First Exoplanet Rotation Clocks In At 8 Hour Day

By James Anderson •  Updated: 05/02/14 •  4 min read

The rotation rate of an exoplanet has been determined, for the first time ever, by observations made from the European Southern Observatory (ESO).

Beta Pictoris b’s day lasts only eight hours, much quicker than any planet in the Solar System.

The exoplanet’s equator is moving at almost 100,000 kilometres per hour. This result applies the relation between mass and rotation seen in our Solar System to exoplanets.

Beta Pictoris

Beta Pictoris b, also known as HD 39060, SAO 234134 and HIP 27321, orbits around the star Beta Pictoris, about 63 light-years from Earth in the southern constellation of Pictor.

The planet, discovered almost six years ago, 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 (eso1024), making it the closest exoplanet to its star ever to be directly imaged.

The observations took advantage of the adaptive optics technique, compensating for Earth’s atmospheric turbulence which may distort images. It allows astronomers to create super-sharp images, almost as clear as those that could be seen from space.

Very Large Telescope

Using the ESO’s Very Large Telescope, astronomers from Leiden University and the Netherlands Institute for Space Research found that the equatorial rotation velocity of exoplanet Beta Pictoris b is almost 100,000 kilometres per hour.

In 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, 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,”

study co-author Remco de Kok said.

Conservation of Angular Momentum

Only about 20 million years old, Beta Pictoris b is an extremely young planet, compared to Earths’ age of 4.5 billion years. This exoplanet is likely to cool and grow smaller over time, making it spin even faster.

This is a result of conservation of angular momentum, the same effect that makes a spinning ice skater turn more rapidly when they bring their arms closer to their body.

Conversely, other processes could be in effect that change the spin of the planet. For example, the spin of Earth is slowing down over time due to the tidal interactions with our Moon.

“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 lead author Ignas Snellen.

High-dispersion Spectroscopy

The astronomers used an exacting technique known as high-dispersion spectroscopy to split light into its constituent colours, the various 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.

The technique is closely related to Doppler imaging, in use for several decades for mapping the surfaces of stars, and more recently that of a brown dwarf star, 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 planet, showing possible cloud patterns and large storms.

Reference: Ignas A. G. Snellen, Bernhard R. Brandl, Remco J. de Kok, Matteo Brogi, Jayne Birkby, Henriette Schwarz. Fast spin of the young extrasolar planet β Pictoris b. Nature, 2014; 509 (7498): 63 DOI: 10.1038/nature13253

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