History of Pluto's

 

Why is Pluto no longer considered a planet?

For a long time, we thought Pluto was unique in the Kuiper Belt. But as astronomers discovered more and more about the Kuiper Belt (and the asteroid belt between Mars and Jupiter), we learned that there are lots of objects like Pluto. More like Pluto, in some ways, than Pluto is like the other planets. Finding all these new objects, it became necessary for astronomers to get more specific about what we mean by the word "planet," and figure out which category Pluto fit into. 

The three rules astronomers of the International Astronomical Union came up with to define a planet are: The object must orbit the sun; the object must be massive enough to be roughly spherical; and the object must have cleared its orbit of any objects of comparable mass to its own (that is, it must be gravitationally dominant in its orbit). Pluto satisfies the first two of these criteria, but not the third. Even one of its own moons, Charon, is about half of Pluto's size. So, rather than being the runt of the planet group, Pluto is now the "king" of the dwarf planet group!
 

What is Pluto made of?

Astronomers believe that Pluto probably has a rocky core. Outside of that, but still deep in the interior, there's likely an ocean of water, covered by another layer of frozen water ice. The surface crust is a layer of various ices, mostly nitrogen ice, with giant mountains of water ice, and traces of methane and carbon monoxide ices. 

Does Pluto have  atmosphere?

Yes. Much like its crust, Pluto's atmosphere is composed of nitrogen, methane, and carbon monoxide. There are also haze particles in Pluto's atmosphere, which scatter blue light. These particles start out high in Pluto's atmosphere as ionized methane and nitrogen. As the ions interact with each other, they combine into more complex molecules, and start to collect an outer shell of volatile ices. As the haze particles get more massive, they start to fall through the atmosphere, collecting more ice.