Exoplanet spin measured for first time

Eight-hour day on Beta Pictoris b is shorter than day of any planet in our solar system

AS THE WORLD TURNS  On Beta Pictoris b (upper right, in an artist’s illustration), a day is just over eight hours long. The exoplanet spins almost twice as fast as Jupiter does (when measured at the equator). The exoplanet’s spin continues a trend seen in our solar system: More massive planets spin faster. 

I. Snellen/Leiden Observatory, adapted by S. Egts

Astronomers have measured the rotation of an exoplanet for the first time. A day on the planet Beta Pictoris b is roughly eight hours long — shorter than on any planet in our solar system, the researchers report in the May 1 Nature.

Beta Pictoris b orbits a young star 63 light-years away in the constellation Pictor. To measure the planet’s rotation, Ignas Snellen, an astronomer at Leiden Observatory in the Netherlands, and colleagues analyzed the planet’s spectrum. The spectrum reveals certain wavelengths of light that are absorbed by carbon monoxide in the planet’s atmosphere. By measuring how much the absorbed wavelengths are Doppler shifted by the rotating atmosphere, the researchers determined how quickly the planet spins.

In our solar system, more massive planets spin faster. Beta Pictoris b, which is about 11 times as massive as Jupiter and roughly 1.65 times as wide, continues that trend, with a spin of 25 kilometers per second. But Snellen’s team expects that the young exoplanet — a mere 21 million years old — will pick up even more speed as it ages. The planet will contract and spin faster, much like figure skaters who pull in their arms to twirl more rapidly.

headshot of Associate News Editor Christopher Crockett

Christopher Crockett is an Associate News Editor. He was formerly the astronomy writer from 2014 to 2017, and he has a Ph.D. in astronomy from the University of California, Los Angeles.

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