Beta Pictoris b is a gas giant exoplanet about 7 times more massive than Jupiter, orbiting its star at about the same distance as Saturn goes around the Sun. It's notable because it's one of the few exoplanets that's actually been directly imaged. This has been possible because it's only about 20 million yrs old (compared with 4.5 billion yrs for our solar system), so it's still glowing brightly at infrared wavelengths while it radiates away the heat from its recent formation. Also, at a distance of 60 lightyears it's quite nearby, resulting in a relatively large angular separation from its host star and thus making it easier to spatially resolve the two.
Now, astronomers have detected the spectral lines of carbon monoxide in its atmosphere, and used the distortion of these line shapes to infer how fast the planet rotates about its axis. It turns out it takes about 8hrs, compared with 10hrs for Jupiter and, of course, 24hrs for Earth. This is a really cool result, and the first time such a measurement has been made for an exoplanet. Information about planet spin rates as a function of properties like mass etc will be particularly useful for constraining our theories of how planets form.
Astronomers have for the first time managed to detect the rotation of an extrasolar planet, by analysing the way its atmosphere filters light.