 Greetings and welcome to the introduction to astronomy. In this week's special topic in astronomy we are going to discuss the planetesimal and what that means in astronomy. So what is a planetesimal? Well, let's look at this. Well, a planetesimal is a bit of material that eventually built up into planets. So it is the material that eventually formed the planets. These are what formed our planets billions of years ago including Earth very early in the history of our solar system. So they would have been the little bits of material as they got built up a little bit larger. Things would tend to grow in size and collect more and more material. And then they would become what we call a planetesimal and those would then build up into the planets. However, these are still around. Not all of these formed into planets. So some are left behind and can still be studied today. So let's take a look at that. And we see here some of the examples of these. And some of those are the objects in the asteroid belt, the Kuiper belt and the Oort cloud. So these are bits of material that were left over. So here, for example, we show the orbits of some of the very outer objects in the solar system. So all of the planets down in here very close to the center. And then we see Pluto out here and another Kuiper belt object. So it's a belt of objects out beyond Neptune. So these are objects that did not form into one of the planets. Either a planet didn't form out there or this is material that was kicked out of the solar system during the stage of planetary formation. The Oort cloud is similar. That is where a lot of the comets are. And that is a much larger area. You can see that the inset there going way down into the very central portion here is where our solar system is. And then the Oort cloud, the outermost parts are again a little bit of that debris that we now see sometimes as comets when they end up in the inner portions of the solar system. In terms of the asteroid belt, we do have samples here. And in fact, this is the asteroid known as Bennu which was recently visited by the Osiris-Rex spacecraft and a sample was collected to return to Earth. And not all that long before this, there was a sample. The sample had been returned and is now being opened for study here on Earth. So that is something that will tell us a little bit about this early material. Why do we want to study these little bits of material? Well, these are the things that made up Earth when the planets were forming billions of years ago. All the little things like Bennu here are part of the material that formed Earth. However, these are essentially unchanged. What's on Earth has been melted and changed since then. And this remains unchanged so we now have a sample of that material to study. Now there's one more we can look at. This is the object known as Arakath out in the Kuiper Belt. And this is another one that has been studied. This was studied by the New Horizon spacecraft which passed by Pluto in 2015 and was then redirected out to study Arakath. And it's another again example of that type of object that was left over that formed the planets. But the material we find out in the Kuiper Belt is generally more icy and was likely what formed the outer planets. Studying what is in the asteroid belt helps us learn about the material that formed Earth. And studying what is out in the Kuiper Belt helps us study what formed the outer planets, the larger planets like Jupiter, Saturn, Uranus, and Neptune. So let's go ahead and finish up with our summary. And what we've looked at today is that the planetesimals are the material that built up the planets billions of years ago. They slowly accreted together to form the larger objects that we see today. But examples remain in many places in the solar system for us to study today. So that concludes this lecture on planetesimals. We'll be back again next week for another special topic in astronomy. So until then, have a great day everyone, and I will see you in class.