The star GJ 504, a twin of our Sun in age, behaves like a true figure skater: it completes a rotation in 3.4 days, shining with X-ray light. Usually, stars of the Sun's age spin sluggishly, but this one seems to have received a cosmic doping.

The secret was revealed by modeling. Light analysis (spectroscopy) showed that normal braking by the stellar wind couldn't have slowed it down. The only explanation: in the past, the star swallowed a exoplanet the size of Jupiter. This 'dance partner' transferred its orbital momentum, and the star spun up like a figure skater after a lift. Without that, it would rotate lazily — a full revolution per month.

A similar 'dancer' has been found at HD 75332—it seems celestial choreography is more common than expected.