Small Bodies

Claudia Alexander U.S. Project Scientist, Rosetta comet mission Jet Propulsion Laboratory, NASA/Caltech When we look out into space and we're looking for what's out there, what kind of junk might be out there, what kind of stones and boulders and other kinds of planetary bodies are out there, we see asteroids and we see comets. But mostly we see asteroids close to the Earth. In fact, we see asteroids between Mars and Jupiter. There's an enormous belt, called the "Asteroid Belt", where we see most of the asteroids. It's almost as if a planet was trying to form there and it couldn't form for some reason, and so all the boulders that we think collapsed together to make a planet in the first place, they never got a chance to collapse together and create a planet right there between Mars and Jupiter. Further away, we start seeing a lot of comets. There are a lot of comets that go around Jupiter. They are called the "Jupiter family of comets," whose orbits have been influenced by close encounters with Jupiter's gravitational influence. And they go around Jupiter, let's say, in five years and they just are sort of randomly oriented, but there's a lot of them, a lot more of them than we used to think. And further out there's a thing called the "Kuiper belt", and the Kuiper belt is outside of the orbit of Pluto. It's another belt of rocky, icy, boulders that are kind of like the asteroid belt but there's a lot more ice there.

Some people think that what we call the planet Pluto may in fact be a large member of the Kuiper belt objects.

Others think that Neptune's large moon, Triton, which orbits backwards round its planet, may be another.

Claudia Alexander U.S. Project Scientist, Rosetta comet mission Jet Propulsion Laboratory, NASA/Caltech But even farther than the Kuiper belt is a thing called the "Oort Cloud." And the Oort cloud is a is a sphere surrounding the solar system beyond the orbit of Pluto. It's a place that's very, very far away and we haven't even seen it with our own eyes yet.