On Jan. 1, 1801, astronomer Giuseppe Piazzi discovered an object in orbit around the sun between Mars and Jupiter in a location where other astronomers predicted there should be a planet. Piazzi at first thought he had found a comet, but astronomers quickly decided it was the predicted missing planet. They named it Ceres.
For half a century, textbooks listed it as a planet. With the subsequent discovery of similar objects in the same part of the solar system, astronomers decided that Ceres no longer deserved the “planet” designation and demoted it to asteroid status. It's still the largest known of that class of objects.
Jump ahead to the early 20th century. Astronomers once again predicted a Planet X, this time beyond Neptune, because there seemed to be a source of gravity affecting the orbits of other planets. Clyde Tombaugh, a young amateur astronomer at Lowell Observatory, was assigned the task of seeking the proposed trans-Neptunian planet. He was rewarded after many nights and days of hard work with the discovery of Pluto on Feb. 18, 1930.
At first, astronomers studying the new planet calculated its mass to be roughly equivalent to Earth's. But subsequent observations with better equipment eventually reduced Pluto's size to smaller than our moon. Like Ceres before it, some astronomers began to doubt the correctness of calling Pluto a planet. In 2005, astronomers at Palomar Observatory discovered Eris, another object in the same region of the solar system as Pluto, the first of several such discoveries. Even larger than Pluto, Eris sparked anew the debate in the astronomical community about what defines a planet. In August 2006, the International Astronomical Union defined “planet” in such a way that removed Pluto from that classification. Pluto became the first of a new class of objects called dwarf planets.
NASA launched the New Horizons space craft, one of the fastest-moving crafts of any kind, in the same year Pluto was demoted to explore that object, which has gone from Planet X to planet to dwarf planet. I suspect that if Tombaugh, whom I met on several occasions, were still alive, he'd have some really mixed feelings about all of this.
For the next few days, Pluto floats almost directly below, and 2,063,524,000 miles away, from brilliant Venus, now the Morning Star, in the sun's dawn twilight. You won't see it without a large telescope and darker skies. It's 5 degrees below Venus, about the width of your fist held at arm's length.
When New Horizons starts sending back data and images of Pluto, we'll tell you about it, but in the meantime, come by and see “Tonight's Sky” daily in the Kirkpatrick Planetarium Star Theater. Learn what's up each night. Visit our website at sciencemuseumok.org or call 602-3761 for information.
The Oklahoma City Astronomy Club meets at the museum on Feb. 14, Valentine's Day. Give your sweetheart the universe!
Planet visibility report: Of all the planets, Jupiter rules the night sky in February. Up at sunset in the eastern sky, it rises higher each day. Mercury makes a very brief appearance in the western sky after sunset the first week of the month, but never really climbs out of the twilight glow. Mars rises around 11 p.m. followed two hours later by Saturn, and Venus dominates the morning sky as the brilliant Morning Star. Full moon occurs Feb. 14, and in this the shortest month, there is no new moon.
Wayne Harris-Wyrick is director of the Kirkpatrick Planetarium at Science Museum Oklahoma. Questions or comments may be emailed to firstname.lastname@example.org.