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Last modified: Wednesday, August 30, 2006


Pluto no longer a planet

Aug. 30, 2006

BLOOMINGTON, Ind. -- Pluto is no longer a planet, but not without a struggle.


After tumultuous discussions at the general assembly of the International Astronomical Union in Prague, Czech Republic, several hundred astronomers have agreed for the first time on a definition of a planet. Three attempts at a definition were needed, and the discussion was sometimes heated, but there was widespread support among astronomers for the final result.

According to the new definition, a planet is a celestial body that (a) is in orbit around the sun, (b) has enough mass to be nearly round and (c) dominates its orbit. The astronomers were trying to define only what "planet" means in our own solar system, without considering planets around other stars.

The new resolution also defines a class of "dwarf planets" that meet the first two criteria above, but not the last. This class includes Pluto as well as a slightly larger object known as 2003 UB313 that was discovered in 2005, and probably several other icy bodies in the Kuiper Belt beyond Neptune. It also includes Ceres, the biggest of the rocky asteroids circling the sun between the orbits of Mars and Jupiter.

Catherine Pilachowski, the Daniel Kirkwood Chair in Astronomy at Indiana University, attended the IAU meeting in Prague. Pilachowski described the events there as discouraging at first. There was a fundamental split between those who wanted to concentrate on the structure of a celestial object (what is its shape?) and those who wanted to emphasize its behavior (how does it move and affect other objects?). There seemed no way to reconcile the opposing groups, she said.

"I've had two decades of involvement with the International Astronomical Union, and it's usually been cumbersome and slow about getting things done," she said. She was gloomy at the start of the meeting because the first draft of the definition of a planet was so flawed. She felt even worse after the intense arguments over the second draft, which was offered just two days before the end of the meeting. "By then I thought we weren't going to get anything passed at all, which would have been embarrassing for all of us," she said.

But she was encouraged when a third draft was presented just a few hours later and the response to it was more positive. "The organizers considered new ideas and made changes in that short amount of time," she said. "It was amazing." And the discussion and revisions led to a final result that she felt was very good.

Some people may feel uncomfortable because one of the facts they were sure of -- that there are nine planets -- has suddenly been taken away. Actually, Pluto's status as a planet has been disputed for years among astronomers, and that disagreement came to a head when Pluto-sized 2003 UB313 was discovered last year. Was this new object also a planet, or was it something else? The debate led to the classification of both Pluto and the new object as dwarf planets.

The new system seems more likely to last. There are now four small rocky planets (Mercury, Venus, Earth, Mars) and four large gas planets (Jupiter, Saturn, Uranus, Neptune). The two groups are separated by the asteroid belt. In addition there are a number of dwarf planets, comets and other smaller objects in the outer region called the Kuiper Belt. Astronomers expect to discover several other dwarf planets in the next few years. Many of these objects are probably debris left over from the formation of the solar system.

If you were taught a particular phrase to help in remembering nine planets in order, such as "My Very Educated Mother Just Served Us Nine Pizzas," you may be wondering what phrase to use now. Astronomy Professor Phyllis Lugger at Indiana University suggests the following: "My Very Educated Mother Just Served Us Nachos."


If you're outdoors on a clear September night, you may see a colorful display in the sky called an aurora ("northern lights"). These silent, shimmering ribbons and curtains of light have an eerie beauty that can be spellbinding.

Auroras can appear whenever the sun is active, but especially from August to October. Eruptions from the sun's surface hurl enormous amounts of material into space. When some of this solar matter heads in our direction, it causes auroral activity that is sometimes visible even from middle latitudes.

Such a display happened on Aug. 7 when auroras were seen as far south as Colorado. For details and photographs, see

You can watch for auroras when they are most likely to happen by checking Web sites such as and Sightings of auroras are reported at

In the Northern Hemisphere, auroras are sometimes seen as far south as Florida and California. They are caused when charged particles from the sun collide with Earth's magnetic field, sending particles down into Earth's upper atmosphere. Molecules of air glow when they are struck by these particles raining down along the lines of the magnetic field. The result is a beautiful aurora.

The color of an aurora depends on the kind of molecule struck by the particles from the sun. High-altitude oxygen is the source of all-red auroras. Oxygen at lower altitudes glows a brilliant yellow-green, the brightest and most common auroral color. Ionized nitrogen molecules produce blue light, and neutral nitrogen glows red. The nitrogens create the purplish-red lower borders and edges of an aurora.


The only bright planet visible in the evening sky during September will be Jupiter, gleaming low in the southwest 30 to 45 minutes after sunset. The brilliant white planet will be easy to find if it's not hidden by trees or buildings.

Venus, the brightest planet, will be very low in the east before dawn, easily blocked by obstructions. Find a spot with a clear view of the eastern horizon to see it. The time between the rise of Venus and sunrise will shrink from 75 minutes to a half hour during September.

Saturn will be low in the east in early dawn at the start of the month, but it will appear higher each day until it rises four hours before the sun by month's end. The yellow planet will be in the constellation Leo the Lion, near the bright white star Regulus. In the latter part of the month, the sky should be dark enough for those with telescopes to get a good view of Saturn's magnificent rings.

Mercury will be very low in the west all month in the Northern Hemisphere, out of sight for anyone who doesn't have a clear view of the western horizon in a dark sky. In the Southern Hemisphere, however, Mercury will be high above the western horizon and easy to spot near the bright white star Spica.

Mars will be out of sight behind the sun during September.

Solar eclipse

Observers in South America and Africa will see a partial eclipse of the sun on Sept. 22. A few lucky persons on the north coast of South America will see an annular eclipse, when a ring of bright sunlight is visible around the black sphere of the moon, but the path of the annular eclipse will then cross the South Atlantic Ocean, not appearing on land anywhere else. A map showing where the eclipse will be visible can be found at


The sun will reach the September equinox on Sept. 23 at 12:03 a.m. EDT (4:03 Universal Time), marking the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere. For the next six months in the Northern Hemisphere, the nights will be longer than the days. Some of the traditions associated with the September equinox are described at

Moon phases

The moon will be full on Sept. 7, at third quarter on Sept. 14, new on the night of Sept. 22 and at first quarter on Sept. 30.