K2-18b
K2-18b is a super-Earth exoplanet orbiting the cool red dwarf star K2-18, located approximately 124 light-years away in the constellation Leo.[1] Discovered in 2015 during NASA's Kepler Space Telescope's K2 mission through the transit method, it completes an orbit every 32.9 days at an average distance of 0.143 AU from its host star, placing it within the habitable zone where conditions might allow for liquid water.[1][2] The planet has a radius of about 2.46 times that of Earth and a mass of approximately 7.2 Earth masses, yielding a density of 2.28 g/cm³, which suggests a composition possibly including a rocky core, a substantial water layer, and a thick hydrogen-helium envelope, classifying it as a sub-Neptune or Hycean world—a type of ocean-covered planet with a hydrogen-rich atmosphere.[3][4][5] Atmospheric observations have made K2-18b one of the most studied exoplanets for potential habitability. In 2019, the Hubble Space Telescope detected water vapor in its atmosphere, marking the first such finding on a non-hot Jupiter exoplanet in the habitable zone and indicating possible steam or a water ocean beneath the hydrogen envelope.[6] In 2023, the James Webb Space Telescope (JWST) identified methane and carbon dioxide, further supporting a hydrogen-dominated atmosphere with a low ammonia abundance that could be consistent with a water ocean, while also hinting at trace dimethyl sulfide (DMS)—a molecule produced solely by life on Earth—though at low confidence.[7][8] A 2025 JWST study reported a stronger potential DMS signal alongside dimethyl disulfide (DMDS), suggesting possible biological activity if confirmed, but subsequent analyses that year found insufficient statistical evidence for these biosignatures, attributing signals to instrumental noise or alternative non-biological sources.[9][10][11] These findings highlight K2-18b's role in advancing exoplanet atmospheric science, though its exact nature—ranging from a mini-Neptune to a habitable ocean world—remains debated pending further observations.Discovery and observation
Initial discovery
K2-18b was first detected in 2015 as part of NASA's K2 mission, an extension of the Kepler Space Telescope's operations after the failure of its second reaction wheel. The planet was identified using the transit method, which measures the periodic dimming of the host star's light as the planet passes in front of it during its orbit. This detection occurred during K2's Campaign 1 observations of a field in the constellation Leo, where two transit events were recorded for the candidate now known as K2-18b.[12] Initial characterization provided an estimate of the planet's radius at approximately 2.3 Earth radii, derived from the depth of the transit signal relative to the star's size. To confirm the planet's existence and measure its mass, radial velocity observations were conducted starting in 2016 using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory's La Silla Observatory in Chile. These measurements detected the gravitational tug of the planet on its star, yielding an initial mass estimate of about 8.6 Earth masses.[13] The discovery and confirmation were formally announced in a paper published in the journal Astronomy & Astrophysics in December 2017, marking K2-18b as a super-Earth in the habitable zone of its M-type dwarf host star. Subsequent observations with the James Webb Space Telescope have further refined our understanding of the system.[13]Key observational milestones
Following its initial detection, K2-18b became a prime target for atmospheric characterization through transmission spectroscopy, with the Hubble Space Telescope (HST) providing the first key post-discovery observation in 2019. Observations using HST's Wide Field Camera 3 (WFC3) captured nine transits of the planet, enabling the detection of water vapor in its atmosphere at a significance of approximately 3σ. This marked the first unambiguous identification of water in the atmosphere of a non-hot Jupiter exoplanet in the habitable zone.[14] In 2020, the Transiting Exoplanet Survey Satellite (TESS) contributed additional photometric data during its primary mission, observing multiple transits that refined the planet's ephemeris and provided transit timing variations (TTVs). These TESS observations, combined with archival K2 data, improved the precision of orbital parameters and helped constrain the presence of any additional companions in the system through TTV analysis. The James Webb Space Telescope (JWST) advanced the observational timeline significantly in 2023 with near-infrared transmission spectroscopy using the Near-Infrared Imager and Slitless Spectrograph (NIRISS) and the Near-Infrared Spectrograph (NIRSpec). These observations, spanning wavelengths from 0.9 to 5.0 μm, confirmed the presence of methane (CH₄) and carbon dioxide (CO₂) in K2-18b's atmosphere at high confidence levels, with absorption features indicating a hydrogen-rich envelope potentially mixed with water. The data also suggested low ammonia levels, supporting models of an ocean-covered world. In April 2025, JWST's Mid-Infrared Instrument (MIRI) low-resolution spectrometer (LRS) observed K2-18b in the 6–12 μm range, detecting a tentative signal of dimethyl sulfide (DMS) at approximately 3σ confidence amid broader mid-infrared features consistent with prior atmospheric constituents. This observation built on the 2023 near-infrared data, probing deeper into the molecular inventory and highlighting potential sulfur-bearing species. However, subsequent analyses in 2025 found insufficient statistical evidence for these biosignatures, attributing signals to instrumental noise or alternative non-biological sources.[9][10][11]Host star and orbit
Stellar properties
K2-18 is a red dwarf star of spectral type M2.5V, situated approximately 124 light-years from Earth in the constellation Leo.[3] As a cool, low-mass main-sequence star, it exhibits typical characteristics of M dwarfs, including subdued nuclear fusion rates and a compact size compared to solar-type stars.[13] The star's physical parameters have been refined through spectroscopic and photometric analyses, including recent 2025 high-resolution spectroscopy.[15] Its effective temperature is 3449 ± 70 K, radius measures 0.468 ± 0.019 solar radii (R⊙), and mass is 0.32 ± 0.06 solar masses (M⊙). Metallicity stands at [Fe/H] = 0.0 ± 0.1, indicating a solar-like composition. Luminosity is 0.0251 L⊙ (log<sub>10</sub>(L/L⊙) = -1.60), consistent with its small size and cool surface, resulting in a bolometric magnitude that renders it faint from Earth at visual magnitude 13.50.[3] These values derive from mass-luminosity-radius relations calibrated for M dwarfs using high-resolution spectroscopy and transit photometry.[13]| Property | Value | Unit | Reference |
|---|---|---|---|
| Spectral type | M2.5V | - | Benneke et al. (2017) |
| Distance | 124 | light-years | NASA Exoplanet Archive |
| Effective temperature | 3449 ± 70 | K | Howard et al. (2025) |
| Radius | 0.468 ± 0.019 | R⊙ | Howard et al. (2025) |
| Mass | 0.32 ± 0.06 | M⊙ | Howard et al. (2025) |
| Metallicity | 0.0 ± 0.1 | [Fe/H] | Howard et al. (2025) |
| Luminosity | 0.0251 | L⊙ | Howard et al. (2025) |
Orbital parameters
K2-18b orbits its M-dwarf host star at a semi-major axis of 0.143 ± 0.006 AU, corresponding to an average separation that places it within the star's habitable zone.[3] The planet completes one orbit every 32.940 ± 0.0001 days, a period determined from transit timing analysis of K2 photometry combined with radial velocity measurements.[13] The orbit is nearly circular, with an eccentricity less than 0.43, consistent with tidal circularization over billions of years for a close-in planet around a cool star. Transit observations indicate an orbital inclination of 89.58 ± 0.01°, nearly edge-on relative to our line of sight, enabling the detection of transits. The transit duration is approximately 2.66 ± 0.02 hours, during which the planet passes in front of the star, producing a transit depth of 0.285%—a measure of the fractional decrease in stellar flux attributable to the planet's silhouette blocking the starlight.[3] Assuming zero Bond albedo and efficient heat redistribution, K2-18b has an equilibrium temperature of 235 ± 9 K, calculated from the incident stellar flux.[3] This temperature positions the planet within the habitable zone of its host star, defined by the inner edge at approximately 0.10 AU (runaway greenhouse limit) and outer edge at 0.26 AU (maximum greenhouse limit), based on stellar effective temperature and luminosity as of 2025. Radial velocity data confirm an inner companion, K2-18c, on a possibly non-coplanar orbit with a period of about 9 days; the system exhibits mean-motion resonance influencing long-term stability.[18]| Parameter | Value | Reference |
|---|---|---|
| Semi-major axis (AU) | 0.143 ± 0.006 | Cloutier et al. (2017) |
| Orbital period (days) | 32.940 ± 0.0001 | Cloutier et al. (2017) |
| Eccentricity | <0.43 | Cloutier et al. (2017) |
| Inclination (°) | 89.58 ± 0.01 | Benneke et al. (2017) |
| Transit duration (hr) | 2.66 ± 0.02 | Benneke et al. (2017) |
| Transit depth (%) | 0.285 | Montet et al. (2015) |
| Equilibrium temperature (K) | 235 ± 9 (A=0) | Howard et al. (2025) |
Physical characteristics
Mass, radius, and density
K2-18b is classified as a sub-Neptune exoplanet based on its bulk properties, with a measured radius of $2.610 \pm 0.087 Earth radii derived from refined transit observations using the Hubble Space Telescope.[19] This value represents an update from earlier Kepler measurements, incorporating higher-precision photometry to better account for limb darkening and stellar variability effects.[19] The planet's mass has been determined through radial velocity observations using the HARPS and CARMENES spectrographs, yielding $8.63 \pm 1.35 Earth masses.[20] This measurement combines multiple seasons of data to mitigate stellar activity signals, confirming K2-18b as significantly more massive than Earth while remaining below the mass threshold for typical ice giants.[20] From these parameters, the mean density of K2-18b is calculated as $2.67^{+0.52}_{-0.47} g/cm³, which is substantially lower than Earth's 5.51 g/cm³ but higher than Neptune's 1.64 g/cm³.[19] This intermediate density suggests an internal structure dominated by a rocky core surrounded by a substantial hydrogen-rich envelope, consistent with formation models for temperate sub-Neptunes.[19] The resulting surface gravity is approximately 12.4 m/s², or about 1.27 times Earth's value, computed as g = GM / R^2 using the planet's mass and radius.[19] Overall, K2-18b is roughly 8.6 times more massive and 2.6 times larger in radius than Earth, placing it in a distinct regime of planetary diversity where extended atmospheres play a key role in overall composition.[19][20]| Parameter | Value | Unit | Reference |
|---|---|---|---|
| Radius | $2.610 \pm 0.087 | R_\Earth | Benneke et al. (2019) [19] |
| Mass | $8.63 \pm 1.35 | M_\Earth | Cloutier et al. (2019) [20] |
| Mean Density | $2.67^{+0.52}_{-0.47} | g/cm³ | Benneke et al. (2019) [19] |
| Surface Gravity | \sim 12.4 | m/s² | Benneke et al. (2019) [19] |