Beta Pictoris b, a very young gas giant that orbits the star Beta Pictoris just 63 light-years away in the southern constellation of Pictor, has been found to have a day that lasts only 8 hours.
This is an artist’s impression of the young exoplanet Beta Pictoris b and its parent star. Image credit: ESO / L. Calçada / N. Risinger, skysurvey.org.
Discovered about 6 years ago, Beta Pictoris b orbits its host star at a distance of only 8 times the Earth-Sun distance.
The exoplanet is about 20 million years old, more than 16 times larger and 3,000 times more massive than our planet.
Astronomers using ESO’s Very Large Telescope have now found that the equatorial rotation speed of Beta Pictoris b is almost 100,000 km per hour. This is much quicker than any planet in our Solar System and corresponds to an 8-hour day.
By comparison, the rotation speed of Jupiter’s equatorial atmosphere is about 47,000 km per hour, while the Earth’s travels at 1674.4 km per hour.
Over time, Beta Pictoris b is expected to cool and shrink, which will make it spin even faster. On the other hand, other processes might be at play that change its spin. For instance, the spin of the Earth is slowing down over time due to the tidal interactions with our Moon.
The astronomers used high-dispersion spectroscopy to split Beta Pictoris light into its constituent colors – different wavelengths in the spectrum.
The principle of the Doppler effect allowed them to use the change in wavelength to detect that different parts of the exoplanet 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. 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,” said Dr Ignas Snellen of Leiden University, the Netherlands, who is the lead author of the paper published in the journal Nature (full paper in .pdf).
This technique is closely related to Doppler imaging, which has been used for several decades to map the surfaces of stars, and recently that of a brown dwarf – Luhman 16B.
The fast spin of Beta Pictoris b means that in the future it will be possible to make a global map of the exoplanet, showing possible cloud patterns and large storms.
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Ignas A. G. Snellen et al. The fast spin-rotation of a young extra-solar planet. Nature, published online April 30, 2014;
