What is the Kuiper Belt?

What is the Kuiper Belt? - I've lost track of who asked this one... sorry!

A plot of solar system object data. Units are in AU, Kuiper
belt objects are green. Not sure who to attribute this to, sorry!
Starting at around the orbit of Neptune*, the last planet from the Sun, extending out to about 50 AU and staying relatively close to the plane of our solar system, there's a region of space that can, in some ways, be compared to the asteroid belt that lies between the orbits of Mars and Jupiter. The asteroid belt is a belt (a region of space roughly and fuzzily circular around the Sun) that contains rather more asteroids than most of the rest of the solar system can be said to.

Now, here's where I wish I'd thought a bit more carefully about that last sentence before I started writing it, because the Kuiper belt isn't, as you may be forgiven now for thinking, a belt that contains rather more Kuipers than the rest of the solar system. It is, however, a belt around the Sun that contains rather more stuff than the rest of the solar system does. This is because a "Kuiper" isn't a thing, but the name of a person.

Anyway, the stuff that the Kuiper belt is made up of is largely different varieties** of ice, mixed up with smaller quantities of rock and clumped in lumps of varying sizes. The dwarf planets Haumea, Makemake and our dearly beloved Pluto are objects fitting this description that reside in the Kuiper belt. Over a thousand Kuiper belt objects have been discovered so far, the first being Pluto in 1930. The belt itself, however, was only discovered in 1992, and the object that caused this discovery was the snazzily named "(15760) 1992 QB1".

While the Kuiper belt was discovered in 1992, the existence of such a region has been put forwards with varying degrees of accuracy by various astronomers since very soon after Pluto was discovered. There is some controversy as to who deserves the credit for predicting it.

The largest known object in the Kuiper belt is Eris*** (bigger than Pluto, and known initially as Xena****). The smallest Kuiper belt object discovered so far that I'm aware of is a bit less than a kilometre across.

* That's about 30 AU from the Sun.
** The methane variety, the ammonia variety and the common-or-garden water variety.
*** Though Eris is technically in the 'scattered disk' part of the Kuiper belt, rather than the nice, stable part that you'd take home to meet your mother.
**** I don't know why, but my guess is that this was a candidate for being the 10th planet. It's arguably Eris/Xena's fault that Pluto isn't a planet any more.


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