Another Planet in the System?
"My Very Educated Mother Just Showed Us Nine Planets -- plus one." That's right. Scientists have recently discovered what many are calling the 10th planet. Scientists at the California Institute of Technology, also known as Caltech, in Pasadena, Calif., saw what may be another planet in our solar system. Their research was sponsored, in part, by NASA. They made the announcement of the new planet earlier this year. Until it gets an official name, the planet is being called 2003 UB313.

Large telescope inside the observatory
Image to right: The Samuel Oschin Telescope photographed the 10th planet. Credit: Danner/Hogg Caltech

Is It a Planet or Not?
Some people are not sure if the object is a planet. So, how do we know? The funny thing about deciding if an object is a planet is that there is no scientific definition for "planet." We have always called the large objects that orbit our sun "planets." Seventy-five years ago, Pluto was discovered and we called it a planet. We know that this new object is larger than Pluto. So, if Pluto is a planet,
2003 UB313 must be too, its discoverers say.

What Do We Know About This Object?
Scientists can make an educated guess about the size of 2003 UB313 by studying its brightness, its distance from the sun and by determining which telescopes can or cannot see it. They can also tell what the object is made of using spectroscopes, which break up reflected light into its different colors, like a prism.

Way, Way Out
Scientists measure the distances between planets in our solar system in astronomical units (AU). One AU is the distance from the sun to the Earth, or about 150 million kilometers (93 million miles). The"new planet" is 97 AUs from the sun. That is more than twice the distance to Pluto. It is currently the farthest known object in the solar system. It orbits the sun in the Kuiper (ky-per) Belt. This is the cold region beyond the planet Neptune. From 97 AUs, the sun would look like the brightest star in the night sky. There would be no daylight.

A drawing showing a rocky planet and its tiny moon with a bright distant star
Can I See It?
If you have a really large telescope in your backyard, you would be able to see 2003 UB313.

Image to left: The sun would look like the brightest star in the sky from this planet. Credit: Robert Hurt, IPAC

The telescope that was used to find it has a mirror that collects light. That mirror is 1.2 meters (48 inches) across. It can collect about 360,000 times the amount of light collected by your eye.

The Name Game
Compared to names like Mars or Venus,
2003 UB313 may sound strange. How will you remember that name for a test? Relax, this is only a temporary name. The name will change when people decide just what the object is. So what does 2003 UB313 mean? The first four digits tell us the year of discovery. The object was first photographed in 2003, though scientists didn't find it in the photographs until 2005. The first letter indicates which half of which month the object was found. All letters of the alphabet are used except "I" and "Z." A "T" would represent the period from Oct. 1-15. The "U" in UB313 means that it was found during the period Oct. 16-31.

The second letter and subscript digits tell us the order in which the object was found during that period of time. An “A” would be given to the first object found, a "B" to the second, and so on. This time, all letters are used except "I." What happens once 25 objects have been found and the end of the alphabet is reached? The 26th object starts over with the letter "A" followed by a subscript numeral "1" …

A-Z represent objects 1-25
A1-Z1 represent objects 26-50
A2-Z2 represent objects 51-75
… and so on.

The number 313 tells us that there were many more than 75 objects found during this two-week period. Can you figure out how many objects were found before 2003 UB313? B313 means that during that half-
Tenth Planet Fast Facts

Discovered by: Michael Brown, Caltech
Chadwick Trujillo, Gemini Observatory
David Rabinowitz, Yale University
Telescope: Samuel Oschin Telescope, San Diego, Calif.
First Photographed: Oct. 21, 2003
Noticed in photos: Jan. 8, 2005
Discovery announced: July 29, 2005
month period, astronomers used all 25 letters 313 times, and then used the letters A and B again. Since 25 times 313 is 7,825, adding two more (for the extra A and B) makes a total of 7,827. Almost all of these were small asteroids in the Main Asteroid Belt between the orbits of Mars and Jupiter. There are hundreds of thousands of asteroids there.

Since 2003 UB313 is a temporary name, who actually gets to name the planet? The scientists who discovered it have selected a name, but the International Astronomical Union will make the final decision.

Something to Think About
Many people say a sentence, called a mnemonic, such as "My Very Educated Mother Just Showed Us Nine Planets" in order to remember the order of planets starting from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. If 2003 UB313 is dubbed a planet, what new sentence will we use to remember it?

Denise Miller/NASA Educational Technology Services