Standard planet-formation models say that a rocky core of around 23 Earth masses is far past the threshold at which a growing planet starts pulling in hydrogen and helium from the surrounding disc and runs away into a gas giant. GJ 523b appears not to have done that. Its measured density points to a body that is mostly rock, with very little atmosphere for its mass. The system is also young, about 169 million years old, so there has been comparatively little time for an envelope to be stripped away afterwards. On top of that, the minimum orbital obliquity of 71.4 degrees places it on a strongly tilted, near-polar orbit rather than in the plane one would expect from quiet formation in a disc, which hints at a disruptive history.

Exoplanet science has moved past counting planets to explaining the ones that should not exist. A dense, nearly atmosphere-free body of 23 Earth masses sits in the gap that formation theory says should be occupied by gas giants, so either the models are missing something about how envelopes are acquired, or this planet lost one through a history violent enough to also account for its tilted orbit. Cases like this are how the theory gets corrected, which is why a single planet draws attention out of proportion to its size.