The Kepler mission revealed something deeply unsettling about our solar system: it appears to be missing the most common type of planet in the galaxy. Worlds between 1 and 4 Earth radii—larger than Earth but smaller than Neptune—dominate exoplanet demographics, yet our own planetary system contains nothing in this size range. The jump from Earth at 1.0 R⊕ to Neptune at 3.9 R⊕ is suspiciously clean, with no intermediate body to bridge the gap.

This absence is not merely a curiosity of planetary bookkeeping. It strikes at the heart of how we understand planet formation, disk evolution, and the conditions that produce habitable worlds. If super-Earths and mini-Neptunes are the default outcome of planet formation around Sun-like stars, then our solar system is the anomaly that demands explanation—not the archetype we long assumed it to be.

Recent advances in exoplanet characterization, atmospheric escape modeling, and protoplanetary disk physics have sharpened this question considerably. The observed bimodality in the radius distribution of close-in exoplanets, the constraints imposed by mass-radius relationships on bulk composition, and the timing of core accretion relative to gas disk dispersal all converge on a narrative that reframes our solar system's architecture as one particular solution among many. Understanding why we lack this planet type may ultimately tell us more about our own origins than the planets we do possess.

The California-Kepler Survey's precise stellar characterization revealed a striking feature in the radius distribution of close-in exoplanets: a deficit of planets near 1.8 R⊕, now widely known as the Fulton gap or radius valley. Planets cluster either below ~1.5 R⊕ as bare rocky cores or above ~2.0 R⊕ as volatile-enveloped worlds. This bimodality is not an observational artifact—it emerges robustly across different stellar types and orbital period ranges, though its precise location shifts with incident flux and host star mass.

Two primary mechanisms compete to explain the gap's origin. Photoevaporation, driven by XUV radiation from the host star during the first ~100 Myr, can strip hydrogen-helium envelopes from cores below a critical mass threshold, converting mini-Neptunes into super-Earths. The alternative—core-powered mass loss—invokes the residual thermal energy of formation itself, with the cooling luminosity of the rocky core driving atmospheric escape over gigayear timescales. Both models predict a radius valley, but they differ in its temporal evolution and dependence on orbital period.

Observational diagnostics are beginning to discriminate between these scenarios. Photoevaporation predicts that the valley's slope in radius-period space should follow R_gap ∝ P^−0.15, while core-powered mass loss predicts a steeper dependence. Recent analyses of the Kepler sample and young stellar clusters suggest that atmospheric stripping occurs early, favoring photoevaporation as the dominant sculptor—though core-powered mass loss likely contributes on longer timescales, particularly for planets at wider separations.

What makes this relevant to our solar system's missing planets is the implication that many super-Earths may be the stripped remnants of former mini-Neptunes. The distinction between these two planet types is not necessarily one of formation pathway but of post-formation atmospheric survival. A planet's fate—rocky super-Earth or gas-enveloped mini-Neptune—depends on the interplay between core mass, envelope mass fraction, orbital distance, and the intensity and duration of high-energy irradiation from its star.

Our solar system never produced close-in rocky cores massive enough to accrete and then lose substantial envelopes. The radius gap physics presupposes a population of cores in the 2–10 M⊕ range at orbital periods less than ~100 days. The absence of such cores in our system is not explained by atmospheric escape alone—it points to a more fundamental difference in how mass was distributed and how migration operated during the first few million years of solar system history.

Takeaway

The division between super-Earths and mini-Neptunes is largely a story of atmospheric survival after formation—the same core can become either planet type depending on irradiation history and envelope mass, meaning the boundary between rocky and gaseous worlds is far more contingent than our solar system's clean categories suggest.

The mass-radius diagram has become the primary tool for inferring the bulk composition of exoplanets in the super-Earth to mini-Neptune regime. Precise mass measurements from radial velocity follow-up, combined with transit-derived radii, allow us to compute mean densities and compare them against interior structure models. The results reveal a striking compositional diversity that has no analog in our solar system's terrestrial or ice giant planets.

Planets below the radius gap generally follow mass-radius relationships consistent with Earth-like rock-iron compositions—silicate mantles overlying iron cores, with Fe/Si ratios broadly similar to solar. Some super-Earths, however, show densities requiring either iron-enriched interiors (suggesting giant impact stripping of mantles) or significant water content. The degeneracy between an iron-poor dry interior and a smaller iron-rich core with a water layer remains one of the fundamental ambiguities in exoplanet characterization.

Above the gap, mini-Neptunes require substantial volatile envelopes—typically 1–10% of their total mass in hydrogen-helium—to explain their inflated radii. This is a remarkably small gas fraction by mass but produces dramatic effects on radius, since H/He at modest pressures has extremely low density. A 5 M⊕ core with just 2% H/He by mass can appear as a 2.5 R⊕ planet, fully twice the radius of the bare core. This sensitivity means that the radius gap corresponds to a very narrow range in envelope mass fraction, reinforcing the idea that atmospheric loss mechanisms operate as a relatively sharp threshold.

JWST transmission spectroscopy is now beginning to probe the atmospheric compositions of these worlds directly. Early results for planets like GJ 1214b and TOI-270d suggest high mean molecular weight atmospheres—possibly water-dominated or heavily enriched in metals—rather than the primordial H/He expected from simple accretion models. This raises the possibility that some mini-Neptunes are better described as water worlds or steam planets, with volatile inventories reflecting formation beyond the snow line followed by inward migration.

These compositional constraints reveal that the super-Earth/mini-Neptune population samples a region of parameter space—moderate core masses with variable volatile fractions—that our solar system simply does not explore. Earth is too small and too dry, Neptune too massive and too gas-rich. The gap between them represents not just missing planets but missing compositions, entire categories of planetary interior that nature readily produces but that our particular formation history failed to generate.

Takeaway

A hydrogen envelope comprising just a few percent of a planet's mass can double its apparent radius—the difference between a super-Earth and a mini-Neptune is often not a difference in core but in a thin veneer of gas, making planetary identity far more fragile and mutable than solid-body thinking suggests.

The most compelling explanation for our solar system's missing intermediate planets involves the timing and location of core assembly relative to gas disk dispersal. In the standard core accretion framework, rocky cores that reach ~10 M⊕ while the gas disk is still present undergo runaway gas accretion, becoming gas giants. Cores that grow too slowly miss this window and remain as bare rocky or icy bodies. The super-Earth/mini-Neptune population occupies a middle ground: cores massive enough to gravitationally bind modest envelopes but not massive enough—or not embedded in dense enough gas—to trigger runaway accretion.

Jupiter's early formation may have been the critical event that prevented this outcome in our system. Isotopic evidence from meteorites suggests Jupiter's core reached ~20 M⊕ within the first million years, early enough to open a gap in the protoplanetary disk and effectively partition the solid reservoir into inner and outer populations. This partitioning starved the inner disk of the pebble flux that would otherwise have built up super-Earth-mass cores. In systems without an early-forming giant, pebble drift continues unimpeded, efficiently concentrating mass at short orbital periods.

The Grand Tack and Early Instability models further reinforce this picture. Jupiter's inward-then-outward migration would have scattered and depleted material in the inner solar system, while an early dynamical instability among the giant planets would have excited eccentricities and prevented the survival of any close-in massive rocky bodies. Population synthesis models consistently show that systems producing Jupiter-mass planets at ~5 AU tend not to produce super-Earths interior to 1 AU, and vice versa—the two outcomes are anti-correlated.

Disk lifetime adds another dimension. Observations of protoplanetary disks show a spread in dispersal timescales from ~1 to 10 Myr. Disks that dissipate quickly leave cores stranded at modest masses with thin or no envelopes—producing super-Earths. Disks that persist longer allow continued gas accretion onto growing cores. Our solar nebula appears to have had a disk lifetime and solid mass distribution that favored the extremes: small terrestrial planets in the inner system and gas/ice giants beyond the snow line, with nothing in between.

This framing recasts our solar system not as a failed super-Earth factory but as a system where giant planet formation preempted the default pathway. The ubiquity of super-Earths and mini-Neptunes in the galaxy reflects the fact that most systems don't form Jupiters early enough to block pebble drift and deplete the inner disk. Our solar system's architecture—with its clean separation between terrestrials and giants and its conspicuous intermediate gap—is the signature of a relatively rare formation history, one that may have profound implications for the prevalence of Earth-like habitable environments.

Takeaway

Jupiter may be the reason we exist and the reason super-Earths don't—its early formation starved the inner solar system of the building material that in most other systems accumulates into the galaxy's most common planet type, making our planetary architecture a consequence of one giant's precocious growth.

The absence of super-Earths and mini-Neptunes from our solar system is arguably the most significant constraint on solar system formation models to emerge from the exoplanet revolution. It tells us that our planetary architecture is not universal but contingent—shaped by the specific timing of Jupiter's core assembly, the dynamics of pebble drift, and the lifetime of our protoplanetary disk.

As JWST characterizes the atmospheres of these intermediate worlds and next-generation radial velocity surveys refine their masses, we will build an increasingly detailed picture of the compositional diversity our system lacks. Each characterized super-Earth or mini-Neptune becomes a data point illuminating the formation pathway our solar system did not take.

The deepest implication may be this: understanding why these planets are missing is inseparable from understanding why Earth exists in the form it does. Our planet's size, volatile inventory, and habitability may all be downstream consequences of the same formation dynamics that excluded the galaxy's most common world from our neighborhood.