Day On Exoplanet Measured, Found To Be Much Shorter Than Earth’s

By Shweta Iyer on April 30, 2014 4:59 PM EDT

planet beta pictoris b
This artist's view shows the planet orbiting the young star Beta Pictoris. This exoplanet is the first to have its rotation rate measured. Its eight-hour day corresponds to an equatorial rotation speed of 100,000 km/hr. (Photo: SO L. Calçada/N. Risinger)

How would a day in a planet outside our solar system be? Would it be a 24-hour day like our's or shorter? These questions were answered recently when Dutch astronomers from Leiden University and the Netherlands Institute for Space Research (SRON) found the equatorial rotation velocity of an exoplanet, according to a press release Wednesday.

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The planet is question is Exoplanet Beta Pictoris B, discovered six years back. It orbits the star Beta Pictoris also known as HD 39060, SAO 234134, and HIP 27321, which lies about 63 light-years from Earth in the southern constellation of Pictor (The Painter's Easel), and is visible to the naked eye. Exoplanet Beta Pictoris b was the first exoplanet to be directly imaged.

The Dutch team used the CRIRES instrument on the VLT. With this high-resolution IR Echelle Spectrometer, the astonomers made use of adaptive optics technique, which allowed them to get clear images without the usual distortions that would occur due to the Earth's atmospheric turbulence.

The astronomers found that the equatorial rotation velocity of exoplanet Beta Pictoris B is almost 100, 000 kilometres per hour. Even though it has a mass between 4 and 11 Jupiter masses and a radius around 65 percent larger than Jupiter's it rotates at double the velocity. Jupiter's equator has a velocity of about 47, 000 km per hour, while the Earth's is 1700 km per hour. In comparison to the Earth, Beta Pictoris B is more than 16 times larger and 3000 times more massive, yet a day on the planet only lasts 8 hours.

"It is not known why some planets spin fast and others more slowly," says 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 relatively young planet at 20 million years old compared to the Earth, which is 4.5 billion years old. According to astronomers the planet will spin faster when it cools over time. But the spin may be influenced by other factors like the presence of a moon. For instance the spin of the Earth is slowing down over time due to the tidal interactions with our Moon.

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," says lead author Ignas Snellen. "Using this technique we find that different parts of the planet's surfaceare moving towards or away from us at different speeds, which can only mean that the planet is rotating around its axis"

This technique is very similar to Doppler imaging, which has been a time-tested technique to map the surface of stars, and recently that of a brown dwarf star called Luhman 16B. Astronomers are optimistic that the fast spin of Beta Pictoris b will allow them to make a global map of the planet, showing possible cloud patterns and large storms.

METIS principal investigator and co-author of the new paper, Bernhard Brandl, says, "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."

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