If you look at the thousands of exoplanets discovered in our galaxy, you'll notice a gap. Planets between about 1.8 and 2 Earth radii — right in the middle of the size range between rocky super-Earths and gas-shrouded sub-Neptunes — are strikingly rare. Scientists have debated for years what causes this 'radius gap.' Now, a newly identified 20-million-year-old planetary system may provide the answer — a cosmic snapshot of planets caught mid-transformation.
Announced by scientists at the SETI Institute and the Astrobiology Center in Tokyo, the discovery has been described as the long-sought evolutionary 'missing link' in planetary formation theory. Dr. John H. Livingston, an exoplanet scientist at the Astrobiology Center, presented the findings on March 10, 2026.
**The Radius Gap: A Puzzle at the Heart of Planet Science**
The most common planets in the Milky Way are compact systems of sub-Neptunes and super-Earths — small planets packed close to their host stars. But within that population, there's a persistent mystery: when you plot the sizes of these planets, you see two clusters with a relative desert between them at around 1.8–2 Earth radii. That desert is the radius gap.
The leading explanation is photoevaporation. Young planets orbiting close to their stars are bathed in intense ultraviolet and X-ray radiation. Planets with a certain mass-to-atmosphere ratio lose their gas envelopes over time. Planets just above the gap threshold lose enough gas to fall below it, becoming bare rocky worlds. Planets with enough gravity hold onto their envelopes and survive as sub-Neptunes. The gap is an evolutionary outcome.
But this model had a problem: where were the young, transitional planets in the gap — the puffed-up worlds not yet stripped down to rocky cores?
**The Missing Link: 20 Million Years Old**
The newly characterised system is just 20 million years old — an infant in cosmic terms. Our Solar System is 4.6 billion years old. Most compact exoplanet systems studied by Kepler and TESS are billions of years old, their atmospheric evolution long since complete.
In this young system, researchers found planets whose sizes and masses place them squarely in the radius gap — worlds still inflated, puffy, and transitional. They represent planets before photoevaporation did its work. They are, in effect, the galaxy's most common planets, caught in the act of becoming what they are.
The masses were measured using transit timing variations (TTVs) — detecting subtle gravitational tugs between planets that cause slight variations in transit timing. Combined with N-body simulations modelling gravitational dynamics, the team precisely characterised the planets and placed them in context with the broader galactic population.
**James Webb Is Already Looking**
The discovery opens an unprecedented opportunity to study atmospheric evolution in near-real time. The James Webb Space Telescope has already begun observing the system, examining atmospheric composition — what gases are present, how thick the envelopes are, how they compare to older planets at different evolutionary stages.
In coming years, as Webb continues to monitor the system, it may be possible to detect atmospheric changes happening on observable timescales. We may be watching a planet change.
**Why This Matters**
The identification of this system resolves a significant theoretical loose end. The photoevaporation model had struggled to account for the scarcity of transitional worlds in the radius gap. This discovery suggests the scarcity is real — the process happens so quickly on cosmic timescales that it's rarely caught in the act. This 20-million-year-old system is the exception.
More broadly, it reinforces the idea that compact multi-planet systems across the galaxy share a common evolutionary origin. The Milky Way is full of such systems, many in the habitable zone where liquid water could exist. Understanding how they formed, and how their planets evolved, is foundational to understanding the conditions for life.
Further observations with James Webb are expected over the next 12 to 24 months. 🪐
*Sources: SETI Institute (March 10, 2026) · Dr. John H. Livingston, Astrobiology Center Tokyo · SETI Live event, March 10, 2026*
