Infrared astronomy forces the planet Neptune to yield a secret

Get out the eraser fluid. Even the latest astronomy books have it wrong. The best estimate they can give of the length of the day on distant Neptune is somewhere between 15 and 20 hours. But astronomers now have a precise (well, fairly precise) figure -- 18 hours give or take 24 minutes.

This measurement reported by Michael J. S. Belton, Sethanne Howard, and Lloyd Wallace of the Kitt Peak National Observatory is a minor triumph for the relatively new science of infrared (IR) astronomy -- the study of the universe by heat radiation.

Neptune, some 30 times more distant from the sun than Earth, is difficult to observe. Although astronomers have been able to estimate that the planet has 17 times the mass and four times the volume of Earth, many characteristics, such as rotation rate, are hard to pin down.

Even now, the Kitt Peak astronomers face a basic uncertainty. Their IR observations, made with the observatory's 1.3-meter telescope, can only detect movement of the planet's cloud tops. The surface itself can't be seen. Since the clouds travel at varying speeds, the astronomers can't pin down the rotation rate exactly and have to live with that 24-minute uncertainty.

There are other such elementary questions which astronomers now are trying to answer with studies that strain the limits of what can be observed from Earth. Does Neptune's orbital motion suggest the presence of a 10th planet, yet to be discovered? Does Neptune have rings as Jupiter and Uranus, as well as Saturn, now are known to possess?

This past spring and summer, several scientists have been looking for signs of such rings. The results, while negative, are tantalizing. They are sensitive enough to rule out any prominent ring system such as that of Saturn, yet not sensitive enough to detect a faint ring such as that of Jupiter.

On May 10, for example, James Elliot of the Massachussetts Institute of Technology, one of the discoverers of the Uranian rings, coordinated a netwook for six telescopes at several sites in the Northern and Southern Hemispheres as Neptune passed close to the position of a star. As in the case of Uranus, it was hoped that any ring system would show up as an unusual winking of that star as it was eclipsed by the rings. No suspicious flickers that could definitely be attributed to rings were observed. A second such observation May 24, made by Jay Elias at Cerro Tololo Observatory in Chile, was likewise negative.

Better studies can be made in 1983 when detectability will improve by a factor of two. However, it already appears that any ring system would be tenuous at best. A thin ring is very hard to detect from Earth. That of Jupiter can't be seen at all. Perhaps if Voyager 2 makes in working condition late in this decade, some of these questions can be cleared up. Meanwhile, at least the rotation rate of this mysteriuos planet is better known.