This image of the dwarf planet Eris and its moon Dysnomia (just to the right of the main body) was taken at the Keck Observatory on September 10, 2005. Click on image for full size (30 Kb) Image courtesy W. M. Keck Observatory.

Eris is a dwarf planet that was discovered in 2005. Eris is a trans-Neptunian object (TNO) that orbits the Sun on the frozen fringes of our Solar System beyond the Kuiper Belt. Eris takes 557 years to travel once around the Sun along its highly elliptical orbit. We don't yet have any clear images of the surface of Eris because it is so far away. Astronomers aren't yet certain how long a day on Eris is; they think it lasts at least eight hours.

Eris is slightly larger than Pluto, having a diameter somewhere between 2,400 and 3,000 km (). The discovery of Eris brought to a head a debate among astronomers as to whether Pluto should be considered a true planet. In 2006 the International Astronomical Union (IAU) declared that Pluto, Eris, and Ceres are the first three members of a new class called "dwarf planets".

Eris was photographed in October 2003, but, because of its slow rate of motion, wasn't recognized as a KBO until January 2005. The object's discovery was officially announced on July 29, 2005, by the scientists who found it (Michael Brown, Chad Trujillo, and David Rabinowitz).

Eris has an unusual orbit. The orbit is tilted at an angle of 44° with respect to the ecliptic, the imaginary plane in which Earth and most other planets move. The orbit is highly eccentric (e = 0.44), bringing the object within 38 AU of the Sun at perihelion and sending it out beyond 97 AU at aphelion. Eris takes 557 years to orbit the Sun at an average distance (semi-major axis) of 68 AU.

Eris has a moderate-sized moon which was first spotted in September 2005. The moon has been given the name Dysnomia.

When Eris was first discovered, it bore the temporary designation of 2003 UB₃₁₃. Astronomers nicknamed it "Xena", after the title character of the television show "Xena: Warrior Princess". The moon was nicknamed "Gabrielle" after Xena's sidekick in the TV series. The name Eris now supercedes all prior designations.

This picture shows the dwarf planet Eris and its moon Dysnomia (just to the right of center). Click on image for full size (30 Kb) Image courtesy W. M. Keck Observatory.

Astronomers have discovered a new object that some people are calling our Solar System's "tenth planet". The object is a large sphere of ice and rock similar to Pluto. It orbits the Sun on the edge of the Solar System. Like Pluto, it is in the area called the Kuiper Belt.

The object has the temporary name "2003 UB₃₁₃". After astronomers decide whether or not to call it a planet they will give it a permanent name. For now, it has been nicknamed "Xena". It was officially discovered in July 2005. It was discovered by Michael Brown, Chad Trujillo, and David Rabinowitz.

Xena looks like it is about 2,700 km (1,700 miles) across. We aren't sure of its exact size because it is so far away and very hard to see, even with the world's biggest telescopes. It is probably larger than Pluto, though, which is 2,390 km (1,490 miles) across. Since Xena is bigger than Pluto, and since most people consider Pluto a planet, some astronomers think Xena is a planet. However, some other astronomers don't think Xena should be called a planet. Right now, there is no official definition of a planet. Scientists are discussing what the definition of a planet should be.

Xena has a small moon. The moon has been nicknamed "Gabrielle".

Xena's orbit is odd. The orbit is not a circle, so Xena moves closer to and further from the Sun as it goes around. It takes Xena 557 years to go around one time! Xena's orbit is also tilted... it doesn't move in the same plane that the other planets orbit in. Xena's strange orbit is another reason some astronomers don't think it should be called a planet.

This picture shows the dwarf planet Eris. It also shows its moon. The moon is the smaller dot to the right of Eris. The moon's name is Dysnomia. Click on image for full size (30 Kb) Image courtesy W. M. Keck Observatory.

Astronomers have found a new object in our Solar System. Some of them are calling the object a new planet. Other scientists don't think it is a planet.

The object is a large ball of ice and rock. It is a lot like Pluto. It orbits the Sun way out on the edge of our Solar System, like Pluto does.

The object has a temporary name of "2003 UB₃₁₃". It won't get a permanent name until astronomers decide whether it is a planet or not. Some scientists have given it the nickname "Xena".

Xena was discovered in July 2005. It was found by Michael Brown, Chad Trujillo, and David Rabinowitz. Xena is hard to see, even using the biggest telescopes in the world, because it is so far away. Xena is probably a little bigger than Pluto.

Xena has a moon. The moon has a nickname, too. The moon's nickname is "Gabrielle".

It takes Xena 557 years to orbit the Sun once! Xena's orbit is strange. The orbit isn't round, and it is tilted a lot.

So, for now, our Solar System officially has nine planets still. If astronomers decide that Xena is a planet, then we'll have ten!