In the following lecture, we are going to look at our own solar system. And after this lecture, you should have an overview over the different objects that make up the solar system and over the different regions that exist in the solar system. During this course on Big History, we are slowly zooming in from the universe to galaxies to stars. And today we will take a look at our own solar system. Let us begin with the tour of the solar system. Even though the solar system is much smaller than the scales of the universe or galaxy, it is still large. A useful measure is the distance between the Sun and the Earth. Astronomers call this distance one astronomical unit. This will be our measuring rod today. An astronomical unit corresponds to a light travel time of 8 minutes and 20 seconds. And the size of the solar system for today's purposes is about 50 astronomical units or 7 hours of light travel. Do you remember the names of the planets from school? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune? There used to be Pluto in this sequence, but currently we do not look at it as a planet in the same right as the other planets. More about that later. In fact, Pluto might be something much better than just the 9th planet. Let's start by asking what a planet actually is. A planet is a massive object orbiting a star. We have the convention that we require it to be round, because gravity is trying to make it round. And when an object is large enough, it will not be strong enough to resist gravity. For example, your body, my body, the International Space Station are strong enough to resist their own gravity. But when an object becomes about 1000 kilometers in diameter, gravity will win over the hardest rock and the object will be round. Except maybe tiny little structures on the surface like the Himalayas on the Earth. So orbiting around the Sun and being round, those are the most important criteria. A few years ago, an additional criterion was added, saying that the planet should be dominating the region of the solar system that it is in. Pluto does not do that, and that is why we no longer call it a planet. Our solar system is clearly structured into two major regions, the regions of terrestrial planets, close to the Sun, and the region of gas giants. The inner region is the domain of terrestrial planets. These are planets like the Earth. Such planets are made of solid material like rock or iron. They have a surface you could walk on, and the density of terrestrial planets is typically around 4 grams per cubic centimeter. All terrestrial planets in our own solar system have a core made of mostly iron, and the mantle made of rock like material. Terrestrial planets often also have an atmosphere. Let's make a quick tour among them. We start with Mercury, the inner most planet, at 0.4 astronomical units from the Sun. Mercury is small, a bit more than 1 third of the diameter of the Earth. Gravity is so low that Mercury has no atmosphere, and its surface is full of craters from impact of large meteorites. Venus would be our next stop at 0.7 astronomical units from the Sun. Venus is almost a twin of the Earth, same size, same mass, but its atmosphere is 90 times thicker than that of the Earth. Venus is too hot, and it never could form an ocean like the Earth, so everything that could have become an ocean stayed in the atmosphere. And now Venus is suffering from an extreme greenhouse effect, and that is setting the surface temperature to an incredible value of 460 degrees Celsius. Lead would be melting on Venus. Let's step over the Earth for now, and move on right to Mars. Half the size of the Earth. Surface gravity on Mars is 1 third of what we have here. You would be able to run across Mars in large strides. And in fact, at some time in the future astronauts may just do that. Of all the planets, Mars is the most likely one to be visited in the future. Of all the planets, it has the best surface conditions, still freezingly cold, and the very thin atmosphere is less than a percent of Earth's pressure, but still. Mars may have had liquid water in the past, which makes it the prime place in the solar system when we would like to look for signs of possibly past extraterrestrial life. Next stop is Jupiter, the King of planets. Jupiter has more mass than all other planets combined. Its powerful presence affects many other bodies in the solar system. With Jupiter, we enter the regime of gas giants, planets that are very different indeed from terrestrial planets. Jupiter does not have a solid surface you could walk on. It is all atmosphere that just gets denser and warmer with depths, but you would never reach a solid surface. Jupiter is made mostly of hydrogen and helium, just like the Sun. Of the heavier elements it contains, a large fraction is confined to the core which is estimated to be about 10 Earth masses. The gas giants formed by first assembling a hardcore, similar to what the terrestrial planet would look like, but then they start attracting gas as well and grow to enormous sizes. Jupiter has more than 10 times the diameter of the Earth, and more than 300 times the mass. Saturn, lord of the rings, at about 10 astronomical units. A slightly smaller version of Jupiter, but most well known for its majestic rings. The rings orbit so close to Saturn that tidal forces prohibit the ring particles from agglomerating into moons. Saturn also has moons, many of them, but those orbit at larger distances from the planet. Let's move on to Uranus at about 20 AU and to Neptune at about 30 AU. Both are also gas giants, but with a much smaller amount of gas in the atmosphere, only about 8 Earth masses. There is around a core of similar mass. So these are the planets that cover it. Is this all in the solar system? Not quite. The solar system also contains a huge amount of smaller objects, which sizes of, let's say a few kilometers. Actually the size distribution of spans from 1000 kilometers down to tiny dust grains, but I would like to focus on the middle sizes because there are so many of them and they are so interesting. These smaller bodies are mostly avoiding the regions where the planets are, because they would be kicked out of the system by a close encounter or they would crash onto a planet rather quickly. So if they are avoiding the regions where the planets are, where else can they be? Well there are three regions. The first one is a so-called asteroid belt, between Mars and Jupiter. Serious at War of Planet is the largest member and lots of smaller objects are there as well. They are called asteroids and they are being pushed around by forces of Jupiter who reach this down into the belt and sometimes sends an asteroid deeper into the solar system where it might collide with the Earth. The second region is the Kuiper belt, right outside the outermost planet Neptune. The bodies here are made to large extent of ice. Sometimes one of them gets pulled into the inner solar system, is heated by the Sun and develops a large tail of gas and dust that can be seen by naked eye, a comet. The Kuiper belt may be the most exciting discovery in the solar system during the last 20 years and this is why I said earlier that Pluto may even be something better than just a 9's planet. Pluto was the first member of the Kuiper belt ever discovered and in that way it is the key to opening a new door onto the history of the solar system, more about that during the next lecture. And finally the third region where these small bodies live is called the Odd Cloud. It's much further away, almost half way to the Neil star, very far away. Potential comets reside here as well and we only know about the Odd Cloud because sometimes a passing star sends down one of them to come close to the Sun and become yet another comet. You might ask why do I spend so much time talking about the smaller bodies in the solar system? This will become clear during the next lecture when we talk about the history of the solar system. Small bodies like asteroids and comets play a big role here.