Which exoplanet was the first whose rotation rate was measured directly?

Beta Pictoris b was the first exoplanet whose rotation rate was measured directly.

In 2014, astronomers used high-resolution infrared spectroscopy to measure the broadening of carbon-monoxide absorption lines in the planet’s atmosphere. One side of the rotating world moves toward Earth while the other moves away, shifting the spectral signal in opposite directions. That Doppler pattern revealed the planet’s spin rather than merely estimating it from theory.

The measurement gave Beta Pictoris b a rotation period of about 8.1 hours, making its day shorter than Jupiter’s roughly 10-hour day. It is a young, massive super-Jupiter orbiting the young A-type star Beta Pictoris, about 63 light-years away in the constellation Pictor.

This result is sometimes described as the first measurement of an exoplanet’s “day.” It does not mean astronomers resolved the planet’s surface like a disk; instead, they inferred rotation from atmospheric gas moving across the visible hemisphere. Beta Pictoris b was discovered by direct imaging in 2008, a separate achievement from the later spectroscopic rotation measurement.