The most abundant planets in the galaxy — and a mystery: where did all the planets between 1.5 and 2 Earth radii go?
One of the most striking discoveries of exoplanet astronomy is the radius gap. Kepler data revealed a deficit of planets with radii between 1.5 and 2.0 Earth radii. Below 1.5 R_⊕, the distribution is densely populated with super-Earths. Above 2.0 R_⊕, sub-Neptunes and Neptunes dominate. But the narrow band between — virtually empty.
This "radius valley" or "radius gap" is not a selection effect. Statistical analyses confirm it is real. Somehow, planets either grow past 2 R_⊕ or remain below 1.5 R_⊕. The gap sits at a boundary: below it, rocky planets with thin atmospheres; above it, planets with thick hydrogen/helium envelopes.
The leading explanation invokes photoevaporation. Young planets, still surrounded by hydrogen envelopes acquired during formation, orbit close to their host stars. Intense ultraviolet radiation from the young star heats the upper atmosphere, causing hydrogen and helium to escape. Planets with masses less than ~5 Earth masses lack sufficient gravity to retain their atmospheres. Their envelopes strip away, leaving naked rocky cores — super-Earths. Planets more massive than ~10 Earth masses retain their hydrogen envelopes and become sub-Neptunes or Neptunes.
Adjust planetary radius to explore the composition and evolution of different worlds.
Classification
Mini-Neptune
Rocky core + thick envelope
Estimated Mass
6.5 M_⊕
Surface Gravity
3.2 g
Planetary structure
Super-Earths and mini-Neptunes likely form through core accretion: gravitational agglomeration of planetesimals, followed by migration. A rock-ice core forms, then gravitationally attracts hydrogen and helium from the protoplanetary disk. The final mass and composition depend on formation location and timing.
Super-Earths, with radii below the photoevaporation boundary, either (a) formed with minimal hydrogen envelope, or (b) had their envelopes stripped. Mini-Neptunes, above the boundary, retained their hydrogen envelopes. The boundary location depends on stellar type: high-energy radiation from young hot stars creates wider valleys.
Interior compositions vary wildly. Some super-Earths are nearly as dense as Earth (suggesting silicate mantles and iron cores). Others are less dense, indicating volatile-rich interiors (water, ammonia, methane). TRAPPIST-1e might be 20% water by mass — an ocean world. Others might be dominated by high-pressure ices.
Kepler-11 g is a sub-Neptune with radius 2.7 R_⊕ orbiting at 0.5 AU from a Sun-like star. Its estimated mass is 8 M_⊕, placing it firmly in the mini-Neptune regime. It orbits within a multi-planet system, exhibiting complex orbital dynamics.
LHS 475 b, one of the first JWST-observed exoplanets, is a rocky world 99% Earth's radius, just 26 light-years away. Its composition and habitability remain uncertain — it might be an ocean world, a Venus-analog, or something entirely unexpected.
The TRAPPIST-1 system offers an unprecedented laboratory. Seven planets, with the three in the habitable zone: TRAPPIST-1e (terrestrial), TRAPPIST-1f (mini-Neptune?), and TRAPPIST-1g (mini-Neptune?). Future observations with JWST will constrain atmospheric compositions and refine habitability assessments.
TRAPPIST-1e
Radius: 0.92 R⊕
Rocky
Kepler-11g
Radius: 2.71 R⊕
Mini-Neptune
LHS 475 b
Radius: 0.99 R⊕
Rocky
Photoevaporation is the dominant mechanism removing hydrogen atmospheres from young planets. Extreme ultraviolet (EUV) radiation ionizes atmospheric particles, which then escape as a hydrodynamic wind. The escape rate depends on stellar EUV luminosity and planet mass.
Higher stellar EUV luminosity causes faster mass loss. Higher planet mass retains atmospheres longer.
The question of when photoevaporation occurs remains open. Does it happen while the protoplanetary disk is still present (first few million years)? Or after disk dispersal (5-100 million years)? The timing dramatically affects planetary outcomes.
JWST is beginning to characterize super-Earth and mini-Neptune atmospheres. Early observations suggest dramatic diversity: some have thick atmospheres, others have thin or absent atmospheres. Some show hydrogen, others show water and CO2. Each system is a puzzle.
Future ground-based and space telescopes will probe deeper, measuring atmospheric compositions, masses, and densities with unprecedented precision. These observations will test photoevaporation models and constrain planet formation theory.
Super-Earths and mini-Neptunes dominate the exoplanet population. Yet they are rare in our solar system. Understanding why — how formation and evolution processes differ between our system and others — remains one of the great unanswered questions in planetary science.