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Starwatch NewsletterMinnesota Starwatch is a newsletter describing the night sky in the Midwest.
It is updated monthly, and is produced by the |
Minnesota Starwatch for September 2006
This September will be a sparse month for viewing the planets. Jupiter will be setting early but Uranus and Neptune can be seen in the southern sky with binoculars. If you know where to look, you might even find Pluto with a telescope. Ah, but is Pluto still considered a planet? Recently the International Astronomical Union recommended that Astronomers finally get around to defining what it means to be a planet. We all learned about the nine planets, which included Pluto, when we were growing up, but there was never any formal definition of just what made a planet. With the discovery of many large icy worlds beyond the orbit of Neptune, the IAU decided it was time settle the issue before we had dozens, perhaps hundreds, of new objects that could be considered planets.
The IAU recommended that in order to be a planet, an object had to orbit a star, not another planet, and be massive enough for gravity to make it round (or nearly so). This new definition was to include the eight classical planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), a new class of objects called Plutons (Pluto, Charon and 'Xena'), and the largest asteroid, Ceres. This proposal was rejected by the main body of the IAU and Pluto was demoted to the status of 'Dwarf Planet' along with Ceres. The distinction is made between objects that are massive enough to be round, but are part of a swarm of similar objects that orbit the Sun in the same vicinity. Thus, poor Pluto is now classified as one of the many Trans-Neptuinan objects, no longer a full fledged planet.
So, has Pluto been demoted? Well, a little bit, but it is still considered a planet.
The Minnesota Starwatch is produced by the University of Minnesota Astronomy Department.