2025ApJ...995L..62C

We report the detection of rotation in the old (∼7 Gyr) solar twin HIP 73815. This result provides new insight into the validity of gyrochronology beyond the solar age, a subject of intense debate in recent years. Previous claims in the literature often relied on merged or fragmented time series or on model-dependent proxies, leading to ambiguous interpretations. Thanks to ESPRESSO's service mode, we obtained an unprecedented, single, coherent data set spanning over 80 days and covering more than two full rotation cycles of this old solar twin. We measure a rotation period of ${P}_{{\rm{GP}}}=31.{1}_{-1.1}^{+1.3}$ days, consistent with predictions from gyrochronology, in contrast with recent studies suggesting that these relations break down past the solar age. Our result demonstrates that gyrochronology remains a reliable method for estimating stellar ages for Sun-like stars at least up to ∼7 Gyr, beyond the solar age, where its validity has been under debate. Moreover, we show that with a carefully designed ground-based observational strategy, precise and accurate rotation periods can be measured even in old stars, enabling robust age determinations for Sun-like stars and providing critical calibration for independent dating methods.