ASK DR. FRED
by Dr. Fred Bortz
For more than twenty years, Dr. Fred Bortz worked as a scientist, researcher, and teacher. Now he spends most of his time writing books and articles for young readers like you.
He enjoys both science and writing for the same reason: HE LOVES QUESTIONS. He writes for people your age because he knows you love questions, too.
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How Many Planets are in the Solar System, and Is Pluto One of Them?
BIG NEWS COMING! THE INTERNATIONAL ASTRONOMICAL UNION WILL BE VOTING ON ITS OFFICIAL DEFINITION OF A PLANET ON AUGUST 24, 2006. WE WILL REWRITE AND REPLACE THIS ARTICLE AFTER THAT VOTE. CLICK HERE FOR THE NEW VERSION
NOTE: THIS ARTICLE WAS WRITTEN IN 2002, AFTER THE DISCOVERY OF QUOAOR. SINCE THEN, THERE HAVE BEEN NEW DISCOVERIES, INCLUDING A BODY LARGER THAN PLUTO. WE WILL BE CONTINUING TO ADD NEWS FLASHES TO THIS PAGE TO KEEP IT UP TO DATE.
Scientists like Dr. Fred love questions, especially "open questions" when not everyone agrees about the answer. When scientists don't agree, they argue. It doesn't mean they're angry with each other, it just means they look at the same evidence and draw different conclusions.
Dr. Fred thinks arguments like that are fun, especially when someone disagrees with something that no one has questioned for years, such as the number of planets in the Solar System.
If somene asked you to list the planets in the Solar System, you might answer with this list from nearest the Sun to furthest away: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Those nine, no more, no less, forever and ever. The same nine planets that people have known about for centuries, right?
The ancients knew only five planets, Mercury, Venus, Mars, Jupiter, and Saturn. For thousands of years, people thought that Earth was the center of everything. It wasn't until the early 1500s that some people, like the famous Polish astronomer Nicolas Copernicus, began to argue about that. Earth was a planet, he claimed, just like the other five, each following its own path -- or orbit -- around the Sun.
The argument continued for more than a hundred years before most people realized Copernicus was right. The list of planets then numbered six and stayed that way until 1781, when William Herschel discovered Uranus. Uranus was so far from the Sun that it moved very slowly across the constellations; still its motion was fast enough for Herschel to notice after observing it on different nights.
Once Uranus was known to be a planet, people measured its movements very carefully. By the 1840s, astronomers realized that its path was different from what the law of gravity predicted. Something was tugging it away from the predicted orbit.
Could there be another planet? Yes, and the race was on to find it. In 1846, the noted French astronomer Urbain Jean-Joseph Le Verrier published an article predicting the size and orbit of the unseen planet, and on September 23 of that year, observers at the telescope of the Berlin Observatory found it. Le Verrier wanted the planet named after himself, but it became known as Neptune.
Were there more planets to be found? Astronomers tracking the motion of Neptune discovered some irregularities, but for many years, observers found nothing to cause them. Finally in 1930, Clyde Tombaugh of the Lowell Obsevatory in Flagstaff, Arizona found planet number nine and named it Pluto.
Pluto turned out to be unusual in many ways. The other outer planets are "gas giants" (Jupiter and Saturn) or slushy "ice giants" (Uranus and Neptune). Pluto is a tiny, solid, icy world in a more elongated orbit than the nearly circular paths of the other planets. By the late 1900s, astronomers had found that it was the smallest planet of all, even smaller than Mercury; yet it had a sizeable moon, Charon. Another oddity about Pluto is the inclination of its orbit. All the other planets follow paths in a narrow band within the boundaries of the constellations we call the zodiac, but Pluto's orbit takes it well above and below that band.
Still, despite those peculiarities, no one questioned whether Pluto deserved to be called a planet. Then, in 1992, astronomers began finding smaller Pluto-like bodies in the distant reaches of the Solar System called the Kuiper (KI-per) Belt. Now, nearly 100 Kuiper Belt Objects (KBOs) are known. The largest one discovered so far is named Quaoar (kwa-oh-wahr) after a creation god of the Native American Tongva tribe. When it was discovered in 2002, it was big news, since it is about half the size of Pluto. It orbits the sun once in 288 years and has a more planet-like orbit than Pluto-Charon (more circular and less inclined).
There may well be undiscovered KBOs larger than Pluto but darker and farther away. So in 1998, some members of the International Astronomical Union began to question whether Pluto should be considered a planet or merely the largest known KBO.
MARCH 2004: HOT NEWS FLASH ABOUT A COLD WORLD
Click Here For Even Bigger News Flashes Including the Discovery of a Kuiper Belt Object Larger than Pluto.
IS IT THE TENTH PLANET?
Scientists who study the way planetary systems formed have been speaking for years about a region far beyond the Kuiper Belt where many comets are born. Called the Oort Cloud, it is full of small, icy bodies that orbit the sun at about thirty times the distance of Pluto. In November 2003, astronomers observed a large icy body far beyond Pluto. By March 2004, they recognized that it was the first known Oort Cloud object. They named it Sedna for the Inuit goddess of the ocean, and reported a number of details about it and its orbit.
They were able to spot Sedna because it is now nearing its closest point in a very elongated 10,500-year path around the Sun. It is currently 13 billion kilometers (about 8.5 billion miles) from the Sun, or ten times closer than the main body of the Oort Cloud. Its discoverers suspect that its original round and distant orbit was disturbed into a narrow ellipse by the gravity of a passing star early in the history of the Solar System. It will get closer and brighter for the next seventy-two years, but it will still remain much farther away than the Kuiper Belt. Then it will head back outwards toward the main Oort Cloud. The last time it approached this close to Earth, our planet was just coming out of the last ice age and most of North America was covered by glaciers.
This NASA artist's conception compares the size of Earth, the Moon, Pluto, Quaoar, and Sedna.
Sedna is nearly as red as Mars, but too far away and dim for scientists to determine what is is made of (besides ice). Because it is nearing its closest point to the Sun, Sedna is experiencing a warm spell. Its temperature is probably a balmy 240 degrees below zero Celsius (400 degrees below zero Fahrenheit). At its most distant point, it will be many degrees colder. The best estimate of its size is 1700 kilometers (1000 miles), halfway between Pluto and Quaoar. That's large enough for the NASA news release to call it a "planet-like body."
So how many planets are there in the Solar System? It's an open question, and it can be fun to argue about. (See this newspaper article for more about the argument.) The answer comes down to the meaning of the word "planet," but it doesn't really change anything in the Universe. Pluto-Charon, Quaoar, the KBOs, Sedna, and other Oort Cloud objects are out there, and we have lots to learn by studying them -- no matter what we call them!
Looking forward to wearing a new Kuiper belt,
Dr. Fred has written about discoveries that changed the way people thought about many things in To the Young Scientist: Reflections on Doing and Living Science. He has also written Jupiter and Beyond: Heidi Hammel Explores the Giant Planets, scheduled for publication in the fall of 2005.
Read about the scientists interviewed inTo the Young Scientist
Travel to the Big Island of Hawaii with Dr. Fred as he spent three days with Heidi Hammel observing Uranus, Neptune, and the moons of Mars.
JULY 2005: REALLY BIG NEWS FROM THE KUIPER BELT
On July 29, 2005, Dr. Mike Brown of the California Institute of Technology announced that he and fellow astronomers Chad Trujillo (of the Gemini Observatory in Hawaii) and David Rabinowitz (of Yale University) have found a world bigger than Pluto. On October 31, 2003, they first photographed it using the Samuel Oschin Telescope at Palomar Observatory near San Diego. But it is so far from the Sun that it moves very slowly across the sky. The astronomers only detected that motion when they took a second look at their images in January, 2005. They have been observing it carefully ever since to get a better idea of its size and orbit.
From NASA, this artist's concept shows how the newly discovered object might appear from just beyond its orbit.
The bright star is the very distant Sun.
Because of its brightness, they now know that its diameter is larger than Pluto's 1400 miles (2300 kilometers). But it must be smaller than about 2000 miles (3200 kilometers), because it is invisible to NASA Spitzer Space Telescope, which measures the infrared light (heat waves) from the object.
Its name is temporarily 2003 UB313, but its discoverers have proposed a permanent name that is not being released until it is approved by the International Astronomical Union.
Note added, February 2, 2006:
An article in Nature magazine reports that 2003 UB313 is approximately 3000 kilometers (nearly 1900 miles) in diameter, or almost as large as Earth's Moon.
Nature published this image, comparing the size of the "tenth planet" to other large Solar System bodies from the asteroid Ceres to the four largest moons: Neptune's Triton, Earth's Moon, Saturn's Titan, and Jupiter's Ganymede.
The orbit of this new Kuiper Belt Object is elongated and tilted like Pluto's, and for part of that orbit it will be closer to the Sun than Pluto. That won't happen for another 240 years, and who knows how many bigger objects will be discovered by then!
Should we call 2003 UB313 a planet? It certainly qualifies to be one, even more than its little sibling Pluto. Whatever it is and whatever it is eventually called, it has already made a name for itself in the history of astronomy!
Read more about 2003 UB313 and follow links to a blink movie showing its motion at the Sky & Telescope web site.
Shop for To the Young Scientist.
Text copyright 2002-6 by Alfred B. Bortz, all rights reserved
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