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Astronomers find "madly whirling" exoplanet

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VLT measures the spin of Beta Pictoris b. An ESO photo

LONDON (PTI): Astronomers have for the first time ever measured the rotation rate of an alien planet, and found that the huge Jupiter-like world about 63 light-years from Earth has a day lasting just eight hours.

Observations from European Southern Observatory (ESO)'s Very Large Telescope (VLT) determined that the rotation rate of the exoplanet Beta Pictoris b is much quicker than any planet in the planetary system - its equator is moving at almost 100,000 kilometres per hour.

This new result extends the relation between mass and rotation seen in the solar system to exoplanets.

Beta Pictoris b orbits the naked-eye star Beta Pictoris which lies about 63 light-years from Earth in the southern constellation of Pictor (The Painter's Easel).

This planet was discovered nearly six years ago and was one of the first exoplanets to be directly imaged.

It orbits its host star at a distance of only eight times Earth-Sun distance - making it the closest exoplanet to its star ever to be directly imaged.

A team of Dutch astronomers from Leiden University and the Netherlands Institute for Space Research (SRON) have now found that the equatorial rotation velocity of Beta Pictoris b is almost 100,000 kilometres per hour.

By comparison, Jupiter's equator has a velocity of about 47,000 km per hour, while Earth's travels at only 1700 km per hour.

Beta Pictoris b is more than 16 times larger and 3000 times more massive than Earth, yet a day on the planet only lasts 8 hours.

"It is not known why some planets spin fast and others more slowly," said co-author Remco de Kok, "but this first measurement of an exoplanet's rotation shows that the trend seen in the Solar System, where the more massive planets spin faster, also holds true for exoplanets. This must be some universal consequence of the way planets form."

Beta Pictoris b is a very young planet, only about 20 million years old (compared to 4.5 billion years for Earth).

Over time, the exoplanet is expected to cool and shrink, which will make it spin even faster.

The astronomers made use of a precise technique called high-dispersion spectroscopy to split light into its constituent colours - different wavelengths in the spectrum.

The principle of the Doppler effect (or Doppler shift) allowed them to use the change in wavelength to detect that different parts of the planet were moving at different speeds and in opposite directions relative to the observer.

By very carefully removing the effects of the much brighter parent star they were able to extract the rotation signal from the planet.

"We have measured the wavelengths of radiation emitted by the planet to a precision of one part in a hundred thousand, which makes the measurements sensitive to the Doppler effects that can reveal the velocity of emitting objects," said lead author Ignas Snellen.

The finding appears in the journal Nature.

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