--- title: Ice world summary: 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. science_status: [observed, model, sim] categories: [Planets, Planet classes, Terrestrial planets, Dwarf planets, Outer Solar System] aliases: [TIW, T3-I, Pluto-like world, Icy world, Frozen world, Nitrogen ice world, Terrestrial ice world, Pluto analogue, Triton analogue] infobox: type: planet_class name: Ice world code: TIW legacy_code: T3-I level: type series: "Terrestrial with atmosphere (T)" science_status: [observed, model, sim] image: File:Ice_world_sim.avif literature_equivalent: "Icy dwarf planet or large icy moon with volatile ices (Pluto, Triton, Eris, Makemake); no planet-mass example around another star is confirmed" defining_criteria_sim: "Surface of water ice with nitrogen, methane or carbon monoxide ices; placed in cold outer orbital zones; surface temperature below about 150 K" mass_earth: {observed: "0.0022 (Pluto) to 0.0036 (Triton)", sim: "0.01 to 3.0"} radius_earth: {observed: "0.18 (Eris, 1,163 km) to 0.21 (Triton, 1,353 km); Pluto 0.19 (1,188 km)", sim: "R = 1.2 M^0.27, about 0.35 to 1.6"} density_g_cm3: {observed: "1.85 (Pluto), 2.06 (Triton), 2.52 (Eris)"} surface_gravity_m_s2: {observed: "0.62 (Pluto), 0.78 (Triton), 0.82 (Eris)"} escape_velocity_km_s: {observed: "1.21 (Pluto), 1.46 (Triton), 1.38 (Eris)"} equilibrium_temperature_k: {observed: "about 30 to 40 in the Kuiper belt; below 20 at Sedna's distance"} surface_temperature_k: {observed: "Pluto about 40 (nitrogen ice about 37 to 40); Triton 38", sim: "about 40 to 150"} geometric_albedo: {observed: "0.5 to 0.7 (Pluto, varying by region); 0.76 (Triton); 0.77 (Makemake); 0.96 (Eris)"} surface_pressure_bar: {observed: "about 1e-5 (Pluto, about 10 microbar); 1.4e-5 (Triton); Eris and Makemake have no global atmosphere today"} dominant_gases: "N2 with traces of CH4 and CO, in vapour-pressure equilibrium with the surface ices" clouds_hazes: "Layered hydrocarbon haze on Pluto (about 20 layers) from methane photochemistry; tholins colour the surface red-brown" interior: "Rock core under a water-ice mantle; Pluto may keep a buried ocean" tidal_state: "Pluto and Charon mutually locked; Triton synchronous and retrograde" typical_orbit: "Beyond Neptune (30 to 100 AU for the Solar System's examples), or as captured moons" frequency_in_sim: "422 of 8,742 generated planets (4.8%), in the committed sheets as of 2026-09-27" real_examples: ["Pluto", "Triton", "Eris", "Makemake", "Haumea", "Sedna (smaller, colder)"] subtypes: ["TIW-PR Pristine", "TIW-CR Cracked", "TIW-TN Tholin-stained"] sim_source: "Ice-world physics engine and helpers; science set TIW_00 to TIW_19 and the ice-world science reference" last_verified: 2026-09-27 sim: entity: planet_class.TIW refs: - id: stern2015 type: article-journal author: [{family: "Stern", given: "S. A."}, {family: "Bagenal", given: "F."}, {family: "Ennico", given: "K."}, {family: "Gladstone", given: "G. R."}, {family: "Grundy", given: "W. M."}, {family: "McKinnon", given: "W. B."}, {family: "Moore", given: "J. M."}, {family: "Olkin", given: "C. B."}, {literal: "et al."}] title: "The Pluto system: Initial results from its exploration by New Horizons" container-title: "Science" volume: "350" page: "aad1815" issued: 2015 DOI: 10.1126/science.aad1815 - id: gladstone2016 type: article-journal author: [{family: "Gladstone", given: "G. Randall"}, {family: "Stern", given: "S. Alan"}, {family: "Ennico", given: "Kimberly"}, {family: "Olkin", given: "Catherine B."}, {family: "Weaver", given: "Harold A."}, {family: "Young", given: "Leslie A."}, {family: "Summers", given: "Michael E."}, {family: "Strobel", given: "Darrell F."}, {literal: "et al."}] title: "The atmosphere of Pluto as observed by New Horizons" container-title: "Science" volume: "351" page: "aad8866" issued: 2016 DOI: 10.1126/science.aad8866 - id: moore2016 type: article-journal author: [{family: "Moore", given: "Jeffrey M."}, {family: "McKinnon", given: "William B."}, {family: "Spencer", given: "John R."}, {family: "Howard", given: "Alan D."}, {family: "Schenk", given: "Paul M."}, {family: "Beyer", given: "Ross A."}, {family: "Nimmo", given: "Francis"}, {family: "Singer", given: "Kelsi N."}, {literal: "et al."}] title: "The geology of Pluto and Charon through the eyes of New Horizons" container-title: "Science" volume: "351" page: "1284-1293" issued: 2016 DOI: 10.1126/science.aad7055 - id: mckinnon2016 type: article-journal author: [{literal: "the New Horizons Geology, Geophysics and Imaging Theme Team"}, {family: "McKinnon", given: "William B."}, {family: "Nimmo", given: "Francis"}, {family: "Wong", given: "Teresa"}, {family: "Schenk", given: "Paul M."}, {family: "White", given: "Oliver L."}, {family: "Roberts", given: "J. 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L."}, {family: "Bertrand", given: "T."}, {family: "Lellouch", given: "E."}, {family: "Desmars", given: "J."}, {family: "Forget", given: "F."}, {literal: "et al."}] title: "Lower atmosphere and pressure evolution on Pluto from ground-based stellar occultations, 1988-2016" container-title: "Astronomy & Astrophysics" volume: "625" page: "A42" issued: 2019 DOI: 10.1051/0004-6361/201834281 - id: soderblom1990 type: article-journal author: [{family: "Soderblom", given: "L. A."}, {family: "Kieffer", given: "S. W."}, {family: "Becker", given: "T. L."}, {family: "Brown", given: "R. H."}, {family: "Cook", given: "A. F."}, {family: "Hansen", given: "C. J."}, {family: "Johnson", given: "T. V."}, {family: "Kirk", given: "R. 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L."}, {family: "Assafin", given: "M."}, {family: "Jehin", given: "E."}, {family: "Maury", given: "A."}, {family: "Lellouch", given: "E."}, {family: "Hutton", given: "R. Gil"}, {family: "Braga-Ribas", given: "F."}, {literal: "et al."}] title: "A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation" container-title: "Nature" volume: "478" page: "493-496" issued: 2011 DOI: 10.1038/nature10550 - id: ortiz2012 type: article-journal author: [{family: "Ortiz", given: "J. L."}, {family: "Sicardy", given: "B."}, {family: "Braga-Ribas", given: "F."}, {family: "Alvarez-Candal", given: "A."}, {family: "Lellouch", given: "E."}, {family: "Duffard", given: "R."}, {family: "Pinilla-Alonso", given: "N."}, {family: "Ivanov", given: "V. D."}, {literal: "et al."}] title: "Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation" container-title: "Nature" volume: "491" page: "566-569" issued: 2012 DOI: 10.1038/nature11597 - id: brown2004 type: article-journal author: [{family: "Brown", given: "Michael E."}, {family: "Trujillo", given: "Chadwick"}, {family: "Rabinowitz", given: "David"}] title: "Discovery of a Candidate Inner Oort Cloud Planetoid" container-title: "The Astrophysical Journal" volume: "617" page: "645-649" issued: 2004 DOI: 10.1086/422095 - id: emery2024 type: article-journal author: [{family: "Emery", given: "J.P."}, {family: "Wong", given: "I."}, {family: "Brunetto", given: "R."}, {family: "Cook", given: "J.C."}, {family: "Pinilla-Alonso", given: "N."}, {family: "Stansberry", given: "J.A."}, {family: "Holler", given: "B.J."}, {family: "Grundy", given: "W.M."}, {literal: "et al."}] title: "A tale of 3 dwarf planets: Ices and organics on Sedna, Gonggong, and Quaoar from JWST spectroscopy" container-title: "Icarus" volume: "414" page: "116017" issued: 2024 DOI: 10.1016/j.icarus.2024.116017 - id: grundy2024 type: article-journal author: [{family: "Grundy", given: "W.M."}, {family: "Wong", given: "I."}, {family: "Glein", given: "C.R."}, {family: "Protopapa", given: "S."}, {family: "Holler", given: "B.J."}, {family: "Cook", given: "J.C."}, {family: "Stansberry", given: "J.A."}, {family: "Lunine", given: "J.I."}, {literal: "et al."}] title: "Measurement of D/H and 13C/12C ratios in methane ice on Eris and Makemake: Evidence for internal activity" container-title: "Icarus" volume: "411" page: "115923" issued: 2024 DOI: 10.1016/j.icarus.2023.115923 - id: protopapa2025 type: article-journal author: [{family: "Protopapa", given: "Silvia"}, {family: "Wong", given: "Ian"}, {family: "Lellouch", given: "Emmanuel"}, {family: "Johnson", given: "Perianne E."}, {family: "Grundy", given: "William M."}, {family: "Glein", given: "Christopher R."}, {family: "Müller", given: "Thomas"}, {family: "Kiss", given: "Csaba"}, {literal: "et al."}] title: "JWST Detection of Hydrocarbon Ices and Methane Gas on Makemake" container-title: "The Astrophysical Journal Letters" volume: "991" page: "L34" issued: 2025 DOI: 10.3847/2041-8213/adfe63 - id: canup2005 type: article-journal author: [{family: "Canup", given: "Robin M."}] title: "A Giant Impact Origin of Pluto-Charon" container-title: "Science" volume: "307" page: "546-550" issued: 2005 DOI: 10.1126/science.1106818 - id: beaulieu2006 type: article-journal author: [{family: "Beaulieu", given: "J.-P."}, {family: "Bennett", given: "D. P."}, {family: "Fouqué", given: "P."}, {family: "Williams", given: "A."}, {family: "Dominik", given: "M."}, {family: "Jørgensen", given: "U. G."}, {family: "Kubas", given: "D."}, {family: "Cassan", given: "A."}, {literal: "et al."}] title: "Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing" container-title: "Nature" volume: "439" page: "437-440" issued: 2006 DOI: 10.1038/nature04441 images_wanted: - file: File:Ice_world_sim.avif subject: "A TIW planet from the sim: pale ice surface with darker tholin-stained regions, lit low from one side" source: sim note: "Shot list: render a TIW plate on the lit sphere; label 'Sim render'. Confirm the plate generator's licence." - file: File:Pluto_New_Horizons_PIA19952.jpg subject: "Pluto in enhanced colour from New Horizons, 14 July 2015, showing the bright nitrogen-ice plain of Sputnik Planitia and dark red tholin-coated terrain" source: nasa page_url: https://science.nasa.gov/photojournal/the-rich-color-variations-of-pluto credit: "NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute" licence: "Public domain (NASA)" note: "PIA19952. Label 'Observation (enhanced colour)'." - file: File:Triton_Voyager2_PIA00317.jpg subject: "Global colour mosaic of Triton from Voyager 2 (1989), with the pinkish south polar cap and dark plume streaks" source: nasa page_url: https://science.nasa.gov/photojournal/global-color-mosaic-of-triton credit: "NASA/JPL/USGS" licence: "Public domain (NASA)" note: "PIA00317. Label 'Observation'." --- An **ice world** is a world cold enough that the familiar gases of Earth's air freeze solid. Its crust is water ice as hard as granite, and on top of it lie softer ices of nitrogen, methane and carbon monoxide that flow as glaciers, sublimate in sunlight and fall again as frost. Pluto and Neptune's moon Triton are the best-studied examples, with the dwarf planets Eris and Makemake close behind. In Pax Abyssi the class is also generated as planets in the outer reaches of other systems, many of them far larger than Pluto. ::figure{src="File:Pluto_New_Horizons_PIA19952.jpg" size=wide alt="Pluto in exaggerated colours: a pale heart-shaped plain on the right, deep red terrain along the equator, and blue and yellow patches elsewhere." caption="Observation, enhanced colour: Pluto from New Horizons on 14 July 2015. The pale lobe of the heart is Sputnik Planitia, a basin filled with nitrogen ice; the dark red belt is coated in tholins."} ## Characteristics ### Cold enough to freeze air At Pluto's average distance of 39.5 AU the Sun delivers less than a thousandth of the light Earth receives. With a Bond albedo around 0.7, the equilibrium temperature (see [[Planet classification]]) is only about 32 K. The measured surface is a little warmer, about 40 K, because dark regions absorb more light and heat is stored in the ground. At these temperatures water ice behaves as bedrock and can hold up mountains several kilometres high, while nitrogen ice, which melts at 63 K, is soft enough to flow. The atmosphere is set by the ices. Nitrogen at the surface evaporates until the gas pressure matches the vapour pressure of the ice, which depends very steeply on temperature. On Pluto that balance gives about 10 microbar, a hundred-thousandth of Earth's surface pressure, with traces of methane and carbon monoxide :cite[gladstone2016]. A change of a couple of kelvin in the temperature of the nitrogen ice changes the pressure several-fold, so the atmosphere follows the seasons. Pluto's surface pressure roughly tripled between 1988 and 2016 as its long northern spring advanced :cite[meza2019], and far from the Sun it may largely freeze onto the ground. ### Keeping the ices Whether a small body can keep nitrogen and methane at all depends on how fast those molecules escape. The key number is the Jeans escape parameter, the ratio of a molecule's gravitational binding energy to its thermal energy at the top of the atmosphere: $$ \lambda = \frac{G M m}{k_B T r}, $$ where $M$ is the body's mass, $m$ the molecular mass, $T$ the temperature and $r$ the radius at which molecules can escape. When $\lambda$ is large, few molecules are fast enough to leave. Schaller and Brown (2007) worked through this over the age of the Solar System and showed that only the largest, coldest Kuiper belt objects (Pluto, Eris, Makemake and perhaps Quaoar) should retain nitrogen, methane and carbon monoxide at their surfaces, which matches what telescopes see :cite[schaller2007]. Smaller bodies at the same distance keep only water ice. ### Colour and chemistry Ultraviolet light and charged particles break methane and nitrogen apart in the upper atmosphere and on the surface. The fragments recombine into larger organic molecules, and eventually into a reddish-brown residue called **tholin**. On Pluto the process runs through about twenty stacked layers of haze reaching more than 200 km above the ground :cite[gladstone2016]. The haze settles out and stains the old, stable terrain dark red, while fresh nitrogen frost keeps other regions bright; the result is one of the most varied surfaces in the Solar System. ### Geology New Horizons found Pluto far more active than expected :cite[stern2015]. Its heart-shaped Sputnik Planitia is a basin about 1,000 km across filled with nitrogen ice several kilometres deep. The ice slowly overturns in convection cells tens of kilometres wide, erasing craters so thoroughly that the surface must be younger than about 10 million years :cite[mckinnon2016] :cite[moore2016]. Nitrogen glaciers flow down into it from the surrounding uplands, and ranges of water-ice mountains float in it like icebergs. The basin's position, almost exactly opposite Charon, suggests that a dense mass beneath it tipped the whole planet over, which is easiest to explain if a liquid water ocean lies under the shell :cite[nimmo2016]. A thin layer of gas hydrates at the top of that ocean could insulate it well enough to survive to the present :cite[kamata2019]. Triton, captured by Neptune, is younger still. Voyager 2 photographed dark plumes rising about 8 km and trailing downwind for more than 100 km, driven by sunlight warming nitrogen ice from below :cite[soderblom1990]. ::figure{src="File:Triton_Voyager2_PIA00317.jpg" size=wide alt="A mosaic of Triton showing a large pinkish southern polar cap streaked with dark marks, and a bluish-green band near the equator." caption="Observation: Triton from Voyager 2 in 1989. The pinkish southern cap is frozen nitrogen and methane; the dark streaks are deposits from plumes."} ## Formation Ice worlds formed far enough from their stars for water, and further out nitrogen, carbon monoxide and methane, to condense as solids. In the Solar System they are survivors of the population of small bodies that formed beyond Neptune. Most kept their original orbits in the Kuiper belt, but some were thrown into strange paths as the giant planets migrated. Pluto and Charon are thought to have formed from a giant impact between two such bodies :cite[canup2005]. Triton orbits Neptune backwards, the mark of capture: the leading idea is that it was one of a pair of Kuiper belt objects that passed close to Neptune, where its partner was ejected and Triton was left behind :cite[agnor2006]. Around other stars, larger ice-rich planets presumably form the same way beyond the snow line, as the cores that in other circumstances would have grown into giant planets. Gravitational microlensing, which can detect cold planets far from their stars, has found planets of a few Earth masses in orbits where their surfaces should be around 50 K, such as OGLE-2005-BLG-390Lb, about 5.5 Earth masses :cite[beaulieu2006]. Their compositions cannot yet be measured. ## How we know Almost everything known about ice worlds in detail comes from two flybys: Voyager 2 at Triton in August 1989 and New Horizons at Pluto on 14 July 2015. Between them, ground-based astronomers have watched stars pass behind these bodies. These **stellar occultations** are how Pluto's atmosphere was discovered and how its pressure has been tracked since 1988 :cite[meza2019], and they give precise sizes: an occultation in 2010 measured Eris's radius as 1,163 km and showed it has no global atmosphere today, with a geometric albedo of 0.96, among the brightest surfaces in the Solar System :cite[sicardy2011]. A similar event showed that Makemake also lacks a Pluto-like atmosphere :cite[ortiz2012]. The James Webb Space Telescope now takes spectra of these distant bodies. It has found ethane, acetylene and ethylene ices, products of methane chemistry, on Sedna, Gonggong and Quaoar :cite[emery2024], and deuterium-to-hydrogen ratios in the methane of Eris and Makemake that point to methane produced or processed inside the bodies rather than inherited unchanged from the Sun's birth cloud :cite[grundy2024]. Webb has also detected a trace of methane gas above Makemake :cite[protopapa2025]. ## Notable examples | Body | Radius | Mass | Density | Surface temperature | Distance from the Sun | |---|---|---|---|---|---| | Pluto | 1,188 km | 1.30 × 10²² kg | 1.85 g/cm³ | about 40 K | 29.7 to 49.3 AU | | Triton (Neptune) | 1,353 km | 2.14 × 10²² kg | 2.06 g/cm³ | 38 K | 30 AU, with Neptune | | Eris | 1,163 km | 1.66 × 10²² kg | 2.52 g/cm³ | about 30 to 55 K | 38 to 98 AU | | Makemake | about 715 km | about 3 × 10²¹ kg (estimate) | about 1.9 g/cm³ (estimate) | about 40 K | 38 to 53 AU | | Sedna | about 500 km | unknown | unknown | about 12 to 35 K | 76 to about 940 AU | Sedna, discovered in 2003, spends most of its long orbit far beyond the Kuiper belt and is too small and cold to keep the volatile ices of its larger relatives :cite[brown2004]. :::callout{type=sim title="In Pax Abyssi"} The sim places ice worlds in the cold outer zones of a system and generates them as planets from 0.01 to 3 Earth masses. Even the smallest is several times Pluto's mass, and the typical one is closer to Earth's, so these are icy planets rather than dwarf planets. Radius follows a law for a mix of rock and ice. Depending on mass, temperature and internal heat, a world gets anything from a near-vacuum or a thin Pluto-like atmosphere to a nitrogen-methane atmosphere of a few tenths of a bar, and occasionally a denser Titan-like one; worlds with enough internal heat keep a buried ocean. Three visual subtypes are assigned by surface temperature: **pristine** (TIW-PR), **cracked** (TIW-CR) and **tholin-stained** (TIW-TN). In the committed system sheets, 422 of 8,742 generated planets are ice worlds. Airless ice balls such as Saturn's moon Tethys belong to a separate type, the pure ice world (TPI). In the game each ice world is drawn as a prebaked texture plate chosen by its subtype code. ::: ## See also - [[Subsurface ocean world]] - [[Super-Earth]] (icy super-Earths, SEI) - [[Asteroid belt]] - [[Natural satellite]] - [[Titan]] - [[Frost lines]] - [[Atmospheric escape]] - [[Planet classification]]