This artist’s view shows the planet orbiting the young star Beta Pictoris. Image Credit: ESO/L. Calcada/N. Risinger

For the first time ever, astronomers have measured the rotation rate of an exoplanet. With the help of the European Southern Observatory (ESO)’s Very Large Telescope (VLT), astronomers have determined that a day on Beta Pictoris b lasts a mere eight hours – this is much shorter than any planet in our solar system. Beta Pictoris b’s equator is moving at speeds of up to 62,137 miles per hour (100,000 kilometers per hour), showing the relationship between mass and rotation.

Approximately 63 light-years away in the constellation Pictor (the Painter’s Easel), the exoplanet Beta Pictoris b orbits its host star Beta Pictoris at a distance of only eight times the distance from Earth to the Sun. This earns Beta Pictoris b the distinction of the closest exoplanet to its host star ever imaged.

Dutch astronomers from Leiden University and the Netherlands Institute for Space Research (SRON) used the CRIRES instrument on VLT in order to determine the exoplanet’s equatorial rotation velocity. In comparison, Jupiter’s equatorial rotation velocity is 29,240 mph (47,000 km per hour) while the Earth’s is only 1,056 mph (1,700 km per hour). Beta Pictoris b is 3,000 times more massive and 16 times larger than the Earth, yet a day only lasts eight 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.

This graphic shows the rotation speeds of several of the planets in the Solar System along with the recently measured spin rate of the planet Beta Pictoris b. Image Credit: ESO/I. Snellen

Beta Pictoris b is very young, about 20 million years old, compared to the Earth’s 4.5 billion years. During its lifespan, the planet will cool and shrink, thus increasing its rate of rotation. However, there could be outside forces that affect the spin. Our own planet’s spin has been slowing over time thanks to tidal interactions with the Moon.

In order to determine the planet’s rotational signal, astronomers used a technique known as high-dispersion spectroscopy. By splitting the light into its constituent colors/different wavelengths and applying the Doppler effect, astronomers were able to detect changes in wavelengths to observe that different parts of the planet were moving at different rates and in opposite directions in relation to the observer. Astronomers then carefully removed the effects of the parent star, allowing them to extract the planet’s rotation signal.

Doppler imaging has been used by astronomers for several decades to map stellar surfaces and even the surface of Luhman 16B, a brown dwarf. Since Beta Pictoris b has a fast spin, astronomers will be able to construct a global map in the near future, even showing any potential cloud patterns and large storms.


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