May 2011

Tony Martinez

Each month I will describe sights of interest in the night skies of South Carolina. These sights will be broken down into three sections; what you can see with the naked eye, with binoculars, and with a small telescope. The best time to view the night sky is at and around the times when the Moon is not visible, what is known as a New Moon; which will occur this month on May 3rd. For May, your best viewing nights will be from May 1st through May 9th and May 20th through May 31st. The Star chart below is set for Florence, SC on May 15th at 9 pm.

Pluto and Friends:

A 10-year-old girl named Jillian recently asked me why Pluto is no longer a planet. It was a good question, and I gave Jillian the short answer. I then decided to use this column to give the long answer.

By the late 1800’s, astronomers felt that there was another planet out beyond Neptune that was causing some disturbances to the orbit of Uranus. This “Planet X” was finally discovered in 1930, and called Pluto. Later information proved that Pluto was too small to have any effect on Uranus, and thus the whole Planet X/Uranus theory was disproven. Therefore, it was only by accident that Pluto was discovered and not because of theoretical calculations. Below is an image of the actual photos they used in 1930 when they discovered Pluto.

So for seventy-six years, our solar system was listed as having nine planets. Then in 2006, Pluto was removed from planetary status; why? The short answer is that as our telescopes improved, we began to locate other “small planets” beyond Pluto: Quaor is half the size of Pluto, Sedna is almost the size of Pluto and Eris is equal or larger then Pluto. Therefore, the number of planets in our solar system could possibly increase yearly. It was much easier to demote Pluto and the new “Pluto-like” objects to dwarf planet status then to continually increase our solar system planet count. This approach would then leave us with eight permanent planets, with Mercury being the smallest at twice the size of Pluto.

Below is an image of some of the major dwarf planet compared in size to the Earth. Note that Pluto has now been found to have three moons!

Now astronomers knew that they could not just arbitrarily remove Pluto as a planet. Instead, they set up a list of three rules that would be used to describe a planet.

1. Must be in orbit around the Sun

2. Must be large enough to be a sphere due to its own gravitational force.

3. Must have cleared the neighborhood around its orbit.

All of the newly assigned dwarf planets, including the asteroid Ceres fit the first two rules. However, neither Pluto nor any of the other mentioned dwarf planets fit the third rule. As our telescopes improved we were able to detect thousands of small objects orbiting the Sun in the region of Pluto and beyond. Now if you remember the layout of our solar system, each planet circles the Sun all alone in its orbit except for the tiny meteors we run into each day. Soon after the discovery of Pluto, some astronomers actually thought that there might be an area beyond Neptune, possibly including Pluto that contained millions of small asteroid type objects forming a “halo” around the outer planets. Gerald Kuiper studied this theory in the 1950’s, and it became known as the Kuiper Belt. It wasn’t until the 1990’s that the Kuiper Belt was found, and understood. The Kuiper Belt starts beyond Neptune, and extends outward for over 2 billion miles. Like the asteroid belt between Mars and Jupiter, the Kuiper Belt contains leftover material/debris from the formation of our solar system. However, the asteroid belt contains primarily rocky or metallic objects, while the Kuiper Belt contains mainly frozen gases and water objects. Pluto and some of our newly found dwarf planets are located in this belt. Therefore, Pluto has not cleared the neighborhood of its orbit.

Note: the Kuiper Belt is sometimes confused with the Ort Cloud. The Ort Cloud can be thought of as the source for most of our comets, and the Ort Cloud is a halo extending 1000 times farther out from the Sun beyond the Kuiper Belt!

The following images should help to explain the origin of the Kuiper Belt. After our Sun formed, it was surrounded by a disk of billions of pieces of debris that were pulled into the Sun’s gravitational neighborhood. This is called an accretion disk, and looked somewhat like the below image #1. The larger objects in the disk pulled close by smaller objects into themselves, and then became much larger objects finally forming our planets; image #2 below.

Much like a rolling snowball, the larger the planet grew, the more debris it pulled into itself, see image #3 below. Over a period of billions of years, this “gravitational vacuuming” cleared away all of the objects in the paths of the planets, as is seen in image #4. Note: the four planets shown in these images represent Jupiter, Saturn, Uranus and Neptune.

Beyond the orbit of Neptune, there were never any sufficiently large objects to initiate this planet forming and vacuuming, so all that remains is a debris field, the Kuiper Belt, with the largest object being the newly classified dwarf planet Pluto.

Naked Eye Sights: The best grouping of planets this year will occur on the eastern horizon on May 11th, just before sunrise. Four of the five visible planets will be grouped together; Mercury, Mars, Venus, and Jupiter.

Binocular Sights (7 to 10 power): On any month that there are not any special sights, use this time to find more Messier Objects.

Telescope Sights (60-100mm): Spend your time observing the planet Saturn, which is moving higher into the night sky this month.

See you next month!

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