fter the sun formed, the dust and gas left over from its natal cloud slowly swirled into the eight planets we have today. Small, rocky things clung close to the sun. Gigantic gas worlds floated in the system’s distant reaches. And around countless stars in the galaxy, a version of this process repeated itself, forging plentiful planets in a spectrum of sizes — except, apparently, worlds just a tad bigger than Earth.

While NASA’s newest planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), steadily tallies more exoplanets, a mysterious gap in their sizes, first identified in 2017, has persisted. The gap shows that scientists need some new ideas to explain how planets are made, both in the broader cosmos and in our backyard.

Astronomers have used TESS to find hundreds of possible planets around the nearest stars since its launch in April 2018, including 24 confirmed worlds so far. The galaxy seems to host a lot of small planets, especially ones measuring between two and four times the size of Earth and others in Earth’s ballpark. But for some reason, planets with radii between 1.5 and two times that of Earth are rare.

The paucity of planets in that range, known as the “Fulton gap” after the lead author of the paper that pointed it out, first appeared in the findings of the Kepler Space Telescope, which hunted exoplanets for nearly a decade before passing the torch to TESS. While TESS doesn’t yet have enough planets in its statistics bin to confirm or disprove the Fulton gap, the trend has continued, and astronomers say they don’t expect the gap to disappear.