{{short description|Planet smaller than Neptune with a low density envelope}} [[File:NASA's James Webb Space Telescope Primed to Lift the Haze Surrounding Sub-Neptunes (51685676732).jpg|thumb|An artist's concept of TOI-421 b, a sub-Neptune.]] A '''sub-Neptune''', also known as a '''mini-Neptune''', is a type of exoplanet smaller in radius than Neptune, but larger than the small planet radius gap. Based on their low bulk densities, these planets likely possess a low density envelope surrounding a rocky core. Despite being one of the most numerous types of exoplanets discovered as of 2026, no sub-Neptune is known to exist in the Solar System.
The exact nature of sub-Neptunes is uncertain. It is debated whether they are "gas dwarfs" with a gas envelope of hydrogen and helium over a rocky core, "water worlds" with large amounts of volatiles such as water, or smaller versions of ice giants with both gas and volatiles. In order to discern between these scenarios, several sub-Neptunes such as GJ 1214 b and K2-18 b have been observed by telescopes such as the Hubble Space Telescope and the James Webb Space Telescope in order to infer their compositions and formation histories.
== Terminology ==
The terms "sub-Neptune" and "mini-Neptune" are used inconsistently in literature. Many research papers use either term to refer to the small planet population located above the radius gap,<ref name=Fulton2017/><ref name=Gillis2026/> possessing a low density envelope,<ref name=Zhang2022/> in contrast to super-Earths, which are smaller and lack an envelope. Other terms used to refer to this planet population include "gas dwarf",<ref name=Wang2015/><ref name=Winn2018/> which now usually specifically means water-poor sub-Neptunes, and "small Neptune".<ref name=Fressin2013/><ref name=Mortier2016/><ref name=Leconte2024/>
"Sub-Neptune" is occasionally used to collectively refer to any planet between 1–4 Earth radii, including super-Earths and mini-Neptunes.<ref name=Zhang2022/><ref name=Rigby2024/> This population is also known as "small planets",<ref name=Fulton2017/> or "Kepler planets".<ref name=Kutra2021/><ref name=Yang2023/> "Mini-Neptune" is sometimes used to refer to specific subsets of sub-Neptunes. The term has been used to describe sub-Neptunes with a massive hydrogen atmosphere and no defined surface,<ref name=Wogan2024/> or water-rich sub-Neptunes.<ref name=Barnes2009/><ref name=Calder2025/>
== Occurrence ==
Sub-Neptunes occur much more frequently than giant planets in the inner planetary system.<ref name=Fulton2018/> A 2013 study using data from the Kepler space telescope found that planets between 2–4 Earth radii orbiting within 245 days can be found around 31% of sun-like stars, while less than ten percent of such stars host larger planets orbiting within 418 days.<ref name=Fressin2013/> Subsequent works show that their occurrence increases with orbital period until around 12–13 days, where the occurrence rate flattens out.<ref name=Petigura2018/><ref name=Kunimoto2020/> Microlensing surveys, which are sensitive to planets located at large orbital separations, indicate that long period sub-Neptunes and super-Earths may also be common.<ref name=Zang2025/>
Short period sub-Neptunes are common around G-type and K-type stars, and their occurrence peaks around early M-type stars. Their occurrence decreases around stars hotter than the Sun, as well as around cooler stars.<ref name=Hardegree-Ullman2025/> Sub-Neptunes are rare around mid-to-late M dwarfs less massive than around 0.4 solar masses, with an occurrence rate of 0.148 ± 0.045 planets orbiting within 30 days per star, while super-Earth-size planets remain much more common.<ref name=Gillis2026/>
Sub-Neptune occurrence weakly correlates with the host star's metallicity, unlike larger giant planets.<ref name=Wang2015/><ref name=Petigura2018/><ref name=Kutra2021/>
In addition to a broad suppression in small planet occurrence, stellar companions with separations of less than around 100 au suppress the occurrence rate of sub-Neptunes.<ref name=Sullivan2025/>
== Characteristics ==
=== Orbit ===
Sub-Neptunes are frequently found with other sub-Neptunes (or super-Earths). They tend to have similar sizes as other sub-Neptunes in the system. Such systems are known as "peas in a pod" systems.<ref name=Millholland2021/>
Most sub-Neptunes are not in mean motion resonance with their neighbours. However, there is an excess of sub-Neptunes near first-order resonances, especially just wide of it.<ref name=Fabrycky2014/><ref name=Choksi2020/> Transiting sub-Neptunes near mean motion resonances, such as those orbiting Kepler-223<ref name=Mills2016/> and HD 110067,<ref name=Luque2023/> exhibit strong transit timing variations, which allows measurements of their masses.<ref name=Leleu2024/>
The sub-Neptune planet population's average eccentricity is lower than larger planets. The transition between the two populations occurs at a radius of around 3.5 Earth radii.<ref name=Gilbert2025/>
=== Size ===
The radius range of the main sub-Neptune population lies between the small planet radius gap, at around 1.7–1.8 Earth radii,<ref name=Fulton2018/> and the radius cliff, the sudden drop in planet abundance beyond approximately 3–4 Earth radii.<ref name=Fulton2018/><ref name=Kite2019/><ref name=Lee2025/> This definition is not rigid; some sub-Neptune-sized planets with masses exceeding Neptune's have also been referred to as sub-Neptunes,<ref name=Roy2022/><ref name=Barat2026/> as well as planets larger than Neptune which is less massive than it.<ref name=Beatty2024/>
Young sub-Neptunes, such as the transiting sub-Neptunes orbiting Kepler-51<ref name=Masuda2024/> and V1298 Tauri,<ref name=Livingston2026/> can be significantly larger than their older counterparts, some being as big as gas giants even though they are much less massive. As they age and radiate heat away, they undergo Kelvin–Helmholtz contraction, shrinking in radius until they reach typical sub-Neptune sizes.<ref name=Livingston2026/>
=== Composition ===
Although the low density of sub-Neptunes is a clear evidence that these planets possess an envelope made of low density materials in addition to a rocky core, the materials that comprise this envelope is disputed.
Atmospheric escape mechanisms which best explain the small planet radius gap for planets around sun-like stars generally predict most sub-Neptune cores to have Earth-like compositions, with low to no contribution from water.<ref name=Gupta2019/><ref name=Rogers2021MNRAS.503.1526R/> The hydrogen-helium envelope makes up a few percent of the planet's mass.<ref name=Rogers2021MNRAS.503.1526R/> Measurements of young sub-Neptunes' radii tentatively corroborate this model.<ref name=Vach2024/><ref name=Rogers2025MNRAS.539.2230R/><ref name=Livingston2026/> Some planet formation models also predict such dry sub-Neptunes.<ref name=Bean2021/>
On the other hand, some works, especially planet formation models, predict that sub-Neptunes are water-rich worlds. Such simulations predicted that sub-Neptunian planets formed beyond the water-ice line, before migrating inwards.<ref name=Izidoro2017/><ref name=Emsenhuber2023/><ref name=Burn2024/><ref name=Venturini2024/><ref name=Shibata2025/> Strong irradiation on a water-rich planet could produce an inflated steam atmosphere, consistent with observations.<ref name=Mousis2020/><ref name=Chakrabarty2026/> Some of these water-rich worlds may also possess hydrogen and helium in their envelopes.<ref name=Wanderley2025/><ref name=Chakrabarty2026/> There is also a possibility that water worlds which accreted beyond the water-ice line would also accrete large amounts of refractory organic carbon, or "soot".<ref name=Li2026/>
It is possible that sub-Neptunes around M dwarfs are different in nature from sub-Neptunes around sun-like stars, as some studies find the shape of the radius gap for planets around such stars to be consistent with it being created by planet formation processes rather than envelope loss mechanisms, unlike for sun-like stars.<ref name=Cloutier2020/><ref name=Bonfanti2024/><ref name=Ho2024/> Others find the radius gap to be consistent with envelope loss mechanisms, however.<ref name=VanEylen2021/> The minimum mass for sub-Neptunes appears to increase with stellar mass, and small sub-Neptunes around M dwarfs seem to be significantly less dense than their counterparts around FGK stars.<ref name=Parc2024/>
Chemical interactions between the envelope and the core can alter a sub-Neptune's composition after its formation.<ref name=Werlen2025ApJ...988L..55W/> Reactions between the hydrogen atmosphere and the magma ocean can produce water and other volatiles endogenously,<ref name=Kite2020/><ref name=Misener2023/> potentially explaining the presence of volatiles found in some sub-Neptunes' atmospheres without requiring accretion of icy materials beyond the water-ice line.<ref name=Nixon2025/> Water can also dissolve in magma ocean and be sequestered into the interior<ref name=Luo2024/> or converted to hydrogen, causing even sub-Neptunes which form with large amounts of water to develop a relatively water-poor envelope.<ref name=Werlen2025ApJ...991L..16W/>
=== Internal structure ===
Sub-Neptunes are generally believed to be at least partially differentiated, featuring an interior rich in denser materials such as silicates and metals engulfed by an envelope of lighter materials such as "gas" (hydrogen and helium) and/or "ice" (volatiles such as water, methane, and carbon dioxide). However, the nature of the envelope, the degree of differentiation, and the partition of different materials across the planet are not fully understood.
Assuming that sub-Neptunes are gas dwarfs, many may be expected to have a magma ocean. This is because the core, heated by planet formation, is blanketed by the envelope, which prolongs its cooling timescale to billions of years.<ref name=Vazan2018/><ref name=Tang2025/> Some sub-Neptunes may have sufficiently high atmospheric pressure to solidify the magma ocean.<ref name=Breza2025/> However, studies on the miscibility of hydrogen, silicate, and metals at high pressures and temperatures found that under pressures relevant to sub-Neptune interiors, all three materials are miscible above temperatures of approximately 3600 Kelvin, suggesting that a sub-Neptune's interior would not differentiate into a discrete iron core and a silicate mantle, and a significant amount of hydrogen would reside within a sub-Neptune's core as well.<ref name=Young2025/><ref name=Gilmore2026/> As the planet cools, the binodal surface, which delineates regions where the two substances are miscible, recedes deeper into the interior, causing silicate to rain out and hydrogen to enter the envelope, although some hydrogen would remain in the core even after this.<ref name=Rogers2025MNRAS.544.3496R/>
thumb|Internal structures of sub-Neptunes at various temperatures, as proposed by Benneke et al. (2024)
At high temperatures and pressures, hydrogen and water become fully miscible,<ref name=Soubiran2015/> and methane's solubility in water also increases.<ref name=Pruteanu2017/> Therefore, hydrogen, water, and other volatiles within a sub-Neptune's envelope may become miscible. For warmer sub-Neptunes, the entire envelope could be fully miscible, and the composition of the observable upper atmosphere would reflect the relative abundances of hydrogen and volatiles.<ref name=Benneke2024/> For colder sub-Neptunes, water could rain out of the uppermost layers, leaving a hydrogen-dominated atmosphere above the miscible region deeper in the interior.<ref name=Gupta2025/> For very cold sub-Neptunes with very thin hydrogen atmospheres, water may completely condense out to form a liquid ocean below a hydrogen-dominated atmosphere, becoming a hycean planet.<ref name=Innes2023/>
=== Atmosphere ===
Spectroscopic characterization has been performed on a number of sub-Neptunes. These works reveal diversity in their atmospheres, with different metallicities and compositions, presence of clouds and hazes, and albedo, possibly influenced by factors such as size, bulk composition, temperature, and age. Future data, methodologies, and theoretical studies may further refine or refute conclusions drawn from these observational studies.
The atmospheric metallicities of sub-Neptunes appear to vary by several orders of magnitude, ranging from low metallicity atmospheres consistent with the Sun's metallicity, such as TOI-421 b,<ref name=Davenport2025/> to very high metallicity atmospheres with enrichment hundreds or thousands of times greater than solar levels, such as TOI-270 d's<ref name=Benneke2024/> or GJ 1214 b's.<ref name=Schlawin2024/>
Bond albedo, which determines a planet's surface temperature, also varies from planet to planet. GJ 1214 b has a high Bond albedo of 0.51,<ref name=Kempton2023/> while TOI-824 b appears to be darker, with Bond albedo lower than 0.26.<ref name=Roy2022/>
==== Factors ====
Temperature may play a major role in determining the abundances of different gas species in a planet's atmosphere. The abundance of methane is expected to fall above 800–1100 K as carbon preferentially forms carbon monoxide at high temperatures. Therefore, photochemical hazes which form from photolysis of methane are not expected on hot planets.<ref name=Fortney2013/> At temperatures lower than 400 K, haze removal becomes more efficient, resulting in a less hazy atmosphere.<ref name=Yu2021/> This is supported by observations, which found that the atmospheres of warm sub-Neptunes with equilibrium temperatures between 500–800 K feature thick high-altitude aerosols, obscuring absorption signatures of atmospheric gases.<ref name=Brande2024/> Such aerosols may be responsible for "flat" featureless spectra observed in several warm sub-Neptunes, such as GJ 1214,<ref name=Nascimbeni2015/><ref name=Schlawin2024/> TOI-836 c,<ref name=Wallack2024/> and HD 15337 c.<ref name=Wallack2026/> Hotter planets such as TOI-421 b,<ref name=Davenport2025/> and colder planets such as TOI-270 d,<ref name=Benneke2024/> appear to have more clarified atmospheres. At even higher temperatures, a study on HD 86226 c, a small sub-Neptune with equilibrium temperature of 1300 K, found a featureless spectrum, contradicting both the aforementioned trend and predictions for giant planets.<ref name=Kahle2025/><ref name=Gao2020/> However, a high mean molecular mass envelope can also hinder detection of absorption features on this planet.<ref name=Kahle2025/>
Disequilibrium chemistry can further modify the atmosphere. On GJ 3470 b, a sub-Neptune with equilibrium temperature of 600 K, processes such as vertical mixing enhanced by tidal heating may be responsible for reducing the abundance of methane, causing the planet's atmosphere to become more clarified than other planets with similar temperatures.<ref name=Beatty2024/> Additionally, sulfur dioxide, detected on warm sub-Neptunes GJ 3470 b and TOI-1130 b, as well as on some hot Jupiters, is expected to form in the upper atmospheres as a result of disequilibrium photochemistry. This process becomes more efficient at higher temperatures and metallicities.<ref name=Barat2026/>
A sub-Neptune's atmospheric composition may also evolve as the planet ages, as lighter gases like hydrogen and helium can escape more readily than heavier gases such as water or carbon dioxide, possibly increasing the atmospheric metallicity over time. Observation of V1298 Tauri b, an approximately 20 million years old sub-Neptune progenitor planet, revealed an atmosphere with metallicity similar to the Sun's, lower than older sub-Neptunes.<ref name=Barat2025/> Continued depletion of hydrogen may also allow a sub-Neptune's atmosphere to become oxidizing over time, allowing oxygen buildup in the atmosphere and removing reduced species such as methane and ammonia. This may explain the lack of haze on GJ 9827 d, a small, warm sub-Neptune.<ref name=Piaulet-Ghorayeb2024/>
Smaller sub-Neptunes, located near the radius gap, may be more susceptible to atmospheric mass loss, possibly resulting in a greater diversity in atmospheric compositions among this population.<ref name=Piaulet-Ghorayeb2024/> Observations of small sub-Neptunes GJ 9827 d,<ref name=Piaulet-Ghorayeb2024/> TOI-270 d,<ref name=Benneke2024/> TOI-776 c,<ref name=Teske2025/> and GJ 3090 b,<ref name=Ahrer2025/> indicate all four planets possess high mean molecular mass envelopes, with significant compositional variations between them.<ref name=Wallack2026/>
There appears to be a diversity in the underlying bulk compositions of sub-Neptunes, which may be responsible for differences in detected atmospheric metallicities and gases between planets with otherwise similar conditions.<ref name=Roy2025/> Observations and modelings of TOI-270 d,<ref name=Benneke2024/><ref name=Nixon2025/> LP 791-18 c,<ref name=Roy2025/> and TOI-421 b<ref name=Davenport2025/> suggest that these planets may have rocky, water-poor compositions, while planets such as TOI-824 b<ref name=Roy2022/> and TOI-1130 b<ref name=Barat2026/> appear to be more likely to have water-rich bulk compositions. However, it is not necessarily certain which bulk composition best describes the detected features. Various models, including ones implying water-poor<ref name=Shorttle2024/> and water-rich compositions,<ref name=Luu2024/> are capable of explaining the detected atmospheric gases on K2-18 b.
==== Evolution ====
thumb|Diagram of how the two classes of small planets, mini-Neptunes and super-Earths, formed.
The volatile atmosphere of a sub-Neptune undergoes escape over time. The most likely mechanisms driving the envelope loss are photoevaporation and core-powered mass loss.<ref name=Rogers2021MNRAS.508.5886R/> Photoevaporation is driven by high energy radiation (specifically X-ray and extreme ultraviolet) heating the upper atmosphere of the planet and inducing a hydrodynamic outflow.<ref name=Owen2013/> Core-powered mass-loss, on the other hand, is driven by thermal energy from the core.<ref name=Ginzburg2016/> Although both mechanisms occur simultaneously, it is not known which, if any, mechanism dominates the envelope loss of sub-Neptunes,<ref name=Bean2021/> as predictions made by both are consistent with observations.<ref name=Fulton2017/><ref name=Gupta2019/><ref name=Sandoval2021/> One possibility is that core-powered mass loss dominates for planets with lower surface gravity and higher equilibrium temperature, while photoevaporation dominates for planets with higher surface gravity and lower equilibrium temperature, and a planet can transition between one regime to another during its evolution.<ref name=Owen2024/>
The rate of atmospheric loss may reduce dramatically if significant amounts of water is present in the upper atmosphere, as water is effective at radiating in infrared, and efficiently cools the upper atmosphere.<ref name=Yoshida2025/> Since water is expected to only be present in the upper atmospheres on sufficiently warm sub-Neptunes, this effect may explain the lower occurrences of cooler sub-Neptunes compared to warmer ones.<ref name=Gaidos2024/> As water can be produced through reaction between the atmosphere and the magma ocean,<ref name=Kite2020/> this effect can be relevant even for sub-Neptunes that formed dry.<ref name=Kobayashi2026/>
The loss of the envelope appears to occur over billions of years, and gradually converts sub-Neptunes into super-Earths. The result in the decrease of sub-Neptune occurrence and the corresponding increase of super-Earth occurrence with age, which has been observed,<ref name=Sandoval2021/><ref name=Gaidos2024/> although it is possible that this instead reflects changes in primordial planet populations formed at different times.<ref name=Christiansen2023/>
== See also ==
* Super-Neptune * Steam world * Super-Earth * Mega-Earth
== References ==
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</references>
{{Exoplanet}}
Category:Types of planet