Why was Pluto removed from the planet category and then reinstated?

– Xochielt Sanchez

Answer by Robert Frost:

Pluto has not been reinstated as a planet. I recommend utilizing more responsible news sources.

The International Astronomical Union (IAU) published their official definition of what constitutes a planet in this memo: https://www.iau.org/static/resol… in 2006.  In short, it said that:

1) a planet directly orbits the Sun.  That excludes moons because although they indirectly orbit the Sun, they directly orbit a planet.

2) a planet is massive enough that gravity overcomes rigid body forces and results in a round, almost spherical shape.  That excludes the asteroids.


3) a planet has used its mass to sweep up the trash in its neighborhood.  This is where tiny Pluto becomes excluded.

Technical terms lose their effectiveness when they are not precisely defined.  For a long time, the word planet was loosely defined.  The IAU felt it needed to be tightened, so they did so.  The result was Pluto didn't meet the criteria.  That was not their objective, but it was a result.

Obviously, not everyone was happy with the result.  Two weeks ago, at a Harvard panel, Pluto was discussed and the audience was polled.  The audience voted in favor of being a planet.  Various media outlets ran with that and used click bait headlines, but nothing official has changed.  The IAU definition has not changed.  It may, some day, but not yet.

Why was Pluto removed from the planet category and then reinstated?


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