--- title: Barren rock world 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. science_status: [observed, model, sim] 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 name: Barren rock world code: TBR legacy_code: T0-B1 level: type series: "Barren (T0)" science_status: [observed, model, sim] image: File:Barren_rock_world_sim.avif literature_equivalent: "Airless rocky planet or bare-rock exoplanet" defining_criteria_sim: "Rocky world with no significant atmosphere (an exosphere at most); cratered, volcanic or scarped silicate surface" mass_earth: {observed: "0.055 (Mercury)", sim: "0.3 to 1.5"} radius_earth: {observed: "0.383 (Mercury); 1.30 (LHS 3844 b)", sim: "about 0.75 to 1.3"} density_g_cm3: {observed: "5.43 (Mercury)"} surface_gravity_m_s2: {observed: "3.70 (Mercury)"} escape_velocity_km_s: {observed: "4.3 (Mercury)"} equilibrium_temperature_k: {observed: "440 (Mercury); 805 (LHS 3844 b)", sim: "about 250 to 1,050 (5th to 95th percentile of mean surface temperature)"} surface_temperature_k: {observed: "Mercury about 100 at night to 700 at noon"} bond_albedo: {observed: "0.068 (Mercury)", sim: "0.04 to 0.14"} surface_pressure_bar: {observed: "below 5e-15 (Mercury's exosphere)"} 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" interior: "Mercury: iron core about 2,020 km in radius, about 83% of the planet's radius, under a thin silicate mantle" tidal_state: "Close-in exoplanets are tidally locked; Mercury is in a 3:2 spin-orbit resonance" typical_orbit: "Close to the star, or small enough to lose any atmosphere; see the cosmic shoreline" 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" real_examples: ["Mercury", "LHS 3844 b", "TRAPPIST-1 b (bare rock or thin CO2 atmosphere)", "GJ 367 b"] subtypes: ["TBR-HC Heavily cratered", "TBR-SC Scarped", "TBR-VP Volcanic plains", "TBR-DK Dark"] sim_source: "Barren rock physics engine and surface module; 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B."}, {family: "Morley", given: "Caroline"}, {family: "Hu", given: "Renyu"}, {family: "Schaefer", given: "Laura"}, {family: "Deming", given: "Drake"}, {family: "Stevenson", given: "Kevin B."}, {family: "Dittmann", given: "Jason"}, {literal: "et al."}] title: "Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b" container-title: "Nature" volume: "573" page: "87-90" issued: 2019 DOI: 10.1038/s41586-019-1497-4 - id: greene2023 type: article-journal author: [{family: "Greene", given: "Thomas P."}, {family: "Bell", given: "Taylor J."}, {family: "Ducrot", given: "Elsa"}, {family: "Dyrek", given: "Achrène"}, {family: "Lagage", given: "Pierre-Olivier"}, {family: "Fortney", given: "Jonathan J."}] title: "Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST" container-title: "Nature" volume: "618" page: "39-42" issued: 2023 DOI: 10.1038/s41586-023-05951-7 - id: zieba2023 type: article-journal author: [{family: "Zieba", given: "Sebastian"}, {family: "Kreidberg", given: "Laura"}, {family: "Ducrot", given: "Elsa"}, {family: "Gillon", given: "Michaël"}, {family: "Morley", given: "Caroline"}, {family: "Schaefer", given: "Laura"}, {family: "Tamburo", given: "Patrick"}, {family: "Koll", given: "Daniel D. B."}, {literal: "et al."}] title: "No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c" container-title: "Nature" volume: "620" page: "746-749" issued: 2023 DOI: 10.1038/s41586-023-06232-z - id: ducrot2025 type: article-journal author: [{family: "Ducrot", given: "Elsa"}, {family: "Lagage", given: "Pierre-Olivier"}, {family: "Min", given: "Michiel"}, {family: "Gillon", given: "Michaël"}, {family: "Bell", given: "Taylor J."}, {family: "Tremblin", given: "Pascal"}, {family: "Greene", given: "Thomas"}, {family: "Dyrek", given: "Achrène"}, {literal: "et al."}] title: "Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b" container-title: "Nature Astronomy" volume: "9" page: "358-369" issued: 2024 DOI: 10.1038/s41550-024-02428-z - id: zhang2024 type: article-journal author: [{family: "Zhang", given: "Michael"}, {family: "Hu", given: "Renyu"}, {family: "Inglis", given: "Julie"}, {family: "Dai", given: "Fei"}, {family: "Bean", given: "Jacob L."}, {family: "Knutson", given: "Heather A."}, {family: "Lam", given: "Kristine"}, {family: "Goffo", given: "Elisa"}, {literal: "et al."}] title: "GJ 367b Is a Dark, Hot, Airless Sub-Earth" container-title: "The Astrophysical Journal Letters" volume: "961" page: "L44" issued: 2024 DOI: 10.3847/2041-8213/ad1a07 - id: goffo2023 type: article-journal author: [{family: "Goffo", given: "Elisa"}, {family: "Gandolfi", given: "Davide"}, {family: "Egger", given: "Jo Ann"}, {family: "Mustill", given: "Alexander J."}, {family: "Albrecht", given: "Simon H."}, {family: "Hirano", given: "Teruyuki"}, {family: "Kochukhov", given: "Oleg"}, {family: "Astudillo-Defru", given: "Nicola"}, {literal: "et al."}] title: "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*" container-title: "The Astrophysical Journal Letters" volume: "955" page: "L3" issued: 2023 DOI: 10.3847/2041-8213/ace0c7 - id: koll2019 type: article-journal author: [{family: "Koll", given: "Daniel D. B."}, {family: "Malik", given: "Matej"}, {family: "Mansfield", given: "Megan"}, {family: "Kempton", given: "Eliza M.-R."}, {family: "Kite", given: "Edwin"}, {family: "Abbot", given: "Dorian"}, {family: "Bean", given: "Jacob L."}] title: "Identifying Candidate Atmospheres on Rocky M Dwarf Planets via Eclipse Photometry" container-title: "The Astrophysical Journal" volume: "886" page: "140" issued: 2019 DOI: 10.3847/1538-4357/ab4c91 - id: nasa_mercury type: webpage author: [{family: Williams, given: David R.}] title: "Mercury Fact Sheet" container-title: "NASA Space Science Data Coordinated Archive" accessed: 2026-09-27 URL: https://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html - id: esa_bepi type: webpage author: [{literal: "European Space Agency"}] title: "BepiColombo" container-title: "ESA Science and Exploration" accessed: 2026-09-27 URL: https://www.esa.int/Science_Exploration/Space_Science/BepiColombo images_wanted: - file: File:Barren_rock_world_sim.avif subject: "A TBR planet from the sim: grey-brown heavily cratered surface with bright ray craters, hard terminator, no atmosphere" source: sim note: "Shot list: render a TBR-HC plate on the lit sphere; label 'Sim render'. Confirm the plate generator's licence." - file: File:Mercury_MESSENGER_PIA15160.jpg subject: "Mercury's globe from MESSENGER's mapping, centred on 0 N, 0 E, with the rayed crater Debussy and the peak-ring basin Rachmaninoff" source: nasa page_url: https://science.nasa.gov/photojournal/mercury-globe-0n-0e credit: "NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington" licence: "Public domain (NASA)" note: "PIA15160. Label 'Observation'." - file: File:Cosmic_shoreline.svg subject: "Diagram: escape velocity against stellar insolation for Solar System bodies and rocky exoplanets, marking which have atmospheres, with the I proportional to v_esc^4 shoreline" source: other note: "To be drawn by us from NASA fact sheets and the NASA Exoplanet Archive; label 'Diagram'." --- 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. ::figure{src="File:Mercury_MESSENGER_PIA15160.jpg" size=wide alt="The grey globe of Mercury, densely cratered, with a bright rayed crater near the bottom." caption="Observation: Mercury from NASA's MESSENGER spacecraft. The bright rays near the bottom come from the young crater Debussy."} ## 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 :cite[zahnle2017]. 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 :cite[nasa_mercury]. Its equilibrium temperature, the average from the formula on [[Planet classification]], 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 :cite[koll2019]. ### 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 :cite[peplowski2016], and its Bond albedo, the fraction of all sunlight reflected, is only 0.068 :cite[nasa_mercury]. 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 :cite[byrne2014]. 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 :cite[blewett2011]. ### 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 :cite[hauck2013], and part of the core is solid :cite[genova2019]. 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 :cite[anderson2011]. Magnetised rocks in the crust show the dynamo has run for at least 3.7 billion years :cite[johnson2015]. ## 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 :cite[benz2007]. 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 :cite[peplowski2011]. Its surface is also rich in sulfur and poor in iron, a sign that it formed from unusually oxygen-poor material :cite[nittler2011]. 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 :cite[lawrence2013]. 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 :cite[killen2007] :cite[nasa_mercury]. 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 :cite[esa_bepi]. **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 :cite[kreidberg2019]. JWST has since measured several more. TRAPPIST-1 b's dayside, about 500 K at 15 micrometres, first suggested bare rock :cite[greene2023]; 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 :cite[ducrot2025]. Its neighbour TRAPPIST-1 c, at about 380 K, has no thick carbon dioxide atmosphere like Venus's, though a thin one is possible :cite[zieba2023]. ## 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 :cite[goffo2023], 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 :cite[zhang2024]. See [[Exotic worlds]] for iron planets. :::callout{type=sim title="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|lava world]]. 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]] - [[Arid world]] - [[Exotic worlds]] - [[Atmospheric escape]] - [[Red dwarf]] - [[JWST and rocky exoplanet atmospheres]] - [[Sol]] - [[Planet classification]]