The Crowded Universe by Boss Alan

Author:Boss Alan [Alan, Boss]
Language: eng
Format: epub
ISBN: 9780786743674
Publisher: Avalon Publishing Group
Published: 2009-02-05T00:00:00+00:00

FIGURE 25. Pluto and its satellites Charon, S/2005 P1, and S/2005 P2, as observed by the Hubble Space Telescope. [Courtesy of NASA, ESA, Harold Weaver (Johns Hopkins University, Applied Physics Laboratory), Alan Stern (SwRI), and the HST Pluto companion search team.]

But if 2003 UB313 was larger than Pluto, was it a planet too? If so, it would be up to the IAU to decide, because the IAU has the ultimate responsibility for categorizing and naming celestial bodies. If the IAU agreed, Michael Brown, Chad Trujillo, and David Rabinowitz had just found the tenth planet. Whereas Tombaugh’s planet had been named Pluto, after the mythological god of the underworld, Brown had playfully given 2003 UB313 the name Xena, after the warrior princess on a popular television series. Soon after the initial discovery, astronomers realized that Xena had a satellite, and Brown named the satellite Gabrielle, for Xena’s television sidekick.

The IAU had been wrestling with the issue of planethood well before Xena made her appearance on stage. A Working Group on the Definition of a Planet had been created in June 2004 to deal with the problem of Pluto’s planethood. It had become clear that Pluto was the largest known member of the swarm of comets in the Kuiper Belt, a region named after the same Gerard Kuiper who had first suggested gravitational instability as a means for forming gas giant planets. Many Kuiper Belt objects have orbits similar to that of Pluto, and they had been found by the hundreds, making the situation similar to that of Ceres in the asteroid belt. Ceres was discovered in 1801 by the Sicilian astronomer Giuseppe Piazzi. At first it appeared that Ceres was the long-sought “missing planet” between Mars and Jupiter, but then another object with a similar orbit, Pallas, was found in 1802; a third, Juno, in 1804; and a fourth, Vesta, in 1807. By this time it was clear that Ceres was just one of a number of objects orbiting between Mars and Jupiter, and these became known as the minor planets in recognition of their diminutive stature compared to the eight major planets. We now know that the asteroid belt is populated by many hundreds of thousands of bodies large enough to be detected from Earth.

The same fate had befallen Pluto. It was merely the first—and so far the largest, until Xena—body detected in the Kuiper Belt. By the same reasoning that had led to the demise of Ceres’s claims to planethood in the early 1800s, it seemed that Pluto deserved a similar demotion.

The IAU Working Group on Extrasolar Planets had agreed in 2001 on a “working definition” of a planet that could be applied to extrasolar planetary systems. However, we were focused on the upper-mass end: How massive could an object in orbit around a star be and still be called a planet? What if a Jupiter-mass object were not in orbit around a star? Would it still be a planet? Our main debate had centered on whether planets should be defined by their formation mechanism or by their ability to undergo nuclear fusion.

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