When astronomers mapped thousands of exoplanets using NASA’s Kepler telescope, they expected to find a smooth distribution of planet sizes—small rocky worlds gradually blending into larger gaseous ones.

Instead, they found something unexpected: a clear and persistent shortage of planets with radii between about 1.5 and 2 Earth radii.

This region, now widely known as the planetary radius gap or Fulton gap, appears as a distinct “valley” in the size distribution of exoplanets. It is not a subtle statistical fluctuation—it is a real, repeatable feature seen across multiple surveys, and it has forced astronomers to rethink how planets form and evolve.

In this episode, we explore what the gap actually shows in the Kepler data, the leading explanations for why it exists, and why the debate is still very much unresolved.

We begin with the observational result itself. When plotting thousands of confirmed exoplanets by size, two dominant populations emerge: smaller, likely rocky super-Earths below ~1.5 Earth radii, and larger sub-Neptunes above ~2 Earth radii, with a noticeable dip in between. This suggests that planets are not simply distributed along a smooth continuum, but instead tend to cluster into two distinct categories.

The first major explanation is photoevaporation. In this model, young planets initially form with thick hydrogen-helium atmospheres. However, high-energy radiation—particularly XUV (X-ray and ultraviolet) emission from the host star—can strip away these gaseous envelopes over time. Planets close enough to their stars lose their atmospheres entirely, shrinking down into bare rocky cores. Slightly larger or more distant planets retain part of their atmospheres, remaining inflated. The result is a natural separation that creates the observed gap.

The second leading explanation is core-powered mass loss. In this scenario, the energy driving atmospheric escape does not come primarily from the star, but from the planet itself. After formation, a planet’s hot interior gradually cools and releases energy. That internal luminosity can be enough to drive atmospheric escape, particularly for low-mass envelopes. Like photoevaporation, this mechanism can also produce a division between stripped rocky cores and partially gas-rich sub-Neptunes.

What makes the situation scientifically intriguing is that both models can independently reproduce the observed radius gap, at least in broad terms. This means that current data alone does not clearly distinguish whether stellar radiation or intrinsic planetary cooling is the dominant process—or whether both operate together.

This has led to an emerging hybrid interpretation: a sequential two-phase stripping process, where early atmospheric loss is dominated by intense stellar radiation during the star’s active youth, followed by slower, long-term mass loss driven by internal planetary cooling. In this view, the radius gap is not the result of a single mechanism, but the combined imprint of multiple evolutionary stages.

Adding further complexity, recent studies show that the position of the radius gap is not fixed. It appears to shift depending on stellar mass and type, suggesting that the efficiency of atmospheric loss processes depends strongly on the host star’s energy output history. This creates a measurable “fingerprint” across different stellar populations.

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