The distant planets Uranus and Neptune spin in space at about the same rate the earth does, which means their days are about as long as earth days and they were probably formed out of the same debris the earth was.
"Their spin rate suggests they are solid ice-like bodies and not big balls of gas," said Dr. Michael J. S. Belton of Arizona's Kitt Peak National Observatory, whose 158-inch telescope was used to measure the spin rates. "This also suggests Uranus and Neptune are more like earth and Mars than Jupiter and Saturn, which is surprising since they're so much closer to Jupiter and Saturn."
For more than 40 years, scientists have thought that Uranus and Neptune had similar spin rates to those of Jupiter and Saturn. Astronomers have long thought Uranus spun once on its axis every 10.8 hours to turn once on its axis.
Saturn and Jupiter are the fastest rotating planets in the solar system. Saturn turns once every 10 hours and 38 minutes while Jupiter rotates once every 9 hours and 55 minutes.
Fresh measurements by Kitt Peak astromers Belton and Sethanne Hayes show that Uranus turns once every 25 hours and Neptune every 22 hours. Both planets have "days" that are about as long as a day on earth is.
"This finding is startling to me because we got so used to what we thought were hard facts, that the rotations of all the outer planets were the same," Dr. Belton said. "Now it seems that Uranus and Neptune are distinctly different from Jupiter and Saturn."
The Kitt Peak astromers observed Uranus for two nights and Neptune for four nights last spring, using thwo world's second largest opitical telescope and a new spectographic technique of measuring a planet's rotation.
Uranus is 1.8 billion miles from the sun and Neptune is 2.8 billion miles from the sun, which makes them the most outermost planets except for Pluto.
Using a smaller telescope at Kitt Peak, a separate four-member scientific team said it observed what it believes are the first signs of weather patterns in the upper atmosphere of Neptune.
According to Kitt Peak's Dr. Richard Joyce, Neptune's reflection increased markedly from April, 1975, to March, 1976, the time that Joyce and three University of Hawaii scientists observed Neptune with the 84-inch telescope at Kitt Peak. The increased reflection suggests the presence of thin, transient clouds high in Neptune's atmosphere.
Astromers have long assumed that Neptune underwent no atmospheric changes and have therefore have used the distant planet as a standard measure for brightness in the entire solar system.