First measurement of an exosolar planet’s rotation

Nature : The second planet orbiting β Pictoris is a gas giant that rotates at 25 km/s at its equator, 50 times as fast as Earth. Although the planet’s diameter is 16 times that of Earth’s, its day is just slightly longer than 8 hours. β Pictoris b is one of the first planets found by direct observation. The discovery was possible because the planet is large, still strongly radiating in the IR wavelengths, just 65 light-years away, and orbits far from its parent star. That combination of characteristics, along with measurements of the planet’s atmospheric composition, allowed Ignas Snellen of Leiden University in the Netherlands and his colleagues to measure its rotation. Rotating bodies shift the spectrum of emitted radiation, and Snellen’s team was able to determine the effect of the shift from the stretching of the carbon monoxide absorption line in the planet’s spectrum.