How a Swallowed Planet Spun Up a Star ⚡ экспресс
The star GJ 504 is an evolved (2 Gyr) solar analogue with a rotation period of 3.4 days and intense magnetic activity, contradicting expectations for its stage. A nearby giant planet, migrating due to tidal forces, is thought to have been engulfed by the star, spinning it up and rejuvenating the magnetic dynamo. To test this, large-scale, non-axisymmetric dipolar magnetic field with an average strength of 5.3 G was reconstructed from spectropolarimetric data from ESPaDOnS using Zeeman Doppler imaging. This field was then fed into 3D MHD stellar wind models to estimate the angular momentum loss rate. Comparison with rotational evolution tracks shows that only the planet-engulfment scenario explains all observations (rotation, X-ray luminosity, and momentum loss rate corrected for field strength underestimation). Additionally, the star HD 75332 has been identified as another candidate for such an event, suggesting the phenomenon may be widespread.
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.
🎯 The star GJ 504 completes a full rotation in 3.4 days—seven times faster than our Sun.