GJ 504 is a Sun-like star, about two billion years old. Its age is established by multiple independent clocks: spectroscopic indicators, chemical composition, evolutionary modeling. By every standard measure, it's a middle-aged star that should be spinning slowly and magnetically quiet — the inevitable consequence of magnetic braking, which bleeds angular momentum into the stellar wind over billions of years.
But GJ 504 rotates every 3.4 days, roughly seven times faster than the Sun. Its magnetic field measures 5.3 gauss, mapped by Zeeman-Doppler imaging. It has the activity level of a star less than a hundred million years old. Every age indicator except rotation agrees: two billion years. The rotation says: young.
Bellotti, Pezzotti, Buldgen, Vidotto, Evensberget, and Magaudda (arXiv 2602.22979, February 2026) show that the anomaly is explained by planetary engulfment. GJ 504 ate a Jupiter-mass planet. The consumed planet deposited its orbital angular momentum into the stellar convective envelope, spinning the star up. The faster rotation reactivated the magnetic dynamo, which strengthened the magnetic field, which intensified the stellar wind, which — under normal circumstances — would drain the angular momentum back out. But the injection was recent enough that the braking hasn't yet caught up.
The 3D magnetohydrodynamic simulations of the stellar wind quantify the angular momentum loss rate. The key test: only the planetary engulfment scenario reproduces the observed combination of age, rotation, and magnetic field strength simultaneously. A normal evolutionary track for a two-billion-year-old star predicts slow rotation and weak fields. A star that swallowed a giant planet at the right time matches all the data.
The rejuvenation is temporary. Magnetic braking is still operating. The star will spin down again, eventually returning to the rotation rate appropriate for its age. The planet bought time, not youth — the stellar clock wasn't reset, just wound forward and released.
HD 75332 shows similar signatures: anomalously fast rotation for its established age, enhanced magnetic activity. Another candidate for planetary consumption. If the mechanism is common, the implications extend beyond individual stars: the population of anomalously active old stars may be a population of planet-eaters, and their excess angular momentum is a fossil record of planetary systems that no longer exist.
The star looks young because it ate its planet. The planet is gone. The evidence is in the spin.