Astronomers Measure Exoplanet’s Day For The First Time

ESO L. Calçada/N. Risinger

Astronomers are able to determine the length of the orbit of an exoplanet based on when it transits their parent star, but because of the distance, it has been much more challenging to determine how long it takes a planet to rotate once and have a day. A new study using the ESO’s Very Large Telescope (VLT) has yielded the first results of the length of an exoplanet day. The research was conducted by a team of researchers led by Bernhard Brandl and the results will be published in Nature.

Beta Pictoris b is a 'hot Jupiter' exoplanet about 63 light years away from Earth in the constellation Pictor. If this planet sounds familiar, it’s because it was directly imaged in 2008 and was one of the first. Scientists were able to determine that the equator rotates at nearly 100,000 km/h (compared to Earth’s 1700 km/h). Beta Pictoris b is 16 times larger than Earth, which equates to a day that lasts only 8 hours.

In order to determine how fast the planet spins, the researchers were able to use a very sophisticated instrument that acted like a prism and broke the visible light down into its components. The Doppler effect then allowed them to analyze the changes in the light wavelengths. Not all parts of the planet were seen moving at the same speed, which confirmed that the planet was spinning on an axis.

Beta Pictoris b was formed a mere 20 million years ago (around the same time Earth was in the early Miocene and modern birds were present) so it is still undergoing structural changes. It is expected that the planet will shrink as it cools, and will have days that are even shorter. This is assuming present conditions persist and there are no outside forces acting on the planet, such as tidal forces from a satellite.

“It is not known why some planets spin fast and others more slowly,” co-author Remco de Kok said in a press release, “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.”

The method by which the rotation speed was calculated is very similar to one routinely used to analyze the surface of stars. Because of Beta Pictoris b’s incredibly fast rotation, it is hoped that they will be able to map the surface of the planet to learn more about its weather. 

Additionally, astronomers hope to duplicate the technique with other exoplanets. Advances in technology, like the Mid-infrared E-ELT Imager and Spectrograph expected to be put into use next year, will also allow them to study the rotation speed of exoplanets that are much smaller, perhaps closer to the size of Earth.

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