3rd May 2014 Length of exoplanet day measured for first time Astronomers have measured an exoplanet's length of day for the first time. Beta Pictoris b was found to have a day that lasts only eight hours.
Our ability to glean information from distant planets beyond our own Solar System has taken yet another step forward. Researchers at the European Southern Observatory (ESO) have, for the first time, determined the rotation rate of an exoplanet. Beta Pictoris b was found to have a day that lasts only 8.1 hours. This is much quicker than any known planet – its equator is moving at almost 100,000 kilometres per hour. For comparison, Jupiter rotates at 45,300 km/h and Earth rotates at just 1,674 km/h. This new result extends the relation between mass and rotation seen in the Solar System to exoplanets. Similar techniques will allow astronomers to map exoplanets in detail in the future using the European Extremely Large Telescope (E-ELT). Beta Pictoris b orbits the naked-eye star Beta Pictoris, which lies about 63 light-years away in the southern constellation of Pictor (The Painter's Easel). This planet was discovered nearly six years ago and was among the first exoplanets to be directly imaged. It orbits its host star at a distance of only eight times the Earth-Sun distance (8 AU), making it the closest exoplanet to a star ever to be directly imaged. It is 16 times larger and 3,000 times more massive than the Earth.
Using the CRIRES instrument on the Very Large Telescope (VLT), a team of Dutch astronomers from Leiden University and the Netherlands Institute for Space Research (SRON) employed a precise technique called high-dispersion spectroscopy. This was done to split light into its constituent colours – different wavelengths on 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," says lead author Ignas Snellen. "Using this technique we find that different parts of the planet's surface are moving towards or away from us at different speeds, which can only mean that the planet is rotating around its axis." The fast spin of Beta Pictoris b means that in the future, it will be possible to make a global map of the planet, and others, showing cloud patterns and large storms: "This technique can be used on a much larger sample of exoplanets with the superb resolution and sensitivity of the E-ELT and an imaging high-dispersion spectrograph. With the planned Mid-infrared E-ELT Imager and Spectrograph (METIS) we will be able to make global maps of exoplanets and characterise much smaller planets than Beta Pictoris b with this technique", says co-author, Bernhard Brandl. Beta Pictoris b is a very young planet, only about 20 million years old (compared to 4.5 billion years for Earth). Over time, it is expected to cool and shrink – making it spin even faster. On the other hand, other processes might be at play that change the spin of the planet. For instance, the spin of the Earth is slowing down over time due to the tidal interactions with our Moon.
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