Planet lovers take note: Venus is spinning even slower than astronomers thought, according to new data from a European space probe.

In the early 1990s scientists with NASA's Magellan mission calculated that a single rotation of Venus takes 243.015 Earth days, based on the speed of surface features passing beneath the orbiting spacecraft.

But scientists now mapping Venus's surface with the European Space Agency's Venus Express orbiter were surprised to find the same features up to 12.4 miles (20 kilometers) from where they were expected to be, based on the previous measurements.

According to the new data, Venus is rotating 6.5 minutes slower than it was 16 years ago, a result that's been found to correlate with long-term radar observations taken from Earth.

"When the two maps did not align, I first thought there was a mistake in my calculations, as Magellan measured the value [of Venus's spin] very accurately," Nils Müller, a planetary scientist at the DLR German Aerospace Centre, said in a statement.

"But we have checked every possible error we could think of."

Planet Slowed by Dense Atmosphere?

One possible cause for the slowed spin is friction caused by Venus' thick atmosphere and high-speed winds. The motion of the atmosphere on Earth, for example, has been observed to affect the planet's rotation rate, albeit to a much smaller degree.

Thanks to a heavy blanket of carbon dioxide-laden air, the surface pressure on Venus is 90 times what we experience on Earth at sea level, and opaque clouds of caustic sulphuric acid constantly whip around the planet at hurricane speeds.

Still, "it is difficult to find a mechanism that will cause the average rotation rate to change this much in only 16 years," Venus Express project scientist Håkan Svedhem told National Geographic News.

"The origin of this could lay in the solar cycle or in long-term weather patterns that modify the atmospheric dynamics. But this puzzle is not yet solved."

Some reports cite an exchange of angular momentum between Venus and Earth as a possible cause for the variation. A moon, for example, can cause a planet to rotate slower than expected, because both objects share angular momentum.

But with 23.6 million miles (38 million kilometers) between the two planets at their closest approach, "there is no exchange of momentum between Venus and the Earth," Svedhem stated.

Instead, he said, further study is needed to discern the cause—or causes—of Venus' slight reduction in speed.