Abstract
The spin-rotation of a planet arises from the accretion of angular momentum
during its formation, but the details of this process are still unclear. In the
solar system, the equatorial rotation velocities and spin angular momentum of
the planets show a clear trend with mass, except for Mercury and Venus which
have significantly spun down since their formation due to tidal interactions.
Here we report on near-infrared spectroscopic observations at R=100,000 of the
young extra-solar gas giant beta Pictoris b. The absorption signal from carbon
monoxide in the planet's thermal spectrum is found to be blueshifted with
respect to the velocity of the parent star by (-15+-1.7) km/sec, consistent
with a circular orbit. The combined line profile exhibits a rotational
broadening of 25+-3 km/sec, meaning that Beta Pictoris b spins significantly
faster than any planet in the solar system, in line with the extrapolation of
the known trend in spin velocity with planet mass.
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