--- title: Mini-Neptune summary: 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. science_status: [observed, model, sim] categories: [Planets, Planet classes, Mini-Neptunes, Exoplanets] aliases: [Mini-Neptunes, Sub-Neptune, Sub-Neptunes, Gas dwarf, MNT, MNC, MNH, Temperate mini-Neptune, Cold mini-Neptune, Hot mini-Neptune] infobox: type: planet_class name: Mini-Neptune code: "MNT (temperate, 200 to 500 K), MNC (cold, below 200 K), MNH (hot, 500 K and above)" level: series series: "Mini-Neptune (MN)" science_status: [observed, model, sim] image: File:Mini-Neptune_sim.avif literature_equivalent: "Sub-Neptune or mini-Neptune: the population of planets above the radius valley, about 1.7 to 4 Earth radii" defining_criteria_sim: "Planets with a hydrogen-helium envelope over a rocky or icy core, typed by equilibrium temperature" mass_earth: {observed: "about 2 to 20; typically 5 to 10", sim: "3 to 25"} radius_earth: {observed: "about 1.7 to 4; the population peaks near 2.4", sim: "2.0 to 3.8"} density_g_cm3: {observed: "about 1 to 3 (GJ 1214 b about 2.2)"} equilibrium_temperature_k: {observed: "from below 300 (K2-18 b) to above 1,000", sim: "types split at 200 and 500 K"} bond_albedo: {observed: "0.51 (GJ 1214 b, JWST)"} dominant_gases: "H2 and He, with H2O, CH4, CO2, CO and NH3 in varying amounts; some envelopes are metal-rich (heavy molecules make up a large share)" clouds_hazes: "Photochemical hazes and clouds common; GJ 1214 b's are thick enough to flatten its transmission spectrum" interior: "Rocky or rock-and-ice core; envelope of hydrogen and helium making up about 1 to 10% of the mass; possibly water-rich layers" typical_orbit: "Most known ones orbit within about 0.5 AU" occurrence_observed: "Among the commonest planets around Sun-like stars and red dwarfs for orbits shorter than about a year" frequency_in_sim: "900 of 8,742 generated planets (MNC 391, MNT 271, MNH 238), in the committed sheets as of 2026-09-27" real_examples: ["GJ 1214 b", "K2-18 b", "TOI-270 d", "TOI-421 b", "Kepler-11 d, e and f"] subtypes: ["MNT-CL Cool temperate", "MNT-MD Moderate temperate", "MNT-WM Warm temperate", "MNC-HI / MNC-MD / MNC-LO High, mid and low albedo cold", "MNH-HZ Hot hazy", "MNH-AK Hot alkali", "MNH-EV Hot evaporating"] sim_source: "Mini-Neptune physics engines (temperate, cold, hot); mini-Neptune science reference" last_verified: 2026-09-27 sim: entity: [planet_class.MNT, planet_class.MNC, planet_class.MNH] refs: - id: fulton2017 type: article-journal author: [{family: "Fulton", given: "Benjamin J."}, {family: "Petigura", given: "Erik A."}, {family: "Howard", given: "Andrew W."}, {family: "Isaacson", given: "Howard"}, {family: "Marcy", given: "Geoffrey W."}, {family: "Cargile", given: "Phillip A."}, {family: "Hebb", given: "Leslie"}, {family: "Weiss", given: "Lauren M."}, {literal: "et al."}] title: "The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets" container-title: "The Astronomical Journal" volume: "154" page: "109" issued: 2017 DOI: 10.3847/1538-3881/aa80eb - id: fressin2013 type: article-journal author: [{family: "Fressin", given: "François"}, {family: "Torres", given: "Guillermo"}, {family: "Charbonneau", given: "David"}, {family: "Bryson", given: "Stephen T."}, {family: "Christiansen", given: "Jessie"}, {family: "Dressing", given: "Courtney D."}, {family: "Jenkins", given: "Jon M."}, {family: "Walkowicz", given: "Lucianne M."}, {literal: "et al."}] title: "The False Positive Rate of Kepler and the Occurrence of Planets" container-title: "The Astrophysical Journal" volume: "766" page: "81" issued: 2013 DOI: 10.1088/0004-637x/766/2/81 - id: lopez2014 type: article-journal author: [{family: "Lopez", given: "Eric D."}, {family: "Fortney", given: "Jonathan J."}] title: "Understanding the mass-radius relation for sub-Neptunes: radius as a proxy for composition" container-title: "The Astrophysical Journal" volume: "792" page: "1" issued: 2014 DOI: 10.1088/0004-637x/792/1/1 - id: weiss2014 type: article-journal author: [{family: "Weiss", given: "Lauren M."}, {family: "Marcy", given: "Geoffrey W."}] title: "The Mass-radius Relation for 65 Exoplanets Smaller Than 4 Earth Radii" container-title: "The Astrophysical Journal" volume: "783" page: "L6" issued: 2014 DOI: 10.1088/2041-8205/783/1/l6 - id: chen2017 type: article-journal author: [{family: "Chen", given: "Jingjing"}, {family: "Kipping", given: "David"}] title: "Probabilistic Forecasting of the Masses and Radii of Other Worlds" container-title: "The Astrophysical Journal" volume: "834" page: "17" issued: 2017 DOI: 10.3847/1538-4357/834/1/17 - id: otegi2020 type: article-journal author: [{family: "Otegi", given: "J. 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Carbon, Nitrogen, and Oxygen" container-title: "Icarus" volume: "155" page: "393-424" issued: 2002 DOI: 10.1006/icar.2001.6740 - id: charbonneau2009 type: article-journal author: [{family: "Charbonneau", given: "David"}, {family: "Berta", given: "Zachory K."}, {family: "Irwin", given: "Jonathan"}, {family: "Burke", given: "Christopher J."}, {family: "Nutzman", given: "Philip"}, {family: "Buchhave", given: "Lars A."}, {family: "Lovis", given: "Christophe"}, {family: "Bonfils", given: "Xavier"}, {literal: "et al."}] title: "A super-Earth transiting a nearby low-mass star" container-title: "Nature" volume: "462" page: "891-894" issued: 2009 DOI: 10.1038/nature08679 - id: cloutier2021 type: article-journal author: [{family: "Cloutier", given: "Ryan"}, {family: "Charbonneau", given: "David"}, {family: "Deming", given: "Drake"}, {family: "Bonfils", given: "Xavier"}, {family: "Astudillo-Defru", given: "Nicola"}] title: "A More Precise Mass for GJ 1214 b and the Frequency of Multiplanet Systems Around Mid-M Dwarfs" container-title: "The Astronomical Journal" volume: "162" page: "174" issued: 2021 DOI: 10.3847/1538-3881/ac1584 - id: kempton2023 type: article-journal author: [{family: "Kempton", given: "Eliza M.-R."}, {family: "Zhang", given: "Michael"}, {family: "Bean", given: "Jacob L."}, {family: "Steinrueck", given: "Maria E."}, {family: "Piette", given: "Anjali A. A."}, {family: "Parmentier", given: "Vivien"}, {family: "Malsky", given: "Isaac"}, {family: "Roman", given: "Michael T."}, {literal: "et al."}] title: "A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve" container-title: "Nature" volume: "620" page: "67-71" issued: 2023 DOI: 10.1038/s41586-023-06159-5 - id: benneke2019 type: article-journal author: [{family: "Benneke", given: "Björn"}, {family: "Wong", given: "Ian"}, {family: "Piaulet", given: "Caroline"}, {family: "Knutson", given: "Heather A."}, {family: "Lothringer", given: "Joshua"}, {family: "Morley", given: "Caroline V."}, {family: "Crossfield", given: "Ian J. 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L."}, {family: "Samra", given: "D."}, {family: "Steinrueck", given: "M. E."}] title: "Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b" container-title: "Astronomy & Astrophysics" volume: "700" page: "A284" issued: 2025 DOI: 10.1051/0004-6361/202555580 - id: vaneylen2021 type: article-journal author: [{family: "Van Eylen", given: "V"}, {family: "Astudillo-Defru", given: "N"}, {family: "Bonfils", given: "X"}, {family: "Livingston", given: "J"}, {family: "Hirano", given: "T"}, {family: "Luque", given: "R"}, {family: "Lam", given: "K W F"}, {family: "Justesen", given: "A B"}, {literal: "et al."}] title: "Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley" container-title: "Monthly Notices of the Royal Astronomical Society" issued: 2021 DOI: 10.1093/mnras/stab2143 - id: holmberg2024 type: article-journal author: [{family: "Holmberg", given: "Måns"}, {family: "Madhusudhan", given: "Nikku"}] title: "Possible Hycean conditions in the sub-Neptune TOI-270 d" container-title: "Astronomy & Astrophysics" volume: "683" page: "L2" issued: 2024 DOI: 10.1051/0004-6361/202348238 - id: piaulet2024 type: article-journal author: [{family: "Piaulet-Ghorayeb", given: "Caroline"}, {family: "Benneke", given: "Björn"}, {family: "Radica", given: "Michael"}, {family: "Raul", given: "Eshan"}, {family: "Coulombe", given: "Louis-Philippe"}, {family: "Ahrer", given: "Eva-Maria"}, {family: "Kubyshkina", given: "Daria"}, {family: "Howard", given: "Ward S."}, {literal: "et al."}] title: "JWST/NIRISS Reveals the Water-rich “Steam World” Atmosphere of GJ 9827 d" container-title: "The Astrophysical Journal Letters" volume: "974" page: "L10" issued: 2024 DOI: 10.3847/2041-8213/ad6f00 - id: davenport2025 type: article-journal author: [{family: "Davenport", given: "Brian"}, {family: "Kempton", given: "Eliza M.-R."}, {family: "Nixon", given: "Matthew C."}, {family: "Ih", given: "Jegug"}, {family: "Deming", given: "Drake"}, {family: "Fu", given: "Guangwei"}, {family: "May", given: "E. 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J."}, {family: "Barrado", given: "D."}, {family: "Correia", given: "A. C. M."}] title: "Mapping the exo-Neptunian landscape" container-title: "Astronomy & Astrophysics" volume: "689" page: "A250" issued: 2024 DOI: 10.1051/0004-6361/202450957 - id: hoyer2023 type: article-journal author: [{family: "Hoyer", given: "S."}, {family: "Jenkins", given: "J. S."}, {family: "Parmentier", given: "V."}, {family: "Deleuil", given: "M."}, {family: "Scandariato", given: "G."}, {family: "Wilson", given: "T. G."}, {family: "Díaz", given: "M. R."}, {family: "Crossfield", given: "I. J. M."}, {literal: "et al."}] title: "The extremely high albedo of LTT 9779 b revealed by CHEOPS" container-title: "Astronomy & Astrophysics" volume: "675" page: "A81" issued: 2023 DOI: 10.1051/0004-6361/202346117 images_wanted: - file: File:Mini-Neptune_sim.avif subject: "Sim renders of the three mini-Neptune types at the same scale: a cold royal-blue MNC, a temperate steel-blue MNT and a hot copper-toned MNH" source: sim note: "Shot list: render the three plates on the lit sphere; label 'Sim render'. Confirm the plate generator's licence." - file: File:K2-18b_Webb_illustration.jpg subject: "Artist's concept of K2-18 b based on JWST data (September 2023)" source: nasa page_url: https://science.nasa.gov/missions/webb/webb-discovers-methane-carbon-dioxide-in-atmosphere-of-k2-18-b/ credit: "Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Nikku Madhusudhan (IoA)" licence: "Public domain (NASA/STScI release; follow the caption)" note: "Shared with the Ocean world page. Label 'Artist's concept'." - file: File:Sub-Neptune_interiors.svg subject: "Diagram: three interiors that fit the same mass and radius: rock with a thin hydrogen-helium envelope, rock with a water layer and steam atmosphere, and a mixed metal-rich envelope" source: other note: "To be drawn by us; label 'Diagram'." --- A **mini-Neptune**, or **sub-Neptune**, is a planet between about 1.7 and 4 times Earth's radius, larger than any rocky planet in the Solar System and smaller than Uranus or Neptune. Most appear to be rocky or icy cores wrapped in a thin envelope of hydrogen and helium, a few per cent of their mass, which makes them far larger than their cores alone. The Solar System has none, yet the Kepler mission found that they, together with the slightly smaller [[Super-Earth|super-Earths]], are among the most common planets in the Galaxy's inner planetary systems. What they are made of, and whether any of them could hold oceans, is one of the busiest questions in exoplanet science. ## Characteristics ### Small cores, puffy envelopes Hydrogen is so light that a little of it goes a long way. A rocky core of 5 Earth masses is about 1.5 times Earth's radius; wrap it in hydrogen and helium weighing just 1 to 2 per cent of the planet and it swells to between 2 and 3 Earth radii. The radius of a sub-Neptune therefore tracks the fraction of its mass in the envelope far more than its total mass, which lets a measured radius be read, with care, as a rough measure of composition :cite[lopez2014]. That is why the population divides so sharply at the [[Radius valley]]: planets that keep some gas are about twice the size of those that lose it all. Above the valley, the number of planets peaks near 2.4 Earth radii and then drops steeply beyond about 3 :cite[fulton2017]. Because the envelope dominates the size, mass and radius are only loosely linked. For planets of 1.5 to 4 Earth radii, measured masses scatter widely around a nearly linear trend :cite[weiss2014]. Across the whole "Neptunian" range, from about 2 Earth masses to about 130, radius grows roughly as $R \propto M^{0.59}$ :cite[chen2017], much faster than for rocky planets; fitting only the volatile-rich planets gives a similar slope :cite[otegi2020]. ### What is inside The same mass and radius can be matched by quite different interiors, and this degeneracy is the central problem of the class: - **Rock with a thin hydrogen envelope**, the standard picture, with the envelope about 1 to 10 per cent of the mass. - **Water-rich cores**, formed beyond the snow line and migrated inward, whose water could be mostly steam or supercritical fluid mixed with hydrogen :cite[zeng2019] :cite[venturini2020] :cite[burn2024]. - **Metal-rich, mixed envelopes**, in which heavy molecules such as water, carbon dioxide and methane make up a large share of the atmosphere. Spectra are the way to tell them apart. A hydrogen-dominated atmosphere is extended, because light molecules make for a tall atmosphere, and gives strong absorption features in a transit spectrum. An atmosphere heavy in water or carbon dioxide is compact and gives weaker features. ### Chemistry and temperature Temperature sets the chemistry. In a hydrogen-rich atmosphere, carbon takes the form of methane at low temperatures and carbon monoxide at high ones, with the switch near 1,000 K at a pressure of about a bar and lower at lower pressures :cite[lodders2002]. Cool sub-Neptunes should therefore show methane, and hot ones carbon monoxide. Ultraviolet light breaks methane apart high in the atmosphere, and the fragments can build hazes of complex organic molecules. ## Formation and evolution A sub-Neptune's core must grow large enough to pull in gas from its star's disc before the disc disperses, but not so fast that it runs away into a gas giant. The result is a planet holding only a few per cent of its mass as hydrogen and helium. From then on, the envelope is under attack. Starlight, especially the strong X-ray and ultraviolet output of young stars, and the planet's own cooling heat can drive the gas away :cite[owen2017]. Planets close to their stars and with small cores lose their envelopes and become super-Earths; those farther out or with heavier cores keep them. This sculpting is the leading explanation for the radius valley. At higher masses and closer orbits, the same erosion clears out Neptune-sized planets altogether. Planets of Neptune's size and mass are scarce on orbits shorter than about three days, the **hot Neptune desert** :cite[mazeh2016]; just outside it lies a "ridge" of planets at periods of about 3 to 6 days, and a more normal "savanna" beyond :cite[castro2024]. The few survivors inside the desert are odd. LTT 9779 b, a Neptune-sized planet on a 19-hour orbit, reflects about 80 per cent of the light that falls on it, making it the most reflective exoplanet known, probably because of metallic clouds :cite[hoyer2023]. ## How we know Sub-Neptunes are found by transits and weighed by radial velocity or, in tightly packed systems, by the way neighbouring planets tug on each other's timing. The six planets of **Kepler-11**, five of them closer to their star than Mercury is to the Sun, were one of the first such systems, and several have densities so low that they must hold substantial envelopes of light gas :cite[lissauer2011]. Kepler's statistics showed that planets of this size are common around stars of many types :cite[fressin2013]. The James Webb Space Telescope has turned sub-Neptunes from sizes into atmospheres: - **GJ 1214 b**, discovered in 2009 around a nearby red dwarf :cite[charbonneau2009], has 8.2 Earth masses and 2.7 Earth radii :cite[cloutier2021]. Its transmission spectrum is flat, hidden by haze, but JWST's measurement of its heat around a full orbit showed a reflective, metal-rich atmosphere with a Bond albedo of about 0.5 :cite[kempton2023]. - **K2-18 b**, 8.6 Earth masses and 2.6 Earth radii in its red dwarf's habitable zone :cite[benneke2019], has methane and carbon dioxide in its atmosphere :cite[madhu2023]. Whether it has a water ocean beneath a hydrogen atmosphere or is a gas-rich planet with no surface is disputed :cite[wogan2024], and a joint reanalysis found insufficient evidence for the claimed dimethyl sulfide :cite[luque2025]. See [[Ocean world]]. - **TOI-270 d**, 4.8 Earth masses and 2.1 Earth radii at about 390 K :cite[vaneylen2021], also shows methane and carbon dioxide :cite[holmberg2024]. - **GJ 9827 d**, about 2 Earth radii, has an atmosphere dominated by water vapour, a "steam world" :cite[piaulet2024]. - **TOI-421 b**, a hot sub-Neptune near 920 K around a Sun-like star, has a clear, hydrogen-rich atmosphere free of haze :cite[davenport2025]. ::figure{src="File:K2-18b_Webb_illustration.jpg" size=wide alt="An illustration of a blue planet with a hazy atmosphere next to a small red star." caption="Artist's concept: the sub-Neptune K2-18 b, whose atmosphere JWST found to contain methane and carbon dioxide. What lies beneath the atmosphere is still argued over."} ## Notable examples | Planet | Radius | Mass | Temperature | Atmosphere (JWST) | |---|---|---|---|---| | GJ 1214 b | 2.74 R⊕ | 8.2 M⊕ | about 550 to 600 K | hazy, metal-rich, reflective | | K2-18 b | 2.61 R⊕ | 8.6 M⊕ | about 250 to 300 K | CH₄ and CO₂; nature disputed | | TOI-270 d | 2.13 R⊕ | 4.8 M⊕ | about 390 K | CH₄ and CO₂ | | GJ 9827 d | about 2.0 R⊕ | about 3 M⊕ | about 620 K | water vapour dominated | | TOI-421 b | about 2.7 R⊕ | about 7 M⊕ | about 920 K | clear, hydrogen-rich | :::callout{type=sim title="In Pax Abyssi"} The sim's mini-Neptunes are planets with hydrogen-helium envelopes over rocky or icy cores, sorted into three types by equilibrium temperature. **Cold mini-Neptunes** (MNC), below 200 K, carry methane and ammonia ice clouds and are subtyped by how much light those clouds reflect: high (MNC-HI), mid (MNC-MD) or low albedo (MNC-LO). **Temperate mini-Neptunes** (MNT), 200 to 500 K, run from cool worlds with thin ice clouds (MNT-CL, 200 to 300 K) through moderate ones with mixed clouds (MNT-MD) to warm, hazier ones (MNT-WM, 400 to 500 K). **Hot mini-Neptunes** (MNH), 500 K and above, are split at the sim's methane-to-carbon-monoxide switch at 800 K: **hazy** worlds below it (MNH-HZ) where methane feeds photochemical haze, **alkali** worlds above it (MNH-AK) with carbon monoxide and sodium and potassium absorption, and **evaporating** worlds (MNH-EV) above 1,200 K or wherever envelope loss is severe, the survivors at the edge of the hot Neptune desert. In the committed system sheets, 900 of 8,742 generated planets are mini-Neptunes. In the game each is drawn as a prebaked texture plate chosen by its subtype code. ::: ## See also - [[Radius valley]] - [[Super-Earth]] - [[Ocean world]] - [[Hycean worlds and K2-18 b]] - [[Ice giant]] - [[Hot Neptune desert]] - [[Atmospheric escape]] - [[Planet classification]]