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Eight-Hour Shift: Exoplanet's Day Gets Measured for the First Time

For the first time, astronomers have measured the length of a day on a planet beyond our own solar system, and they say it lasts just eight hours.
Image: Beta Pictoris b
This artist's view shows Beta Pictoris b orbiting its young parent star.L. Calcada / N. Risinger / ESO /

For the first time, astronomers have measured the length of a day on a planet beyond our own solar system, and it's a fast one. They say one day on Beta Pictoris b takes just eight hours, due to a rotation rate that's more than 50 times faster than Earth's.

The estimate is based on detailed analysis of the light emitted by the giant planet, which lies about 63 light-years away in the southern constellation Pictor.

Using the CRIRES instrument on the European Southern Observatory's Very Large Telescope in Chile, Dutch astronomers were able to characterize shifts in the spectrum of that light so precisely that they could calculate velocities for different patches of the giant planet's surface.

"We find that different parts of the planet's surface are moving toward or away from us at different speeds, which can only mean that the planet is rotating around its axis," Ignas Snellen, an astronomer at Leiden Observatory who's the lead author of a paper on the observations published by the journal Nature, said Wednesday in a news release.

The results show that Beta Pictoris b's equator is spinning at about 56,000 mph (90,000 kilometers per hour). That's significantly faster than the equivalent rate for Earth (1,040 mph or 1,674 kph) and Jupiter (29,000 mph, or 47,000 kph).

"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," said Remco de Kok, an astronomer at the Netherlands Institute for Space Research and one of the Nature paper's co-authors. "This must be some universal consequence of the way planets form."

Beta Pictoris b was identified in 2008, and it's one of the first exoplanets to be directly imaged. It's around seven times as massive as Jupiter, and orbits its host star about eight times farther away than Earth orbits the sun. One year on Beta Pictoris b lasts somewhere between 17 and 21 Earth years. The planet is thought to be relatively young — no more than 20 million years old, compared with the 4.5 billion-year age for our own solar system.

Astronomers expect Beta Pictoris b to cool and shrink as it ages, which would typically cause it to spin even faster. However, other factors may come into play.

Billions of years ago, Earth is thought to have had days lasting only a few hours — but tidal interactions with the moon gradually slowed down our planet's rotation rate to produce the current 24-hour day. That slowdown will continue for millions of years to come.

In a Nature commentary, University of Arizona astronomer Travis Barman said the case of Beta Pictoris b "paves the way for measuring the spins of a large sample of exoplanets."

Beta Pictoris b was a relatively easy target for direct imaging because it shines so brightly, but future telescopes — such as the 128-foot (39-meter) European Extremely Large Telescope — may make it possible to create global maps of exoplanets. Last year, astronomers released the first rough map of an exoplanet's clouds, based on data from NASA's Kepler space telescope.

In addition to Snellen and de Kok, the authors of "Fast Spin of the Young Extrasolar Planet Beta Pictoris B" include Bernhard Brandl, Matteo Brogi, Jayne Birkby and Henriette Schwarz.