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?

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    $\begingroup$ The discoverer Tombaugh passed away 1997. It would've been disrespectful to "unplanet" Pluto while he was still alive and 80+ years old. Now the timing is rather disrespectful to Alan Stern and his team. It would've been nicer to wait with the reclassification until after New Horizons flyby and presented it as partly a result of the mission. $\endgroup$
    – LocalFluff
    Jun 24, 2015 at 11:58
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    $\begingroup$ But the mission results won't influence the decision either way. The trigger wasn't any change in what we know about Pluto, but the discovery of piles of KBOs. $\endgroup$
    – Hobbes
    Jun 24, 2015 at 12:27
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    $\begingroup$ @Hobbes is right, the change in Pluto's status won't change one bit the importance of the scientific results we will get from New Horizons. $\endgroup$ Jun 24, 2015 at 12:41
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    $\begingroup$ @LocalFluff If you believe the "cleared orbit" distinction between planet and dwarf planet is contrived, you might want to review this answer: astronomy.stackexchange.com/a/592/6. $\endgroup$
    – called2voyage
    Jun 24, 2015 at 14:38
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    $\begingroup$ This comment chain, unrelated to the main point of the question, was growing too long. If anyone wishes to continue discussion over the definition of planets, take it to chat. $\endgroup$
    – called2voyage
    Jun 24, 2015 at 15:28

2 Answers 2


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

  1. is in orbit around the Sun,
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
  3. has cleared the neighbourhood around its orbit.

A dwarf planet is a celestial body that

  1. is in orbit around the Sun,
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  3. has not cleared the neighbourhood around its orbit,
  4. 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.

  • $\begingroup$ Does it really say "around the Sun" rather than "around a star" or something like that? $\endgroup$ Jun 24, 2015 at 12:38
  • $\begingroup$ @PepijnSchmitz Yes. Planets around other stars are called exoplanets. I know, I know ... $\endgroup$
    – TildalWave
    Jun 24, 2015 at 12:41
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    $\begingroup$ Re Stern's It's an awful definition; it's sloppy science and it would never pass peer review -- That's a complete lie, and he knows it. It is very well constructed science (it's his constructed science), and it did pass peer review (it's his paper). He is a coauthor of one of the key papers that eventually led to the so-called demotion of Pluto. $\endgroup$ Jun 24, 2015 at 13:25
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    $\begingroup$ @PepijnSchmitz - That's right. Per the current definition, there are eight planets in the entire universe. $\endgroup$ Jun 24, 2015 at 14:03
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    $\begingroup$ @called2voyage - Before discovering any exoplanets, astronomers and astrophysicists thought star systems would look much like ours, with terrestrial planets in nearly circular orbits close in, giant planets in nearly circular orbits further out, maybe some asteroids and outer planets. Then they started finding them. What they found was a bit wonky. Our solar system is nice and neat. For other star systems, that's not necessarily so. Scientists need a better picture of what's out there before they can come up with a classification system that describes what's out there. $\endgroup$ Jun 24, 2015 at 15:12

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.

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    $\begingroup$ It wasn't just Eris. There was also Varuna (discovered in 2000), Quaoar (2002), Sedna (2003), Haumea (2004), Orcus (2004), Makemake (2005), and a host of others. By 2005, it was looking like a repeat of the mid 19th century, when astronomers were discovering new "planets" (now demoted to "asteroids") on a regular basis. $\endgroup$ Jun 24, 2015 at 14:02
  • $\begingroup$ Eris is no longer considered larger than Pluto in volume, but it has a good deal more mass as measured from the orbital motion of their respective moons. That in itself is intriguing, implying a large difference in density and therefore ice/rock ratio in the two objects. $\endgroup$ Dec 4, 2019 at 1:27

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