Bryan Lamb
1024
Answered
at 8:29pm on March 2nd, 2008
Pluto was discovered in 1930 by Clyde Tombaugh. A ninth planet was being sought due to believed discrepancies between the Newtonian-predicted orbit of Neptune (discovered in 1846) and its actual orbit. this was exactly how Neptune was discovered, by analyzing differences between the observed orbit of Uranus (discovered 1781) and its predicted orbit.
however, Pluto's discovery was mostly due to a "mistake". When Neptune was discovered, its mass was estimated, but actually incorrectly, which affected the equations used to predict its path. as a result the discrepancy that came about between observed and predicted was due to this error, and thus an error led to the search for a ninth planet. When Pluto was discovered it was realized that it was far too small to have caused a gravitational effect on Neptune of the magnitude previously believed, and then the error with Neptune's mass was realized.
in 1978, Pluto's largest moon, Charon, was discovered. not long after its discovery, Pluto and charon;s orbital plane coincided with the earth's own orbital plane, allowing us to see Pluto and charon eclipse each other numerous times. this enabled us to learn more things about the two objects. Pluto and Charon have the closest relative size of any object and its moon (Earth and its moon is next). our moon is tidally locked to the Earth (keeps the same face to us all the time, as anyone who has ever looked at the moon can attest to). what is less well known is that this is also true of jupiter's four largest moons (Ganymede, Io, Callisto and Europa) . The same is also true of Charon relative to Pluto, BUT INTERESTINGLY ALSO OF PLUTO RELATIVE TO CHARON. put another way, Both Pluto and charon keep the same face towards each other the whole time. this has happened because of the similar relative size. Charon's gravitational tides effect on Pluto is large enough to slow down Pluto's rotation rate. Our moon is doing the same to the Earth, but at a much slower rate.
the fact that Pluto and Charon are tidally locked and more similar in size than other Planet - moon systems suggests (in addition to the numerous other unusual facts about Pluto -- its highly eccentric orbit, its small size, etc suggests that perhaps it is not really supposed to be a planet. however, a planet it remained until in 2003, something big happened.
The event that eventually led to Pluto being "declassified" as a planet was the discovery of Eris in 2003. Eris is the furthest known object in the solar system. Its size is slightly larger than Pluto. it would be difficult to continue to call Pluto a planet and not to call something bigger than Pluto a planet, so the choice was either call Pluto something else or have a bunch of other things we call planets also.
The Kuiper belt is basically a region in the solar system approximately beyond Neptune to a fair bit further out. Pluto is now considered to be a member of the kuiper belt, the first such object that was discovered. nowadays, there are numerous kuiper belt objects known.
One specific type of kuiper belt object are "Plutons" which are basically objects which are like pluto. one of the key characteristics of plutons is that they have a 3:2 orbital resonance with Neptune (i.e. that they go twice around the sun for every three times Neptune does.) These objects supposedly gradually migrated into such an orbit by regular "tugs" by Neptune, similar to how Jupiter affects a large number of asteroids in the asteroid belt between Mars and Juipter.
Before the discovery of Eris, there wasn't really a formal definition of what a "planet" is. because of the discovery of other objects like Pluto, a definition needed to be created. the definition would either have to exclude Pluto as a planet, or include Pluto and a bunch of other things.
Pluto was excluded by the clause "has not cleared its orbit" which is true because of the existence of other Plutons which orbit the sun in the same range from the sun as Pluto does.
Pluto is one of three "Dwarf Planets" in our solar system. The other two are Eris, which is larger than pluto, and Ceres, the first asteroid ever discovered in the asteroid belt (back in 1801). Ceres contains a large fraction (about 2/3) of all the mass of all the asteroids and it is one of the few which has a relatively spherical shape (which allows it to be also called a dwarf planet instead of an asteroid).
In summary, the solar system has tons of interesting things in it besides the nine (now eight) planets. look around and you can learn a lot. almost every well known object has its own unique features, and despite the fact that technically Pluto is not a planet any more does not take away from the numerous interesting things about it (including the other two moons it has, Nix and Hydra, which were discovered about 2 years ago.
