How Giant Planets Kept Their Rapid Spin ⚡ экспресс
A survey of rotation speeds of 32 companions was performed using high-resolution spectroscopy from Keck/KPIC. Literature data were added—final sample: 43 companions and 54 free-floating objects. Comparison of the fractional breakup rotation parameter at an age of 10 Myr (assuming constant angular momentum) showed that giant planets (2-7 M_Jup) have significantly higher values than brown dwarf companions (10-40 M_Jup): significance 4-4.5σ for aligned orbital inclinations and 1.6-2.1σ for random. A mass ratio <0.8% serves as a clear boundary between regimes. The higher speeds of planets are interpreted as a result of smaller angular momentum losses due to braking by the circumplanetary disk. Dwarf companions spin slower than isolated ones, while planets and planetary-mass objects spin similarly. Analysis of specific angular momentum of 221 objects <0.1 M_sun shows that objects of 5-40 M_Jup retain significantly more angular momentum than bodies of 40-100 M_Jup after 10 Myr.
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.
🎯 A day on Jupiter lasts only 10 hours—faster than any other planet in the Solar System.