Jupiter isn't a planet because it is made out of air but it is classified as a planet - so why is Pluto not a planet?

Answer 1

Jupiter is considered a planet in spite of the fact that it is gaseous because it orbits the Sun, is spherical, and cleared all matter from its orbit. The gas that it is made of is still matter and the center has very dense liquid and frozen solids, as well as a rocky core. Its gravitational pull also influences asteroids in the asteroid belt between Jupiter and Mars by altering their orbits around the Sun. Pluto is smaller than some planetary moons and is closer in size to large asteroids such as Ceres. It is now classified as a dwarf planet, along with several even more- distant icy planetoids. Two such objects, Eris, and Sedna may be larger than Pluto. (See links)

There exists a large number of undiscovered small planetoids (plutoids) that are smaller than Pluto but occupy similar orbits. Some may eventually be designated dwarf planets as well.

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The codified IAU definition of a planet does not consider whether a planet is mostly solid or gas, but all have solid cores. There is a new requirement (primary object in its orbit) that Pluto simply does not live up to. Not only Jupiter, but Saturn, Neptune, and Uranus are also mostly gas by volume. (see discussion)

----- It has not cleared its orbit. It shares its orbit with many other asteroids and dwarf-planets in the Kuiper Belt, where Pluto spends most of its time. The idea of Pluto coming within Neptune's orbit is somewhat misunderstood. Indeed Pluto is closer to the sun than Neptune for very roughly 15 or 20 years of Pluto's 248 year orbit, but there is no danger that the two will collide. Their orbits do not actually intersect. Their movements are harmonically locked. For every three orbits of Neptune, there are two orbits of Pluto. Because of this some say that Pluto could even be considered a very unusual satellite of Neptune. But Pluto's relationship with Neptune is not a reason for it being considered a dwarf planet. As a note, if Neptune could be an obstacle to Pluto's status as a planet, then Pluto would be an obstacle for Neptune to be considered a planet! Because of the harmonic relationship between the two, Pluto is not considered a piece of debris that Neptune has not yet captured. Neptune will never capture it.

Answer 2

Pluto was reclassified as a dwarf planet in 2006 because it did not meet all the criteria set by the International Astronomical Union to be considered a full-fledged planet. The decision was made based on its size, orbit, and inability to clear its orbital path of other debris. Jupiter, on the other hand, is a gas giant made mostly of hydrogen and helium, but its immense size and gravitational influence classify it as a planet.