The planet Eris was discovered in 2004/2005/2006, which is true?

Answer 1

If you read through Mike Brown's page at the link provided by Northstar you'll see that the object was "discovered" on January 5, 2005. This was when the team was reviewing earlier images after having changed the computer program that tried to identify interesting objects in the image. Basically objects a long way from the Sun move very slowly across the sky and it can be hard to reliably pick one out from a distant star.

Confirming images were obtained in the next few days. The data was then submitted to the Minor Planet Center in July 2005 when the team feared they might be "scooped" by other teams looking for distant objects. When the data was submitted to the Minor Planet Center the earliest observations were 3 positions from October 21, 2003 and these were the reason the object received the designation 2003 UB313 (the 7827th object given a designation with discovery data between October 16 and October 31, 2003).

The object was then found in images stretching back to 1954. All the measurements can be found at the website below.

Answer 2

Before it was named Eris was referred to as 2003 UB313 which tells you it was first discovered in 2003. However, it took some time before its size and significance were recognized. When they realized it was larger than Pluto it was declared a planet and eventually given the name Eris. This forced the issue--what is a planet? A definition was agreed upon that excluded not only excluded Eris but also Pluto from being a planet. Both were reclassified as dwarf planets.

Here is a link.

http://www.gps.caltech.edu/~mbrown/planetlila/