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Pluto has been designated a planet in our solar system for years (ever since it was discovered in the last century), but a couple of years ago it was demoted.

What caused this decision? And is there a chance that it could be reversed?

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So cool that we now have the New Horizons data from Pluto! – peSHIr Jul 15 '15 at 10:56
up vote 25 down vote accepted

Pluto is now classified as a dwarf planet. The main difference between a planet and a dwarf planet has to do with the requirement that a planet clear out the material in and near its orbit. Planets do this, dwarf planets do not.

The reclassification was triggered by the discovery of many additional object (the Edgeworth-Kuiper Belt) out beyond the orbit of Neptune. Some of the objects are nearly as big as (and is a few cases, possibly bigger than) Pluto and in very similar orbits. Thus it was realized that Pluto was just the largest of a large number of objects in the outer solar system.

This is simply science at work. At the local university, we have an Astronomy textbook from the 1800's that lists the 12 planets: Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, Uranus, and Neptune. However, as more objects were detected between Mars and Jupiter, it was realized this was a new class of object and the middle four were downgraded from planet status to asteroids. It is the same process at work today out in the outer solar system.

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I love the tidbit about Ceres, Pallas, Juno, and Vesta. I never knew that! – Joe Jun 2 '11 at 4:35
@Joe in a very short period from 1801 through 1807 the 4 largest asteroids were discovered, and it wasn't until nearly 4 decades later in 1845 that another asteroid was discovered. But within a decade after that dozens more asteroids were discovered. You can see the impact of this discovery timeline on planetary nomenclature. – Wedge Jun 2 '11 at 5:57
what do you mean by saying: "planet clear out the material in and near its orbit" ? – Templar Jun 12 '11 at 14:15
It means that they are large enough to gravitationally either acrete or eject the material from the area of their orbit around the sun. There are lots of objects out in the Kuiper Belt that are on similar and nearby orbits to Pluto, it didn't clear these out since it isn't big enough. – dagorym Jun 12 '11 at 14:45

Pluto is still considered a dwarf planet. This was because it did not meet the full criteria for being classified as a planet. Most notably it did not clear its orbit of other debris.

This is still considered controversial as many scientist do not agree with the definition of what a planet is and still consider Pluto to meet planet criteria. So it is possible that this may change especially if the criteria for what is a planet changes due to new discoveries. This is very possible with the work the Kepler telescope is producing. As we discover new planets in large numbers you can be sure they will be finding ever different planets that will force a redefinition of what a planet is.

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I doubt that Kepler telescope would be able to detect any Pluto-sized bodies in other systems. – Tigran Khanzadyan Jun 1 '11 at 22:21
@Tigran, probably not. But it will detect other bodies which may challenge our current definition of a planet. – John Conde Jun 1 '11 at 23:08
@CarsonMyers: The Earth-Moon barycenter is barely inside the surface of Earth; it's about 1000 miles below the surface, and would be above the surface if the Moon were about 35% farther away, or if were about 35% more massive. – Keith Thompson Oct 12 '11 at 2:16
@Keith I thought the barycenter being inside the planet was a requirement, I could be mistaken though. – Carson Myers Oct 12 '11 at 4:22
@CarsonMyers: It isn't, according to the link in the answer. – Keith Thompson Oct 12 '11 at 4:26

As many already said, Pluto is now considered a "dwarf planet"

For your second question, there is no chance that Pluto will be reclassified as a planet again.

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Pluto has been reclassified as a dwarf planet.

It was reclassified as such because a growing number of objects were found similar to Pluto, which exhibited at least one notable difference from the other planets. The choice would have been to accept these other objects as planets or to develop a new class of object.

The primary features of a planet are that it has an orbital path 'clear' of other debris, orbits around the sun, and is massive enough to maintain hydrostatic equilibrium. Pluto (as well as other dwarf planets) fail to meet the first criterion.

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If you are interested, there is an audio recording of the IAU General Assembly session on the definition of a planet

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I don't really care what the IAU voted. Pluto will always be a planet in my book. Astronomy is full of historic inaccuracies that we perpetuate for tradition sake. Some examples that come to mind are early/late-type galaxies, Population I/II/III stars, and brown dwarfs.

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Is Ceres a planet in your book? What about Haumea, Makemake, and Eris? The real question is, would Pluto have been called a planet if we'd known what we know now about the bodies that exist int the Solar System? – Keith Thompson Oct 12 '11 at 2:19

Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn and Uranus are the primary planets in the Solar System.

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secondary planets are also known as "moons". – Dromaeosaur Nov 22 '11 at 10:16
Wait, what? Vesta, Juno, Ceres, Pallas are all primary planets? What's your definition of a primary planet? – LarsTech Nov 23 '11 at 0:13
This answer seems meant to draw attention to the fact that the demotion of Pluto is not a historical first. Several of the large asteroids were treated as planets for a while before they where identified as members of another class. – dmckee Dec 12 '11 at 20:57
If you had posted this answer in the early 19th century, I would have given it a +1. As it is, posted without explanation and not really answering the question, it deserves a -1. – ghoppe Dec 28 '11 at 23:17

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