gas, and a planet close to a young, active star loses whatever it has to stellar X-rays, ultraviolet light and wind. Mercury's large core is the part that needs explaining. The leading idea is a giant impact early in the Solar System's history that stripped away much of a larger planet's rocky mantle [11]. MESSENGER complicated the story. Mercury's surface has a ratio of potassium to thorium, a measure of volatile elements, similar to the other rocky planets, which rules out models in which Mercury was heated so strongly that volatiles boiled off [12]. Its surface is also rich in sulfur and poor in iron, a sign that it formed from unusually oxygen-poor material [13]. How Mercury ended up with so much iron, and yet kept its volatiles, remains open. ## How we know **Mercury.** Mariner 10 made three flybys in 1974 and 1975. NASA's MESSENGER orbited Mercury from 2011 to 2015 and mapped its surface, chemistry, gravity and magnetic field. Neutron measurements showed that craters near the poles, whose floors never see sunlight, hold water ice, despite Mercury being the planet closest to the Sun [14]. The exosphere of sodium, potassium, calcium, magnesium, oxygen, hydrogen and helium has been watched from Earth and from spacecraft, with a total pressure below about $5 \times 10^{-15}$ bar [15] [2]. ESA and JAXA's BepiColombo, launched on 20 October 2018, is due to reach Mercury orbit at the end of 2026, with routine science from April 2027 [16]. **Exoplanets.** A bare rock around another star is found by elimination. When a transiting planet passes behind its star, the drop in infrared light measures the dayside's glow, and a dayside as hot as the bare-rock formula above leaves no room for an atmosphere to carry heat away. The first such result came from the Spitzer Space Telescope: LHS 3844 b, a planet 1.3 times Earth's radius on an 11-hour orbit around a red dwarf, has a dayside of about 1,040 K, as hot as a bare rock can be, ruling out any atmosphere thicker than about 10 bar [17]. JWST has since measured several more. TRAPPIST-1 b's dayside, about 500 K at 15 micrometres, first suggested bare rock [18]; adding a second wavelength showed that the data fit either a bare, dark rock surface or a carbon dioxide atmosphere with a warm upper layer, so the question is still open [19]. Its neighbour TRAPPIST-1 c, at about 380 K, has no thick carbon dioxide atmosphere like Venus's, though a thin one is possible [20]. ## Notable examples | Planet | Radius | Mass | Orbit | Result | | ------------ | -------- | ------------ | ------------------------- | ------------------------------------------------------------ | | Mercury | 0.383 R⊕ | 0.055 M⊕ | 88 days, 0.39 AU | airless; exosphere below 5 × 10⁻¹⁵ bar | | LHS 3844 b | 1.30 R⊕ | not measured | 11 hours, red dwarf | dayside about 1,040 K; bare rock (Spitzer, 2019) | | TRAPPIST-1 b | 1.12 R⊕ | 1.37 M⊕ | 1.5 days, ultracool dwarf | dayside about 500 K; bare rock or thin CO₂ atmosphere (JWST) | | GJ 367 b | 0.70 R⊕ | 0.63 M⊕ | 7.7 hours, red dwarf | dark, airless and very dense (JWST, 2024) | GJ 367 b is a super-Mercury. Its density of about 10 g/cm³ implies an iron core making up most of its mass [21], and JWST found a dayside at about 1,700 K with no sign of heat being carried to the night side, consistent with bare rock and no atmosphere [22]. See [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md) for iron planets. > **In Pax Abyssi** > > Barren rock worlds are airless rocky planets of 0.3 to 1.5 Earth masses, placed wherever an airless world is likely, from hot inner orbits to cold outer ones. Four subtypes are chosen by the geology the physics engine computes: **heavily cratered** worlds (TBR-HC) with saturated highlands, ray craters or old basins; **scarped** worlds (TBR-SC) whose cooling cores have buckled their crusts into lobate scarps; worlds of **volcanic plains** (TBR-VP) where lava has buried old craters; and **dark** worlds (TBR-DK) darkened by graphite or titanium-rich minerals. Each has visual variants for the texture the game draws. Four sister types cover other airless surfaces: iron-rich (TBI), anorthosite highlands (TBS), basalt plains like the lunar maria (TBB) and metallic (TBM). If a barren world's rock begins to melt, the sim moves it to [lava world](https://paxabyssi.com/wiki/Lava_world.md). In the committed system sheets, 407 of 8,742 generated planets are barren rock worlds, and the five barren types together make up about a fifth of all planets. ## See also - [Lava world](https://paxabyssi.com/wiki/Lava_world.md) - [Arid world](https://paxabyssi.com/wiki/Arid_world.md) - [Exotic worlds](https://paxabyssi.com/wiki/Exotic_worlds.md) - [Atmospheric escape](https://paxabyssi.com/wiki/Atmospheric_escape.md) - [Red dwarf](https://paxabyssi.com/wiki/Red_dwarf.md) - [JWST and rocky exoplanet atmospheres](https://paxabyssi.com/wiki/JWST_and_rocky_exoplanet_atmospheres.md) - [Sol](https://paxabyssi.com/wiki/Sol.md) - [Planet classification](https://paxabyssi.com/wiki/Planet_classification.md) ## References 1. Zahnle, K. J. and Catling, D. C. (2017). The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B. The Astrophysical Journal 843, 122. 2. Williams, D. R.. Mercury Fact Sheet. NASA Space Science Data Coordinated Archive. 3. Koll, D. D. B. et al. (2019). Identifying Candidate Atmospheres on Rocky M Dwarf Planets via Eclipse Photometry. The Astrophysical Journal 886, 140. 4. Peplowski, P. N. et al. (2016). Remote sensing evidence for an ancient carbon-bearing crust on Mercury. Nature Geoscience 9, 273-276. 5. Byrne, P. K. et al. (2014). Mercury’s global contraction much greater than earlier estimates. Nature Geoscience 7, 301-307. 6. Blewett, D. T. et al. (2011). Hollows on Mercury: MESSENGER Evidence for Geologically Recent Volatile-Related Activity. Science 333, 1856-1859. 7. Hauck, S. A. et al. (2013). The curious case of Mercury's internal structure. Journal of Geophysical Research: Planets 118, 1204-1220. 8. Genova, A. et al. (2019). Geodetic Evidence That Mercury Has A Solid Inner Core. Geophysical Research Letters 46, 3625-3633. 9. Anderson, B. J. et al. (2011). The Global Magnetic Field of Mercury from MESSENGER Orbital Observations. Science 333, 1859-1862. 10. Johnson, C. L. et al. (2015). Low-altitude magnetic field measurements by MESSENGER reveal Mercury’s ancient crustal field. Science 348, 892-895. 11. Benz, W. et al. (2007). The Origin of Mercury. Space Science Reviews 132, 189-202. 12. Peplowski, P. N. et al. (2011). Radioactive Elements on Mercury’s Surface from MESSENGER: Implications for the Planet’s Formation and Evolution. Science 333, 1850-1852. 13. Nittler, L. R. et al. (2011). The Major-Element Composition of Mercury’s Surface from MESSENGER X-ray Spectrometry. Science 333, 1847-1850. 14. Lawrence, D. J. et al. (2013). Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements. Science 339, 292-296. 15. Killen, R. et al. (2007). Processes that Promote and Deplete the Exosphere of Mercury. Space Science Reviews 132, 433-509. 16. European Space Agency. BepiColombo. ESA Science and Exploration. 17. Kreidberg, L. et al. (2019). Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b. Nature 573, 87-90. 18. Greene, T. P. et al. (2023). Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST. Nature 618, 39-42. 19. Ducrot, E. et al. (2024). Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b. Nature Astronomy 9, 358-369. 20. Zieba, S. et al. (2023). No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c. Nature 620, 746-749. 21. Goffo, E. et al. (2023). Company for the Ultra-high Density, Ultra-short Period Sub-Earth GJ 367 b: Discovery of Two Additional Low-mass Planets at 11.5 and 34 Days\*. The Astrophysical Journal Letters 955, L3. 22. Zhang, M. et al. (2024). GJ 367b Is a Dark, Hot, Airless Sub-Earth. The Astrophysical Journal Letters 961, L44. ## Infobox (planet class) | Field | Value | | --- | --- | | Code | TBR | | Name | Barren rock world | | Image | File:Barren_rock_world_sim.avif | | Level | type | | Series | Barren (T0) | | Interior | Mercury: iron core about 2,020 km in radius, about 83% of the planet's radius, under a thin silicate mantle | | Subtypes | TBR-HC Heavily cratered, TBR-SC Scarped, TBR-VP Volcanic plains, TBR-DK Dark | | Sim source | Barren rock physics engine and surface module; science set TBR_00 to TBR_19 | | Legacy code | T0-B1 | | Tidal state | Close-in exoplanets are tidally locked; Mercury is in a 3:2 spin-orbit resonance | | Last verified | 2026-09-27 | | Real examples | Mercury, LHS 3844 b, TRAPPIST-1 b (bare rock or thin CO2 atmosphere), GJ 367 b | | Typical orbit | Close to the star, or small enough to lose any atmosphere; see the cosmic shoreline | | Dominant gases | Exosphere of atoms knocked or boiled off the surface: Na, K, Ca, Mg, O, H, He | | Magnetic field | Mercury: weak global dynamo, about 1% of Earth's surface field | | Science status | observed, model, sim | | Frequency in sim | 407 of 8,742 generated planets (4.7%); all five barren types together 1,700 (19%), in the committed sheets as of 2026-09-27 | | Defining criteria sim | Rocky world with no significant atmosphere (an exosphere at most); cratered, volcanic or scarped silicate surface | | Literature equivalent | Airless rocky planet or bare-rock exoplanet | ## Related pages - [Arid world](https://paxabyssi.com/wiki/Arid_world.md): A rocky planet with a thin, cold atmosphere and no stable liquid water at the surface, only ice and the traces of ancient rivers. Mars is the Solar System's example. - [Greenhouse world](https://paxabyssi.com/wiki/Greenhouse_world.md): A rocky planet smothered by a thick carbon dioxide atmosphere whose greenhouse effect bakes the surface far above the boiling point of water. Venus is the Solar System's example, a near-twin of Earth in size. - [Dry habitable world](https://paxabyssi.com/wiki/Dry_habitable_world.md): A rocky planet with liquid water on its surface but only a little of it, in lakes, brines and polar seas rather than oceans. Climate models suggest such land planets can stay habitable over a wider range of distances than an Earth-like world. - [Ice world](https://paxabyssi.com/wiki/Ice_world.md): A cold world whose surface is made of frozen water, nitrogen, methane and carbon monoxide, like Pluto and Triton, where ices behave like rock and even the atmosphere can freeze out. - [Ocean world](https://paxabyssi.com/wiki/Ocean_world.md): A planet whose surface is almost entirely water, from Earth-like worlds with a few scattered islands to true water worlds with oceans hundreds of kilometres deep. None is confirmed, but several planets are strong candidates. - [Mixed world](https://paxabyssi.com/wiki/Mixed_world.md): A rocky planet with both continents and oceans, a temperate climate and liquid water at the surface. Earth is the only known example, and the model every search for habitable planets starts from. Categories: [Planets](https://paxabyssi.com/wiki/Category:Planets.md), [Planet classes](https://paxabyssi.com/wiki/Category:Planet_classes.md), [Terrestrial planets](https://paxabyssi.com/wiki/Category:Terrestrial_planets.md), [Barren worlds](https://paxabyssi.com/wiki/Category:Barren_worlds.md)