Using the Keck telescope and spectroscopy (light analysis that reveals rotation speed), astronomers measured 32 objects: from giant planets to brown dwarfs — bodies heavier than planets but lighter than stars that never ignited.

Giant planets spin almost at the breakup limit: their gaseous cocoon at birth barely braked them, like ice under a spinning top. Brown dwarfs, however, lost spin quickly: their disk acted like sand. What's more, dwarfs bound to stars braked more than solitary ones; planets spin equally regardless of companionship.

Surprise: objects with masses 5–40 Jupiters kept more spin than heavier ones (40–100 Jupiters). It seems that during formation, crossing some threshold triggers strong braking. Jupiter with its 10-hour day is just a modest example: many giant planets spin even faster, teetering on the edge of self-destruction.