Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
home | WebAssign | textbook | Course Calendar course calendar


Kuiper Belt Objects (KBOs), also called trans-Neptunian objects, are rocky, icy bodies 30-50 AU from Sun. They formed at the same time as the solar system itself. The Kuiper Belt is the reservoir for short-period comets (such as Halley's comet). When these objects are "knocked" from their orbits into a more eccentric orbit, they pass close to Sun. They give off gas and dust which we see as the tail, and we then call it a comet.

The orbits of 922 KBOs have been determined since 1992. Eleven of these, including Pluto, have diameters of at least 1000 km. They are listed below.

object diameter (km)
2003 UB313 3000+/-300
Pluto 2320
2003 EL61 1200?
2005 FY9 1250?
Charon 1270
Sedna <1500?
2004 DW ~1500
Quaoar 1200+/-200
Ixion 1065+/-165
2002 AW197 890+/-120
Varuna 900+/-140


As you can see, there is only one KBO that has a diameter larger than Pluto. Because Pluto is considered the "ninth planet," this KBO was originally dubbed to be the tenth planet. It is called Eris.

Here's the official press release from July 29, 2005 regarding the tenth planet's discovery. Originally codenamed Xena by the discoverers, this KBO has now been named Eris (as of September, 2006).

I highly recommend reading about Eris and how it was discovered at the official web site for the planet. Also, many press releases regarding the dwarf planet are at www.tenthplanet.info.

In the blinking image below, can you find the object that moves? That's 2003 UB313 (Eris)!

On September, 2005, astronomers used a telescope at the Keck Observatory (Mauna Kea, Hawaii) to look at 2003 UB313 and found that it had a moon. Read about the moon at the official web site announcing its discovery. The moon's name is officially Dysnomia.

It should amaze you that you got to read about this discovery only a few years after it was officially published. Perhaps you will tell your grandchildren how you read about this new dwarf planet that is bigger than Pluto only years after it was officially named. Maybe you will even want to print the original paper that describes the discovery of the planet.

Dwarf Planets

There are three officially recognized dwarf planets: Ceres (in the asteroid belt and formerly known as an asteroid), Pluto, and Eris.

Yet, there are 12 objects that are on the "dwarf planet watchlist" and have yet to be classfied by the IAU. Check out NASA solar system web site that gives a few more details about dwarf planets.






Note: to keep spammers out, the feedback form requires you to type the class name, such as PHY1050, in order to submit feedback.


Class (enter PHY1050):



On a scale of 1 to 5 stars, with 5 stars
being the best, how do you rate this lesson?