"How many planets are there?" Once upon a time that was a pretty simple question to answer. Nine. But near the end of July, scientists announced discovery of an object in the outer Solar System that's at least as big as Pluto and probably even larger. So does that bring the number of solar system planets to ten? A lot of people believe so.
The object in question only has a number for a name at this time. It's called 2003 UB313. Eventually, it will be granted a somewhat more poetic name by the International Astronomical Union (IAU). The IAU takes the responsibility for classifying and naming celestial objects and their features. 2003 UB313 (we'll call it "UB313" for short) lies about 10 billion miles away from the Sun and is just about as far out as it gets in its orbit. It takes 560 years to travel around the Sun just once, and its closest approach will put it 3.5 billion miles away from the Sun. By contrast, Pluto's average distance from the Sun is about 3.6 billion miles.
Since the distance to the object is known, astronomers can make some guesses about its size. If we assumed, for the moment, that UB313 reflects 100% of the sunlight that hits it, we could estimate that this world is about the same size as Pluto. However, no celestial body reflects light nearly that well. So to appear as bright as it does, it logically must be larger than Pluto.
Whether that means this is Planet Number Ten or not is still a matter of opinion. The problem, as some people have recently pointed out, is that the word "planet" is more of a cultural term than a scientific one. The word planet comes from the ancient Greek planetae, meaning "wanderers." Back then all anyone knew about the planets was that they behaved strangely, wandering against the background stars over time. Nobody knew what they were made of, what their shape was, how far away they were, or that we happen to be living on one ourselves.
Fast-forward to today, a time when we know so much about the Solar System. Compare giant Jupiter with huge swirling weather systems, dozens of moons and a faint ring, with puny Mercury, a little round lump of cratered rock. Thanks to ancient history, these wildly different objects get the very same label: planet. The definition of the word is just very loose.
The IAU has recently made a statement that they plan to reconsider the definition of the word "planet," to be based on the object's size. Until then they will not give UB313 a formal name. If they do decide it's to be called a planet, they will follow tradition and name it after a Greek or Roman god.
Giant Jupiter has slipped into the glare of the Sun and will be hidden from view until it reappears in the morning skies of November. Meanwhile, bright Venus remains low in the west just after sunset and will stay that way through the end of the year.
This is a good time to start watching the red planet Mars. Earth is catching up with Mars in its orbit, on the way to a close encounter in late October and early November. As the two planets approach, Mars gets brighter and brighter.
At the beginning of the month, Mars rises late, after 10:15 pm. Every night it rises a little earlier, and you'll be able to see it rise before 8:45 pm by the end of the month. (You'll need to wait an hour or two after it rises if you have trees or buildings along your local horizon.)
Be sure to watch our star parties schedule for upcoming opportunities for you to get a close up look at the red planet!
Interested in astronomy? Want to see Mars close up? Want to check out different telescopes? Ever wonder what a star party is like? Come on out to the Tennessee Star Party (TNSP) October 7-9 at beautiful Camp Nakanawah, located near Crossville, Tennessee.
The Barnard-Seyfert Astronomical Society (BSAS) sponsors TNSP every year, attracting amateur astronomers from middle Tennessee and beyond. This year features half a dozen distinguished speakers plus stargazing every night! Camp Nakanawah offers rustic but comfortable cabins, good meals, and dark skies well away from Nashville's light pollution. The event is inexpensive (basic admission is only $20) and a great way to meet other people interested in astronomy.
For more information about TNSP, visit the BSAS web site!
BSAS meets the third Thursday of every month at 7:30 pm at Adventure Science Center. September features Mark Manner on digital astrophotograpy. October will be William Keel from U of AL on relativity. In November, Jeff Moersch from UT-Knoxville will talk about Martian geology and the incredible Mars Exploration Rovers.
