Astronomers Celebrate Pluto's 75th Birthday

February 18, 2005 | In 1929 Lowell Observatory's director, Vesto M. Slipher, tasked a young Clyde William Tombaugh to search for "what else is out there beyond Neptune." Tombaugh was supposed to find Percival Lowell's predicted Planet X. One year later, on February 18, 1930, he spotted Pluto.

The status of the ninth planet has come under heavy debate in recent years. Astronomers now realize that Tombaugh's find is the largest known member of an entire class of objects known as the Kuiper Belt. These ice-rock bodies, circling the Sun beyond the orbit of Neptune, have helped astronomers better understand the formation of our solar system and other extrasolar planetary systems. But Ceres, the largest asteroid in the asteroid belt, isn't classified as a planet, so why should the largest Kuiper Belt object have such an honorable distinction? Pluto's status remains a hot topic despite the International Astronomical Union's ruling in favor of its current classification as "planet." In December 1994, Tombaugh wrote a letter to the editor published in Sky & Telescope where he explained his opinion on the matter:

"Pluto started out as the ninth planet, a supported fulfillment of Percival Lowell's prediction of Planet X. Let's simply retain Pluto as the ninth major planet. After all, there is no Planet X. For 14 years, I combed two-thirds of the entire sky down to 17th magnitude, and no more planets showed up. I did the job thoroughly and correctly . . . Pluto was your last chance for a major planet."

Winds on Titan

February 17, 2005 | Due to a programming error between the European Space Agency's Huygens probe and NASA's Cassini orbiter, some of the former's observations were thought to be lost forever. One of the missing measurements was the Doppler Wind Experiment, designed to profile Titan's atmospheric wind speeds as Huygens floated toward the moon's surface.

Fortunately, a worldwide armada of radio telescopes also listened for Huygens's signal. From those observations scientists have learned that the wind speed on Saturn's largest moon is weak at the surface but increases slowly to about 60 km. It peaks at 430 kilometers (270 miles) per hour in a zone located about 120 km high.


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