Super-Earths, Sub-Neptunes & the Fulton Gap
The Kepler mission revealed that roughly 30% to 50% of all Sun-like and red dwarf stars host at least one planet with an orbital period under 100 days and a radius between Earth (1.0 R_⊕) and Neptune (3.9 R_⊕)—a size class completely absent from our own Solar System. High-precision California-Kepler Survey radii (Fulton et al. 2017) discovered that this population splits into two sharp peaks separated by a scarcity gap at 1.5–2.0 R_⊕ ('the Fulton Radius Valley'): dense rocky Super-Earths (R_p ≈ 1.0–1.5 R_⊕, ρ ≈ 5–8 g/cm³) and volatile-enveloped Sub-Neptunes (R_p ≈ 2.0–3.5 R_⊕, ρ ≈ 1.5–3 g/cm³).
Physical Mechanism & Dynamics
Why does a tiny 1%–2% by mass hydrogen-helium envelope double a rocky planet's radius from 1.4 R_⊕ to 2.6 R_⊕, leaving a deep valley around 1.8 R_⊕? Both leading physical explanations—XUV Photoevaporation (Owen & Wu 2013, 2017) and Core-Powered Mass Loss (Ginzburg, Schlichting & Sari 2018)—show that planets born with rocky/iron cores of ~3–8 M_⊕ and H₂/He envelopes are thermodynamically unstable at intermediate envelope fractions (~0.1%–0.3% by mass). In XUV Photoevaporation, high-energy extreme-UV and X-ray radiation from the young host star during its first 100 Myr heats the upper atmosphere to ~10,000 K and drives a hydrodynamic Parker wind; in Core-Powered Mass Loss, the cooling luminosity of the molten rocky core itself plus bolometric stellar heating drives atmospheric escape over ~1 Gyr.
Either mechanism bifurcates the population: cores below a critical threshold mass lose their entire H₂/He atmosphere and shrink down to bare rocky Super-Earths at ~1.3 R_⊕, whereas slightly more massive or cooler cores retain a 1%–3% H₂/He envelope that keeps their radius inflated near ~2.4 R_⊕ as Sub-Neptunes. A third compositional class—Water Worlds and 'Hycean' planets (such as K2-18b and TOI-270d)—combines a 20%–50% H₂O ice/steam mantle with a hydrogen-rich atmosphere.
Quantitatively, this mechanism is governed by Energy-Limited Photoevaporation Mass-Loss Rate & Radius Valley Slope (Ṁ_env ≈ (ε_XUV π R_p^3 F_XUV) / (G M_p K_tide), log₁₀(R_valley / R_⊕) ≈ -0.10 log₁₀(P / days) + 0.37). Shows that the Fulton radius valley R_valley shifts to smaller radii at longer orbital periods P (from ~1.9 R_⊕ at P = 3 d to ~1.5 R_⊕ at P = 50 d) because lower irradiation permits smaller cores to retain H₂ envelopes.
Energy-Limited Photoevaporation Mass-Loss Rate & Radius Valley Slope
Ṁ_env ≈ (ε_XUV π R_p^3 F_XUV) / (G M_p K_tide), log₁₀(R_valley / R_⊕) ≈ -0.10 log₁₀(P / days) + 0.37Shows that the Fulton radius valley R_valley shifts to smaller radii at longer orbital periods P (from ~1.9 R_⊕ at P = 3 d to ~1.5 R_⊕ at P = 50 d) because lower irradiation permits smaller cores to retain H₂ envelopes.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Occurrence rate inside P < 100 days (FGK stars) | 30%–50% of stars (often in compact 'peas-in-a-pod' multi-systems) | Kepler DR25 completeness-corrected occurrence (Zhu, Petigura et al.) |
| Super-Earth peak vs. Sub-Neptune peak | Super-Earths: 1.3 R_⊕ | Fulton Valley: 1.7–1.8 R_⊕ | Sub-Neptunes: 2.4 R_⊕ | California-Kepler Survey + Gaia DR3 stellar radii (Fulton et al. 2017) |
| H₂/He mass fraction needed to double radius | f_env = M_H2 / M_core ≈ 1%–2% by mass | Lopez & Fortney (2014) thermal interior structure models |
| K2-18b & TOI-270d JWST detections | CH₄, CO₂ (and H₂O in TOI-270d) in temperate sub-Neptunes | JWST NIRISS/NIRSpec transmission spectra (Madhusudhan et al. 2023; Benneke et al. 2024) |
Observational Evidence & How Scientists Know
JWST transmission spectra of the temperate sub-Neptunes K2-18b (R = 2.6 R_⊕, M = 8.6 M_⊕, Madhusudhan et al. 2023) and TOI-270d (R = 2.1 R_⊕, M = 4.8 M_⊕, Benneke et al. 2024) detected strong methane (CH₄) and carbon dioxide (CO₂) absorption in hydrogen-dominated atmospheres, while JWST 15 μm secondary eclipses of rocky M-dwarf Super-Earths (TRAPPIST-1b/c, LHS 3844b, GJ 1132b) test atmospheric survival.
Across reference datasets (NASA Exoplanet Archive, JWST / MAST Science Archive & Mission Documentation), quantitative benchmarks for super-earths, sub-neptunes & the fulton gap are anchored by: Occurrence rate inside P < 100 days (FGK stars) (30%–50% of stars (often in compact 'peas-in-a-pod' multi-systems), via Kepler DR25 completeness-corrected occurrence (Zhu, Petigura et al.)); Super-Earth peak vs. Sub-Neptune peak (Super-Earths: 1.3 R_⊕ | Fulton Valley: 1.7–1.8 R_⊕ | Sub-Neptunes: 2.4 R_⊕, via California-Kepler Survey + Gaia DR3 stellar radii (Fulton et al. 2017)); H₂/He mass fraction needed to double radius (f_env = M_H2 / M_core ≈ 1%–2% by mass, via Lopez & Fortney (2014) thermal interior structure models); K2-18b & TOI-270d JWST detections (CH₄, CO₂ (and H₂O in TOI-270d) in temperate sub-Neptunes, via JWST NIRISS/NIRSpec transmission spectra (Madhusudhan et al. 2023; Benneke et al. 2024)).
Uncertainties, Systematics & Open Questions
Distinguishing a Sub-Neptune with a rocky core + 2% H₂ envelope from a 'Steam World' with 40% supercritical H₂O requires measuring atmospheric mean molecular weight μ and C/O/S ratios with JWST NIRSpec and Ariel.
Real Astronomical Benchmarks & Cross-Domain Links
K2-18b & TOI-270d
Temperate sub-Neptunes (2.6 and 2.1 R_⊕) whose JWST transmission spectra revealed abundant CH₄ and CO₂ in hydrogen-rich envelopes
GJ 1214b
Archetypal warm sub-Neptune (2.7 R_⊕, 8.2 M_⊕) whose high-altitude reflective aerosols and high-metallicity steam/CO₂ atmosphere were decoded via a JWST MIRI thermal phase curve
Kepler-11 & Kepler-36 Systems
Compact multi-planet systems showcasing 'peas-in-a-pod' uniformity and adjacent orbits straddling the rocky Super-Earth (Kepler-36b, 1.5 R_⊕, 6.8 g/cm³) and gaseous Sub-Neptune (Kepler-36c, 3.7 R_⊕, 0.9 g/cm³) divide
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- NASA Exoplanet Archive (2026-10-01 snapshot) — Akeson, R. L., et al. 2013, PASP, 125, 989; NASA Exoplanet Archive (IPAC/Caltech). Archive / Reference ↗
- JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)) — Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning). Archive / Reference ↗
Have a correction or updated measurement? Review our Epistemic Standards and Corrections Policy.