Neptune's Spin

By tracking atmospheric features on Neptune, a UA planetary scientist has accurately determined the planet’s rotation, a feat that had not been previously achieved for any of the gas planets in our solar system except Jupiter.

A day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds, according to the first accurate measurement of its rotational period made by University of Arizona planetary scientist Erich Karkoschka.

His result is one of the largest improvements in determining the rotational period of a gas planet in almost 350 years since Italian astronomer Giovanni Cassini made the first observations of Jupiter’s Red Spot.



“The rotational period of a planet is one of its fundamental properties,” said Karkoschka, a senior staff scientist at the UA’s Lunar and Planetary Laboratory. “Neptune has two features observable with the Hubble Space Telescope that seem to track the interior rotation of the planet. Nothing similar has been seen before on any of the four giant planets.”

The discovery is published in Icarus, the official scientific publication of the Division for Planetary Sciences of the American Astronomical Society.

Unlike the rocky planets – Mercury, Venus, Earth and Mars – which behave like solid balls spinning in a rather straightforward manner, the giant gas planets – Jupiter, Saturn, Uranus and Neptune – rotate more like giant blobs of liquid. Since they are believed to consist of mainly ice and gas around a relatively small solid core, their rotation involves a lot of sloshing, swirling and roiling, which has made it difficult for astronomers to get an accurate grip on exactly how fast they spin around.

Neptune 3



“If you looked at Earth from space, you’d see mountains and other features on the ground rotating with great regularity, but if you looked at the clouds, they wouldn’t because the winds change all the time,” Karkoschka explained. “If you look at the giant planets, you don’t see a surface, just a thick cloudy atmosphere.”

“On Neptune, all you see is moving clouds and features in the planet’s atmosphere. Some move faster, some move slower, some accelerate, but you really don’t know what the rotational period is, if there even is some solid inner core that is rotating.”

In the 1950s, when astronomers built the first radio telescopes, they discovered that Jupiter sends out pulsating radio beams, like a lighthouse in space. Those signals originate from a magnetic field generated by the rotation of the planet’s inner core.

No clues about the rotation of the other gas giants, however, were available because any radio signals they may emit are being swept out into space by the solar wind and never reach Earth.



“The only way to measure radio waves is to send spacecraft to those planets,” Karkoschka said. “When Voyager 1 and 2 flew past Saturn, they found radio signals and clocked them at exactly 10.66 hours, and they found radio signals for Uranus and Neptune as well. So based on those radio signals, we thought we knew the rotation periods of those planets.”

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