Why is Pluto excluded as a planet?

Answer 1

Because it was too small and it seemed to be following some other orbit rather than our sun. Its orbit was off from ours by too much time. It was faster than it should have been. ______________ I considered editing the question, but left it because it might resonate with some popular misconceptions. Pluto is still out there, and our struggles to categorize it have not diminished it one bit. It remains part of our solar system and will remain so as long as the solar system exists, barring some unforeseen disaster. It will continue to be a fascinating and inspiring object and one worth studying. Neither its size nor the speed of Pluto's orbit are factors in Pluto's categorization, although many point out that the region that far out in the solar system seems not to be the right place for a planet so small. That part of the solar system is where the gas giants live, and the smaller more rocky planets live near us. Pluto is simply no longer considered a major planet. It is a minor planet. In order for a body to be considered a planet it must orbit a sun or the remnant of a sun. Pluto does that. It must be large enough so that by force of its own gravity it has formed itself into a sphere-like shape. [Planets are not perfect spheres.] Pluto has done that too. So far so good. It must also have cleared its neighborhood of debris. No planet has done this completely; there are chunks of rock and tons of dust and ice floating around. But the major planets have no objects in the vicinity of their orbits large enough to challenge their primacy in their orbits. Here is where Pluto runs into trouble. Pluto's orbit is roughly 248 earth years. During that time Pluto spends roughly 12-14 years in the part of its orbit that takes it closer to the sun than Neptune, the outermost major planet. The rest of the time it is in what is called the Kuiper Belt, a region beyond Neptune that is filled with icy chunks, some of them rivaling the size of Pluto. Probably many of the comets that we sometimes observe spend most of their time there. In fact, Pluto may be very much like a large comet, and might act like one if it got close enough to the sun. It is all the time that Pluto spends among the many other objects in the Kuiper Belt that disqualifies it from being a major planet. Often, Pluto is referred to as a Kuiper Object. So Pluto fans, do not despair. Pluto is in reality eveything it ever was. We just don't include it on the list of major planets any more.

Answer 2

Pluto was re-classified as a dwarf planet in 2006 due to the discovery of more and more objects of a similar size, out beyond the orbit of Neptune. The question was to either call these planets (adding to the list of nine), to keep the list of nine as it was, or to redefine what a planet actually was. The International Astronomical Union (IAU) chose the latter, which meant that Pluto had to be downgraded, as it no longer met the new definition. This will maintain the list of planets at 8 for the foreseeable future.

A Planet has to be approximately round, has to orbit the sun directly (not like a moon that orbits around another body) and has to have cleared its neighborhood of all other objects - so that at that distance from the sun, there are no other sizable bodies. Pluto is not large enough to have done the latter, while all other remaining planets have, so it was downgraded to a dwarf planet. There are now eight planets recognized and five dwarf planets, of which Pluto is one. New candidates for dwarf planet status will be considered at regular intervals.

Answer 3

The discovery of other similar objects to Pluto in more recent years has meant that astronomers either had to keep adding planets to the list, or we had to redefine what a planet actually was. In 2006, a new definition for a planet was established. It had to be approximately round, it had to orbit the sun and it had to have cleared its neighbourhood of all other objects - so that at that distance from the sun, there were no other sizable bodies.

Pluto is not large enough to have done the latter, while all other remaining planets have, so it was downgraded to a dwarf planet. There are now eight planets recognised and five Dwarf planets, of which Pluto is one.

Answer 4

Pluto was excluded because of a new classification of planets:

1. must be big enough to have enough gravity to be (roughly) spherical.
2. must orbit the Sun directly.
3. must be in a clear orbit around the Sun. That means it must be the dominant object in the neighborhood of its orbit.

The only thing that Pluto doesn't have is a clear orbit. This is because it is in the Kuiper Belt. But, I still think Pluto's a planet and many other people believe it to be a planet. I hope you do too.

Answer 5

Pluto has not been excluded from the solar system.

However, Pluto is no longer considered to be a "planet" by the International Astronomical Union. The IAU had never formally established a definition for "planet", and with the recent discoveries of several planet-like objects beyond the orbit of Pluto, the IAU decided that it was time to establish some rules for "What is a planet".

The "What is a planet" rules they came up with in 2006 have been a bit controversial, because by that definition, Pluto didn't qualify.

So Pluto, the asteroid Ceres and three other bodies have been reclassified as "dwarf planets". It seems quite likely that the number of "dwarf planets", now at five, is likely to grow dramatically with the development of increasingly more powerful space-based telescopes.