What planet was discovered in 1999?
HD 209458 b was discovered on November 5th 1999
From 1930 (when Pluto was discovered) to 1978, Pluto was the most distant planet. In 1978, Pluto slipped inside Neptune's orbit. Neptune was the farthest planet from 1978 to 1999, and Pluto (which has a fairly high eccentricity) was farther away from 1999 to 2006. In 2006, the International Astronomical Union "demoted" Pluto from Planet to Dwarf Planet, meaning that Neptune is once again the most distant planet.
In our solar system the outermost planet 'til March 1999 was considered the same as the outermost planet after March 1999 - it was Pluto. In 2006, the IAU reclassified Pluto as a minor or dwarf planet and no longer a true planet - this demotion meaning Neptune would thereafter be considered the outermost planet in our solar system. It is of course possible there is a planet out there not yet discovered, which is…