Astronomers have for the first time managed to detect the rotation of an extrasolar planet, by analysing the way its atmosphere filters light. This technique could also provide clues about planet formation.
Ignas Snellen and his colleagues at Leiden University in the Netherlands report in Nature1 that a gaseous planet orbiting the star β Pictoris rotates at 25 kilometres per second at its equator — faster than any planet in the Solar System and about 50 times faster than Earth. A day on the planet, called β Pictoris b, lasts just over eight hours, even though the planet has a diameter more than 16 times that of Earth's and carries more than 3,000 times Earth's heft.
Exoplanets are typically hidden by the glare of their parent star and can only be discovered indirectly, but β Pictoris b was one of the first to be found by direct imaging2. This was possible because the massive planet, at around 20 million years of age, is relatively young and still warm, so it radiates strongly at infrared wavelengths. In addition, the star system is only about 20 parsecs (65 light years) from Earth — the star β Pictoris, part of the constellation Pictor in the southern sky, is visible to the naked eye — and the planet orbits a long way away from the star, about twice Jupiter’s distance from the Sun.
Those properties, as well as the composition of the star and the planet, enabled Snellen's team to home in on the infrared light from β Pictoris b, using the Very Large Telescope in the Atacama Desert of northern Chile. “It’s exciting to see another kind of insight into the properties of extrasolar planets,” says astronomer Thayne Currie of the University of Toronto.
The researchers calculated the planet's rotation rate by measuring how the infrared light is filtered by carbon monoxide in its atmosphere.
At any given time, half the planet is rotating towards an Earth-based observer, so for that part of the planet's light the spectrum shifts towards bluer (shorter) wavelengths; meanwhile the other half rotates away from the observer, and shifts the spectrum of its light towards redder (longer) wavelengths. No telescope has enough resolution to distinguish the two sides of a planet, and light from the two sides combines into essentially a single pixel. But the blueshift and redshift together broaden the carbon-monoxide absorption line in its spectrum. From the amount of broadening, the team determined that at its equator, β Pictoris b rotates about twice as fast as Jupiter.