According to NASA "New Horizons launched on Jan. 19, 2006" according to Wikipedia this is about the same time Pluto started on the path to loss of planet status. Is it just coincidence that both of these happened at the same time or is there a relationship?
The official trigger for what caused Pluto's demotion as a planet was the discovery of Eris, in October 2005. For a number of years, starting with the discovery of non-Pluto Kuiper Belt Objects (KBOs), and later with the discovery of other large objects in the Kuiper Belt. When Eris was discovered, the IAU decided that it needed to decide what a planet was. New Horizons had absolutely nothing to do with the team that ultimately discovered Eris (Although they probably have some interest as the post-Pluto mission to visit a KBO).
The discussion had been happening for some time before, but ultimately New Horizons had nothing to do with the vote that demoted Pluto as a planet, and had far more to do with the discovery of an object larger than Pluto in the Kuiper belt.
It might be worth mentioning that essentially the same thing happened with the Asteroid belt. The first asteroids discovered were considered planets, and after a large number of them had been found, they were re-classified. Ceres was the first, discovered in 1801. By 1851, they were classified as Asteroids. See Wikipedia for a bit more history.
It's just a coincidence, and officially, the dates aren't that close. The IAU (International Astronomical Union) General Assembly in Prague, Czech Republic where the new definition of a planet was endorsed and with it Pluto losing its planetary status, happened in late August, 2006. Final draft that was voted on states:
A planet is a celestial body that
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
- has cleared the neighbourhood around its orbit.
A dwarf planet is a celestial body that
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- has not cleared the neighbourhood around its orbit,
- is not a satellite.
All other objects except satellites orbiting the Sun shall be referred to collectively as Small Solar-System Bodies.
The New Horizons launch was more than seven months earlier, in mid January that same year.
How this IAU process happened is explained in this Harvard-Smithsonian's Center for Astrophysics (CfA) lecture, presented by David A. Aguilar:
It's unclear how many scientists that voted had vested interest in New Horizons mission, but apparently not enough, if that would have skewed the vote at all. I'm not saying that it would or that it should, but New Horizons Principal Investigator (PI) Alan Stern made his stance clear on several occasions that he stands behind the notion that Pluto is and should be considered a planet:
It's an awful definition; it's sloppy science and it would never pass peer review – for two reasons. Firstly, it is impossible and contrived to put a dividing line between dwarf planets and planets. It's as if we declared people not people for some arbitrary reason, like 'they tend to live in groups'. Secondly, the actual definition is even worse, because it's inconsistent.
But he didn't get to vote in Prague:
I was not allowed to vote because I was not in a room in Prague on Thursday 24th. Of 10,000 astronomers, 4% were in that room - you can't even claim consensus.