Is Pluto A Planet?

**Guillaume** · 08-03-2005, 09:43 AM

I don't know any more! I'm confused.

**manker** · 08-03-2005, 10:15 AM

Originally Posted by lynx

Even Dr Brian Marsden seems a little confused. "we have eight planets and only an object bigger than Mars could be considered to be a planet in the future". But Mercury is smaller than Mars, so surely that throws his theory out of the window already. Or perhaps he has plans to reduce the planet count to seven. Have we discovered the true identity of Ming the Merciless?

I suspect he believes that the number of new planets discovered may get a little large if Mercury was used as a benchmark, so he decided on Mars. The key is that he said in the future. To my knowledge, Mercury was discovered some time ago.

I didn't even consider the possibility that the director of the International Astronomy Union's minor planet centre was unaware that Mars was bigger than Mercury.

**Barbarossa** · 08-03-2005, 12:22 PM

Originally Posted by Rat Faced

Do we have two moons too?

moons are another matter entirely!!

**lynx** · 08-04-2005, 10:46 AM

Originally Posted by Barbarossa

Originally Posted by lynx

Surely, since Pluto has already been accepted as a planet, then it is a planet. If we need to make distinctions about later discoveries then I would suggest that Pluto be taken as the benchmark.

Hmm, isn't that sort of against the principles of scientific research?

When more knowledge becomes available, a more informed decision can be made about things.

Scientists shouldn't be scared to admit they get it wrong sometimes!

No, it is called shifting the goalposts to fit with your beliefs. All other things being equal, the only thing determining whether something is a planet is its size, and that is a purely subjective consideration. Why is there a need to downgrade Pluto and make Mars the future benchmark, especially when Mercury is already smaller than Mars? That can only lead to confusion.

If Mars is to be the future benchmark, then all bodies smaller than Mars should be downgraded, it doesn't make sense to downgrade some bodies and not others. But since it is extremely unlikely that other members of the Astronomical Union (and more importantly the general puplic) would agree to the downgrading of Mercury to a minor planet it would make more sense for a body the size of Mercury or smaller to be the future benchmark.

So if Mars doesn't make sense to be the benchmark, and Pluto is currently considered a planet it actually makes most sense that Pluto should be used as the future benchmark.

What does it matter if we find that the number of planets becomes rather large? Is there some sort of competition? Will it affect the planning conditions for the new inter-galactic super-highway? We should be told.

**Barbarossa** · 08-04-2005, 12:17 PM

There is already precedent for a "planet" to be reclassified. As further knowledge becomes available, definitions can change.

From http://en.wikipedia.org/wiki/Definition_of_planet

When the word was originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky (asteres planetai "wandering stars"). This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon.

Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and, after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified. A "planet" could then be defined as "any object that orbited the Sun, rather than another object".

Also, from http://en.wikipedia.org/wiki/planet

The eight rocky and gaseous planets are universally recognized as major planets. Ceres was called a planet when first discovered, but was reclassified as an asteroid when many similar objects were found. Given recent discoveries of many trans-Neptunian objects which are very similar to Pluto in orbit, size and composition, many people think it should be similarly redefined as a minor planet. For example, Mike Brown of Caltech defines a planet to be: any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit [2] Using this definition, neither Pluto nor Sedna would be a major planet.