The 12 planets in our solar system

Save our skies

Be a considerate neighbor: Reduce nighttime glare. Shield all your outside lights downward (or turn them off completely) and enjoy the beautiful, starry night sky.

Night Sky September 2006

—CHARLIE CHRISTMANN

A GROWING FAMILY
With the imminent addition of several more “planets” to our night sky, it seems that every science text book in the schools will have to be revised, models will have to be reworked, and two aging spacecraft will have to be hauled back for revision.

With the International Astronomical Union about to finally define formally what a planet is, there could be an immediate addition of three planets to our solar system, which would then include Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313. Several more objects will possibly meet the new definition and could continue to expand the number of planets.

The word “planet” comes from Greek, meaning “wanderer.” These were stars that moved across the night sky against a steady, unmoving background. Recently, the IAU has arbitrarily decided what is and is not a planet. Now, with the new definition, two conditions must be satisfied for an object to be called a planet. First, the object must be in orbit around a star, while not being a star itself. Second, the object must be large enough for its own gravity to pull it into a nearly spherical shape. The shape of objects with a diameter greater than about five hundred miles would normally be large enough to form a spherical shape. Borderline cases are bound to crop up and would have to be decided by observing the object's shape. Potentially, there could be hundreds of new planets just waiting to be found.

So, let us meet the new members of our solar family.

Ceres was the first object to be spotted in the asteroid belt between Mars and Jupiter. It is still the largest known object in the area. Its diameter of about 590 miles makes it big enough to be a planet by the new definition. Originally, it was classified as a planet when it was discovered, and was later reclassified as an asteroid over 150 years ago. In Roman mythology, Ceres was the goddess of growing plants (particularly cereals) and of motherly love. She was the daughter of Saturn and Ops, wife-sister of Jupiter, mother of Proserpina by Jupiter, and sister of Juno, Vesta, Neptune, and Pluto.

Pluto and Charon would now be the tenth and eleventh planets in our solar system. Charon's diameter is about 750 miles, just over half that of Pluto’s. And, unlike its companion, Charon does not appear to have an atmosphere. Because of their sizes being so nearly the same, Pluto and Charon will be considered a double planet. That is because the center of gravity, the point around which the two objects rotate, is located between the two planets. These objects orbit every 6.39 days and are about 12,100 miles apart. And this double planet has two newly discovered moons of its own. Hopefully, when the New Horizons spacecraft gets to Pluto/Charon in July 2015, we will learn more about these two planets.

The name 2003 UB313 is temporary. Its permanent name has not yet been assigned by the IAU, as they have been waiting for the new definition of a planet to see where this object will fall. Its Mount Palomar-based discovery team and many media sources are calling this object Xena. 2003 UB313 was discovered by Michael Brown, Chad Trujillo, and David Rabinowitz on January 5, 2005. Its size is estimated by Hubble telescope images to be about fifteen hundred miles in diameter. It is so far out in the fringes of the solar system that it takes 556.7 years to circle the Sun. Currently, lies at almost its maximum possible distance (aphelion) from the Sun. It is currently the most distant known solar-system object from the Sun, at 6.293 billion miles. Additional observations reveal this planet also has a moon. In keeping with the Xena television theme, the moon was nicknamed Gabrielle by its discoverers.

Oh, and those two NASA spacecraft that need revision …. It is going to be very difficult to correct this mistake. When the two Voyager spacecraft were launched in 1977, we only had nine planets. Today we have at least twelve. The plaques affixed to these craft depict many things about us from pictures of human forms, music from several eras, and many other things deemed interesting about Earth and its inhabitants. They also pictured the nine planets. But don't expect any corrections to the plaques. The Voyager 1 spacecraft is the most distant human-made object in the universe—9.3 billion miles from the Sun. At this distance, signals from Voyager 1 take more than thirteen hours to reach Earth. And Voyager 2 is not far behind it, at 7.5 billion miles from the Sun. Correcting this problem in the spacecraft information will take a bit more than your normal road trip.

Now for something you will find really bizarre. Earth's Moon will someday be a planet in its own right. When the Moon was born, it started out very close to the Earth. But it has been moving away from us ever since. It's currently drifting farther away by about 1.5 inches every year. In a few billion years, assuming the Earth and the Moon survive, the center of gravity will be outside the confines of the Earth surface. By the new definition, the Earth and Moon will be a double planet just like Pluto and Charon.

THE PLANETS AND MOON
• Mercury will be hidden in the Sun this month.
• You'll need to get up about an hour before sunrise to see Venus in the eastern sky.
• Like Mercury, Mars is hiding in the Sun.
• Ceres is generally too dim to be seen by the naked eye, but binoculars can show this new planet. Look east-southeast at 9:00 p.m. on September 15
• Look for Jupiter setting in the west about 10:00 p.m. early in the month, but earlier in the evening as the month progresses.
• Saturn rises around 4:00 a.m. in the east.

If you have a question, comment, or suggestion for Charlie, e-mail him at: k5cec@yahoo.com.