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Neptunian desert
Neptunian desert
Neptunian desert
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Distribution of mass versus orbital period for planets with a measured mass. Black lines represent the Neptunian desert. NGTS-4b is shown as a red cross.

The Neptunian desert or sub-Jovian desert is broadly defined as the region close to a star (period < 2–4 days) where no Neptune-sized (> 0.1 MJ) exoplanets are found.[1] This zone receives strong irradiation from the star, meaning the planets cannot retain their gaseous atmospheres: they evaporate, leaving just a rocky core.[2]

Neptune-sized planets should be easier to find in short-period orbits, and many sufficiently massive planets have been discovered with longer orbits from surveys such as CoRoT and Kepler.[1] The physical mechanisms that result in the observed Neptunian desert are currently unknown, but have been suggested to be due to a different formation mechanism for short-period super-Earth and Jovian exoplanets, similar to the reasons for the brown-dwarf desert.[1]

Candidates

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NGTS-4b

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Main article: NGTS-4b

LTT 9779 b

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LTT 9779 b is an ultra-hot Neptune in the Neptunian desert. It has an unusually high albedo of 0.8, and likely has a metal-rich atmosphere.[3]

Vega b

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Vega b, reported in 2021, is a candidate ultra-hot Neptune with a mass of ≥21.9 M🜨 that revolves around Vega every 2.43 days, a mere 0.04555 AU (6,814,000 km) from its luminous host star. The equilibrium temperature of the planet is a white-hot 3,250 K (2,980 °C; 5,390 °F) assuming a Bond albedo of 0.25, which, if confirmed, would make it the second-hottest exoplanet after KELT-9b.[4]

See also

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Notes

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Neptunian desert

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from Grokipedia
The Neptunian desert, also known as the hot Neptune desert or sub-Jovian desert, refers to a sparsely populated region in the parameter space of exoplanets, characterized by the scarcity of Neptune-sized planets (typically 4–10 radii or greater than 0.1 masses) orbiting very close to their host stars, with orbital periods shorter than approximately 2–4 days. This dearth arises primarily from intense stellar irradiation that heats the planets' atmospheres to temperatures exceeding 900°C (1,700°F), causing rapid where gaseous envelopes are stripped away by stellar winds and radiation, often leaving behind smaller super-Earths or mini-Neptunes. The boundaries of the Neptunian desert have been refined through statistical analyses of large catalogs, such as Kepler's DR25, revealing a sharp deficit for in the 4–10 radii range at orbital periods ≲ 3.2 days, with only about 0.1% of close-in falling within these confines at a 3σ level. Beyond the desert lies a transitional "ridge" of elevated occurrence (5.5–8.5 radii) at periods between 3.2 and 5.7 days, where the occurrence rate is roughly 8 times higher than in the itself (at 4.7σ significance), potentially reflecting that have migrated inward or survived partial atmospheric loss. Further out, at periods ≳ 5.7 days, the "savanna" region shows a moderate population of Neptune-like worlds, with an occurrence rate about 3 times that of the (4.7σ significance), indicating more stable conditions for retaining hydrogen-helium atmospheres. Recent TESS-based analyses as of 2025 further refine these boundaries through homogeneous samples of close-in Neptunes. Despite the general paucity, rare "survivors" like NGTS-4b—a Neptune-sized exoplanet (3 times Earth's radius, 20 times its mass) orbiting its star every 1.3 days with a surface temperature of ~1,000°C—demonstrate that intact atmospheres can persist under specific conditions, such as recent migration into the desert or gradual mass loss, challenging models of planetary evolution. More recent examples include sub-Neptunes in the TOI-1117 system (discovered 2025), one of which resides in the desert around a Sun-like star. Neptune-like exoplanets, composed of rocky or icy cores enveloped in thick gaseous layers similar to our Solar System's Neptune and Uranus, are estimated to be up to 10 times more common than Jupiter-mass giants beyond the snow line (around 2.7 AU) as of surveys up to 2016, underscoring the desert's role in highlighting dynamical processes like high-eccentricity migration or photoevaporation that sculpt close-in planetary architectures. Ongoing programs, such as the ATREIDES collaboration using the ESO Very Large Telescope's ESPRESSO instrument and the Next-Generation Transit Survey, aim to map these regions further by targeting systems like TOI-421, which hosts planets in both the ridge and savanna, to probe formation mechanisms and atmospheric retention.

Definition and Characteristics

Definition

The Neptunian desert is a region in the exoplanet parameter space defined by orbital periods shorter than approximately 2–4 days and planetary radii between roughly 4 and 10 Earth radii, where there is a marked depletion of Neptune-sized planets orbiting close to their host stars. This gap appears in the period-radius plane and extends to the period-mass plane for planets with masses around 0.03–0.3 Jupiter masses, highlighting a scarcity of volatile-rich worlds in ultra-short orbits. The term "Neptunian desert" was introduced by Szabó and Kiss in 2011 to describe this observed dearth of short-period, low-density exoplanets analogous to , drawing a parallel to other "deserts" in planetary distributions and contrasting sharply with the relative abundance of hot Jupiters in similar orbital regimes. Exoplanets are broadly classified by size and composition, with super-Earths typically featuring rocky interiors and radii less than about 2 radii, sub-Neptunes or mini-Neptunes possessing extended hydrogen-helium envelopes and radii of 2–4 radii, Neptunian planets exhibiting gaseous atmospheres similar to with radii of 4–10 radii, and hot Jupiters being massive gas giants exceeding 10 radii. The Neptunian desert occupies the transitional zone between super-Earths/sub-Neptunes and hot Jupiters at the shortest periods, underscoring distinct population behaviors. This underdensity was initially derived from transit surveys conducted by the , which revealed a statistical paucity of candidates in this parameter space among thousands of detected exoplanets, and has been corroborated by the (TESS), further mapping the desert's boundaries across a broader stellar sample.

Boundaries and Extent

The Neptunian desert is delineated in the planetary radius-period plane, with an inner radial boundary near approximately 4 radii (R⊕), separating it from the more populous regime below this size, and an outer boundary around 10 R⊕, beyond which hot Jupiters become more common. These radial limits correspond to planetary masses of roughly 10–100 masses (M⊕), or 0.03–0.3 masses, encompassing the Neptune-like domain where the dearth of close-in planets is most pronounced. The boundaries are not abrupt but form a sloped "cliff" structure, with the upper edge exhibiting a negative (Rp ∝ P^{-0.33}, where Rp is planetary radius and P is in days) and the lower edge a positive , as mapped from Kepler . The desert primarily occupies orbital periods less than 3.2 days, corresponding to semi-major axes under 0.03 for solar-type stars, though this extends to 4–5 days for cooler hosts where irradiation is lower. Recent analyses of TESS data reveal subtle asymmetries in this geometry, with the desert narrowing more sharply at shorter periods and transitioning into a "ridge" of enhanced occurrence at 3.2–5.7 days, indicating a population-level rather than a uniform void. This shape contrasts with the related but distinct radius valley—a broader gap in planet sizes around 1.7–2 R⊕ at intermediate periods (5–30 days)—which highlights bimodal distributions in planet radii but does not specifically target the short-period Neptunian regime. Stellar properties significantly influence the desert's extent, with sharper boundaries around hotter, more massive F- and G-type stars due to intensified high-energy that enhances atmospheric loss. In contrast, the desert appears fuzzier and less depleted around cooler M-dwarfs, where lower stellar fluxes allow more sub-Neptunes to persist at short periods, as evidenced by occurrence rates that decrease more gradually with decreasing stellar . These dependencies underscore the role of host-star in sculpting the desert's parametric edges across diverse stellar populations.

Explanatory Mechanisms

Photoevaporation

Photoevaporation serves as the leading explanatory mechanism for the Neptunian desert, where intense high-energy radiation from the host star in the and (XUV) wavelengths heats the outer layers of exoplanets' atmospheres, triggering hydrodynamic escape of their hydrogen/ envelopes. This atmospheric stripping preferentially affects close-in planets, transforming volatile-rich sub-Neptunes into denser super-Earths by eroding their extended gaseous layers. Theoretical models of photoevaporation distinguish between XUV-driven processes, in which stellar radiation supplies the dominant energy for escape, and core-powered mass loss, where residual heat from the planet's cooling rocky core sustains outflows after the initial XUV peak. Both mechanisms effectively reduce the envelope mass fraction to below 1% in surviving , delineating the transition from sub-Neptunes to super-Earths and carving out the observed paucity of intermediate-sized worlds at short orbital periods. The mass loss rate in the energy-limited regime, applicable to highly irradiated atmospheres, is approximated by M˙=πFXUVRp3GMpK,\dot{M} = \frac{\pi F_{\rm XUV} R_p^3}{G M_p K}, where M˙\dot{M} denotes the escape rate, FXUVF_{\rm XUV} the impinging XUV flux, RpR_p and MpM_p the planetary radius and mass, GG the , and KK an efficiency parameter incorporating heating depth and recombination effects. This formulation highlights how proximity to the star amplifies flux and thus accelerates envelope removal for sub-Neptunes. The process predominantly occurs within the first 100 million years following formation, aligning with the host star's heightened XUV emission during its active , after which mass loss tapers as stellar activity declines and leaves exposed cores resembling super-s. Supporting emerges from the radius-period distribution, where a pronounced cliff in occurrence—dropping sharply for radii near 4 radii and periods under 3 days—mirrors photoevaporation simulations that predict efficient stripping in this regime. Additionally, the desert's boundaries sharpen around metal-poor host stars, where diminished yields smaller planetary cores and thinner initial envelopes, heightening susceptibility to total atmospheric loss.

High-Eccentricity Migration and Tidal Effects

High-eccentricity migration (HEM) is a dynamical process that can drive sub-Neptune-sized planets from wider orbits into close-in configurations, potentially contributing to the formation of the Neptunian desert. In this mechanism, gravitational interactions with a companion planet or a binary stellar companion excite the planet's orbit to high eccentricity (e > 0.5), causing its periastron to approach the star closely while the apoastron remains distant. Subsequent tidal interactions with the host star then circularize the orbit at short periods, typically through dissipation of orbital energy into planetary tides. This pathway, first detailed in the context of eccentric hot Jupiters, applies to lower-mass planets as well, where disk-planet scattering or secular perturbations from outer companions initiate the migration. During the circularization phase, the highly eccentric orbit leads to repeated close approaches, where strong tidal forces can cause Roche-lobe overflow and significant stripping for sub-Neptunes. with gaseous are particularly vulnerable, as periastron passages within ~3 stellar radii trigger mass loss via dynamical , potentially reducing the planet to a core if the envelope is not fully stripped. This tidal disruption acts as a barrier, preventing Neptune-sized survivors at periods shorter than approximately 3 days, as the energy dissipation timescale becomes too short for retention of substantial atmospheres. Survivors in the desert's upper edge, such as hot Jupiters, "park" at larger radii due to their higher masses and deeper envelopes, which resist complete stripping. A key aspect of this process is the eccentricity damping timescale, which governs the rate of orbital circularization: τe∝M∗1/2a13/2MpRp5\tau_e \propto \frac{M_*^{1/2} a^{13/2}}{M_p R_p^5}, where M∗M_* and MpM_p are the stellar and planetary masses, aa is the semi-major axis, and RpR_p is the planetary . This scaling highlights the sensitivity to planetary and semi-major axis, with closer orbits and larger radii accelerating damping and increasing disruption risk for volatile-rich sub-Neptunes. Scattered survivors just beyond the desert's inner boundary may represent planets caught mid-migration, where incomplete stripping leaves remnant envelopes. HEM contributes to shaping the Neptunian desert by populating short-period orbits selectively, explaining the dearth of intermediate-mass planets while allowing hot Jupiters to persist. Recent hybrid models combine HEM with photoevaporation, where initial tidal stripping reduces masses, making subsequent atmospheric by stellar more efficient for lower-mass planets. Observations from TESS indicate a link to stellar multiplicity, with hosts of Neptunian desert candidates exhibiting binary fractions of 16.7–27.5%, suggesting gravitational in multi-star systems as a primary excitation mechanism.

Observational Evidence

Historical Discovery

The Neptunian desert was first hinted at through analyses of data from NASA's Kepler space telescope, which operated from 2009 to 2018 and detected thousands of exoplanets via the transit method. Initial studies revealed an underdensity of planets with radii between approximately 2 and 4 Earth radii orbiting at periods shorter than 2–4 days, in the period-radius plane. This scarcity was first quantified in 2016 by Mazeh et al., who analyzed over 900 confirmed Kepler planets and identified a distinct "desert" region in both period-mass and period-radius distributions, spanning Neptune-mass planets (roughly 10–20 Earth masses) at close-in orbits. Their work established the statistical significance of this gap, attributing it to potential evolutionary processes rather than observational artifacts alone. The term "Neptunian desert" gained formal usage around 2018–2019 as subsequent studies built on Kepler's legacy, incorporating emerging data from other surveys. The (TESS), launched in 2018, extended observations to brighter, nearby stars, facilitating more precise radius and period measurements for a broader sample. By 2020, TESS analyses had refined the desert's boundaries, confirming the underdensity extended to fainter host stars while highlighting its sharpness in incident flux-radius space. A key milestone came in 2019 from the Next Generation Transit Survey (NGTS), which detected rare candidates like NGTS-4b in the desert, underscoring the region's extreme scarcity through ground-based photometry of bright targets. Refinements continued into the early 2020s with the (JWST), whose initial observations from 2022 to 2023 provided high-resolution spectra of select close-in sub-Neptunes near the desert edges, enabling mass and atmospheric constraints that sharpened models. In 2025, homogeneous reanalyses of TESS data, such as those from the ATREIDES program, confirmed the desert's asymmetric shape and dependence on stellar type, drawing from uniform processing of over 1,000 close-in planet candidates to mitigate selection effects and identifying new constraints on formation processes. Detection of the desert relies primarily on transit surveys like Kepler and TESS, which measure planetary radii and orbital periods directly from light curve dips, combined with follow-up (e.g., via HARPS or HIRES) for mass determinations to populate the period-mass plane. Statistical power stems from catalogs exceeding 1,000 confirmed or candidate close-in planets, allowing robust occurrence rate calculations. Challenges in delineating the desert include observational biases favoring larger, more detectable transits and incompleteness for ultra-short periods under 3 days, where stellar variability and light dilution obscure signals. These factors necessitate careful correction in population studies to distinguish true astrophysical features from instrumental limits. Theoretical models of photoevaporation had anticipated such a radius-period gap prior to Kepler's empirical confirmation.

Confirmed Candidates

The Neptunian desert hosts fewer than 10 confirmed planets, representing rare survivors that challenge the sparsity of Neptune-sized worlds in close orbits around their host stars. These planets typically exhibit radii between 2 and 6 radii and orbital periods under 3 days, placing them in a parameter space where atmospheric retention is uncommon. Key examples include NGTS-4b, discovered in 2019, which has a mass of 20.6 masses, a radius of approximately 3.18 radii, and an of 1.34 days around a K-type ; its high core mass is inferred to contribute to its persistence despite intense stellar irradiation. Another prominent survivor is LTT 9779 b, identified in 2020, an ultra-hot Neptune with an equilibrium temperature of about 2000 K, a high geometric albedo of 0.8, and a metal-rich atmosphere; it orbits every 0.79 days with a radius of 4.72 Earth radii and a mass of 29.3 Earth masses around a Sun-like star. In 2021, the candidate Vega b was reported with a minimum mass exceeding 21.9 Earth masses, an orbital period of 2.43 days, and an equilibrium temperature around 3250 K, though its confirmation remains pending and recent observations (e.g., JWST in 2024) have not detected it, suggesting the signal may be spurious due to the challenges of observing around the bright A-type star Vega. More recent discoveries from 2024 and 2025 highlight ongoing validations by TESS. TOI-1117 c, confirmed in 2025, is a with a of 8.78 and a 4.6-day period in a multi-planet system, positioning it on the edge of both the Neptunian desert and the radius valley around a G-type star. TOI-5800 b, validated in 2025, features an eccentric orbit (e ≈ 0.3) with a 2.63-day period, a radius of 2.44 radii, and a of 9.4 , suggesting it is actively migrating inward toward the desert's core while experiencing around an M-dwarf host. Other TESS-confirmed examples include TOI-2274 b (period 2.7 days, 6.57 , radius ≈ 2.45 radii around an M-type dwarf) and TOI-2768 b (period 1.5 days, 7.41 , radius ≈ 2.63 radii around a K-type star), both skirting the desert's inner boundary. These survivors share common traits, such as orbiting metal-rich host in 26% of cases within homogeneous samples, and often belonging to multi-planet systems; many display evidence of ongoing mass loss through or elevated eccentricities indicative of dynamical interactions.

Implications and Future Prospects

For Exoplanet Formation Theories

Sub-Neptunes are believed to form beyond the in protoplanetary disks, where sufficient volatiles are available to accrete substantial hydrogen-helium envelopes, before undergoing inward migration driven by disk torques to reach close-in orbits. The Neptunian desert arises as a post-formation selection effect, where only a of these migrated planets survive intense stellar without losing their envelopes, sculpting the observed paucity of short-period Neptune-sized worlds. This implies that formation models must account for both the efficiency of migration and subsequent atmospheric retention to explain the desert's boundaries. Integrating photoevaporation and high-eccentricity migration (HEM) into formation frameworks successfully reproduces the bimodal distribution of super-Earths and sub-Neptunes, with the radius valley marking the transition where envelope-stripped cores dominate over survivors. These mechanisms challenge core accretion paradigms for hot Neptunes, as in-situ formation at short periods struggles to build massive gaseous envelopes before disk dispersal, favoring disk migration followed by evolutionary sculpting instead. The Neptunian desert draws analogy to the brown-dwarf desert, a arising from formation barriers that prevent intermediate-mass objects between planets and stars, highlighting how dynamical and physical processes impose gaps in demographics. It further informs the radius valley as a signature of photoevaporation, where the depth of the valley correlates with stellar irradiation levels, distinguishing evaporated remnants from intact sub-Neptunes. Stellar plays a key role, with metal-rich hosts exhibiting deeper protoplanetary disks that enable more massive envelope accretion, enhancing survival rates against photoevaporation and populating the desert's edges. Recent analyses using DR3 and TESS data reveal stellar multiplicity rates of 16.7 ± 5.8% for confirmed and 27.5 ± 2.6% for Neptunian desert planets, higher than for isolated systems, suggesting origins involving by companions that drive HEM. Open questions persist regarding the scarcity of ultra-hot Neptunes, which form fewer than predicted by migration models despite favorable conditions for envelope capture, potentially due to enhanced tidal disruption or unrecognized mass-loss channels. The role of initial envelope mass fractions remains unclear, as variations in accretion efficiency could determine whether cross into the or stabilize as survivors, necessitating refined simulations of disk-planet interactions.

Ongoing Surveys and Predictions

The (TESS) continues its extended mission beyond 2020, focusing on the discovery and characterization of and Neptunian candidates in short-period orbits to probe the boundaries of the Neptunian desert. Recent TESS observations have identified several such planets, including eccentric like TOI-5800 b, which test migration and processes at the desert's edge. Complementing this, the (JWST) is enabling atmospheric spectroscopy of surviving Neptunian planets through transmission spectra, revealing potential signatures of ongoing mass loss or compositional anomalies in hot, close-in worlds. In 2025, the ATREIDES program was launched as an international collaboration led by the and the , in partnership with the Canary Islands Institute of Astrophysics, to map the exo-Neptunian landscape encompassing the desert, ridge, and savanna regions. This initiative employs high-precision (RV) instruments like to observe and model Neptune-sized planets consistently, aiming to identify "lost" exo-Neptunes near the desert and distinguish evolutionary pathways. Additionally, Warwick's dedicated search for these elusive planets integrates multi-wavelength data to refine the desert's structure. Parallel efforts include the NIRPS Guaranteed Time Observing subprogram combined with TESS, targeting around M dwarfs to explore the desert's lower boundary, as demonstrated by the characterization of the TOI-756 system featuring a transiting candidate. High-eccentricity migration (HEM) models predict an increased prevalence of eccentric crossing the desert's edge, potentially explaining survivors through dynamical rather than isolation. Photoevaporation models, in turn, anticipate detectable / escape signatures in the spectra of at the desert's upper boundary, such as enhanced metastable outflows, providing empirical tests of atmospheric stripping efficiency. Addressing observational gaps requires high-precision RV measurements to determine masses and densities of Neptunian candidates, distinguishing between cores and volatile envelopes. Extremely Large Telescopes (ELTs) will be essential for characterizing faint host stars and resolving atmospheric details in these systems. Data Release 3 (DR3) and subsequent releases enable studies of stellar multiplicity around desert planets, revealing higher companion rates that may influence migration and stability. Homogeneous samples from these surveys are expected to refine the desert's boundaries and yield new candidates, enhancing predictions for demographics by 2030.

References

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