The solar system part 1

0:00- [Instructor] When ancient humans looked up at the night sky night after night, they found some stars wandering relative to the rest. So, they call them asteres planetai, which is Greek for wandering stars. But wait, notice the word planetai? Hey, that's where the English word planet comes from, because guess what?

0:21Today, we know these weren't stars at all. These are planets. And we know something even more incredible. All these planets, including the Earth, are part of a single system, the solar system. So, let's explore that in detail. Let's start at the center, the Sun.

0:39The Sun is a star just like all the stars you see in the night sky, except that it's much, much closer to us. And our sun is enormous. You could fit over a million Earth inside it. In fact, the Sun contains most of the mass in our solar system. Because of this, its enormous gravity pulls everything towards it.

0:59In fact, that's why everything in our solar system orbits the Sun, and it's also the reason why we call it the solar system. Sol means sun. But what is the Sun made of? Well, like most stars, the Sun is mostly made of hydrogen, the most common element in the universe.

1:17But its gravity is so strong that, at the core, it squeezes hydrogen atoms together at temperatures over 15 million degrees Celsius. And when that happens, hydrogen fuse together into helium. This process is called nuclear fusion. And fusion releases huge amount of energy.

1:42That's what makes the Sun shine. That's what gives us light, heat, and energy for life. So, in a sense, the Sun is like a giant natural power plant running continuously for billions of years. That's right, scientists believe that Sun formed about 4.6 billion years ago and will continue for another 5 billion years or so.

2:06All right, now, if we move outwards, our solar system has eight planets. The first four are Mercury, Venus, Earth, and Mars, and the next four are Jupiter, Saturn, Uranus, and Neptune. And by the way, this diagram is not drawn to scale. If we try to do that, this is kind of what it it will look like.

2:29See how far Jupiter is compared to the first four planets? And if we zoom out, this is where Saturn is, and Uranus and Neptune are so much farther. And at this scale, we can't really see the planets, or even the Sun, for that matter. So, just gives you a sense of how far planets are in our solar system.

2:53Anyways, let's go back to this picture. Now, you may have heard of Pluto. It used to be considered the ninth planet, but in 2006, it was reclassified as a dwarf planet. So, we'll explore that in the next video. Anyways, so our solar system has these eight planets.

3:09And guess what, there are some very interesting patterns among these planets. The first four planets are small, rocky worlds with solid surfaces. Therefore, we call them terrestrial planets. Terra is a Latin word for Earth, rocky. (laughs) And the next four are massive, so we call them giant planets.

3:31Earlier, they were called gas giants because they don't have hard surfaces, they have a lot of swirling gases. But today, Jupiter and Saturn are called gas giants, whereas Uranus and Neptune are called ice giants. So, for our purposes, we're just gonna call them giant planets.

3:50The largest planet, Jupiter, is so big that you could fit over a thousand Earths inside it. Now, the difference in the size between these two planets also helps us understand another interesting pattern, their moons. Since the terrestrial planets are small, they have weaker gravity compared to the giant ones.

4:11This is one of the reasons why they have very few moons. In fact, Mercury and Venus don't even have any moons. Earth, as you probably know, has one moon, which we just call it the moon. And Mars has two moons, Phobos and Deimos. In contrast, the giant planets, with their strong gravity, can hold on to a lot of moons.

4:30Jupiter has at least 95 moons, and there could be more. Saturn has the most number of moons in our solar system, 146 and counting. And Uranus and Neptune also have a lot of moons. And some of these moons are quite fascinating. For example, Ganymede, the moon of Jupiter, is the largest moon in our solar system.

4:52It's even larger than the planet Mercury. And guess what? Io, another of Jupiter's moon, happens to be the most volcanic object in our solar system. And Europa likely has an ocean beneath its icy surface, making it a strong candidate for life. If you go to Saturn, its most famous moon is called Titan.

5:15One reason it's famous because it's the second largest moon in our solar system. But it also has a very thick atmosphere and even has lakes of liquid methane, making it the only place in our solar system that we know of besides Earth with stable liquids on the surface.

5:33So, this is another possible location for life. Finally, if you look at Triton, one of the moons of Neptune, it orbits in the opposite direction compared to the Neptune's rotation. Why? Well, we think that Triton was an object from a different region of solar system altogether.

5:54It was probably just passing by, it came very close to Neptune, and just got captured by its gravity and started going around it. But wait, there's more. All four giant planets have rings. That's right, it's just that the rings of Jupiter, Uranus, and Neptune are mostly made of fine and dark dust.

6:13That's why they are faint and hard to see, but they're there. Saturn's rings are more prominent because they're mostly made of bright ice, which reflects a lot of sunlight, making them stand out. And that's why we usually think that Saturn is the only planet which has rings, but all four of them have it.

6:31But why is Saturn's rings so different? Well, one leading idea is that one of Saturn's icy moon broke apart quite recently, forming this bright, highly reflective rings. Now, let's talk about their motion. Planets have two kinds of motion. One is that they spin or rotate around their own axis.

6:54Another one is that they go around, or they revolve around, the Sun. Now, one amazing pattern we see is that terrestrial planets tend to spin much slower compared to the giant planets, which means the terrestrial planets have longer days compared to the giant ones.

7:13But of course, because these terrestrial planets are much closer to the Sun, they revolve faster compared to the giant planets, which means these terrestrial planets have shorter years compared to the giant ones. Now, of course, one nuance over here is that when it comes to spin, this is just a general trend that we observe.

7:34It's not a strict rule. A lot of factors decide how fast a planet spins. This include their mass, how they were formed, and also how close or far they are from the Sun. But when it comes to the years, this is a strict rule. The farther you are, the more time it takes for you to go around the Sun, and so, longer the years.

7:58So, let's take some examples. A day on Mercury lasts about two Earth months, which is pretty long, right? But a year takes only about three Earth months, which is quite short. And Venus is even more extreme. A day lasts about eight Earth months while its year is slightly shorter, about seven and a half months, which means a year on Venus is shorter than its day.

8:28Now, compare that with Earth and Mars. Here on Earth, things feel more familiar, right? A day is about 24 hours, or by definition, one Earth day, and a year is about 365 days, or by definition, one Earth year. On Mars, the day is very close to one Earth day, but the year is longer because it's farther away.

8:48So, it's about two Earth years. Now, compare this with the giant planets. Their days are actually shorter than Earth's, often less than 24 hours, but their years are incredibly long. This brings us to another pattern that we can predict, temperature. Quick question.

9:10Which of the two sets of planets do you think has higher temperature? Well, we would expect the terrestrial planets because they're closer to the Sun, right? And they are, they're generally hotter, and these are generally cooler. But which do you think is the hottest planet of our solar system?

9:25Well, I would guess Mercury, because it's the closest to the Sun. But turns out, Mercury is not the hottest planet. It's actually Venus. But how does that make any sense? Well, Mercury is pretty hot. I mean, during the day, temperatures can reach close to 430 degrees Celsius.

9:43But Mercury lacks an atmosphere, so it cannot retain that heat. And as a result, during the nighttime, the temperatures drop to very low values, about negative 180 degrees Celsius. And remember, Mercury spins very slowly on its own axis, so the hot side stays hot for a long time and the cold side stays cold for a very long time.

10:07So, look, on an average, the temperature is somewhat in between. It happens to be about 167 degrees Celsius. It's still pretty hot. Now, compare that to Venus. Venus is farther from the Sun compared to Mercury, but it has a super thick atmosphere made of carbon dioxide with clouds of sulfuric acid.

10:30This traps heat through something called the greenhouse effect, making it hot enough to melt lead. That's why Venus is the hottest planet in our solar system. If you move outwards, Earth has a balanced atmosphere, allowing liquid water to exist, one of the key reasons why life exists on Earth.

10:52And Mars has a very thin atmosphere, so it's much colder. It has polar ice caps, it also has seasons, and there's evidence of past water. But today, it's a cold and dry world. Now, what do you think about the temperatures of the giant planets? Well, since they are much farther away, they receive very little energy and are extremely cold, and their average temperatures are way below negative 100 degrees Celsius.

11:20And their atmospheres are pretty active. I mean, Jupiter has the Great Red Spot, a storm that has lasted for centuries. Now, it's shrinking in size, but as of today, it's as big as the Earth. And Neptune turns out to have the fastest winds in the solar system.

11:39Its winds can reach up to 2,000 kilometers per hour. Finally, notice that unlike most planets, which spin like a top with a little bit of tilt, Uranus is tilted by almost 90 degrees. Why? Well, one leading idea is that Uranus experienced a massive collision a long time ago with some large object, and that impact may have knocked it over.

12:03So, to sum it all up, our solar system has these eight planets. The inner four planets are small, rocky terrestrial planets. They have very few moons, spin more slowly, revolve more quickly, and tend to be hotter. The outer four are giant planets. They have many moons, they have ring systems, they spin more quickly and revolve more slowly, and are much colder.

12:27Now, what blows my mind is that those wandering stars that ancient humans saw weren't wandering randomly at all. They were following precise patterns, all shaped by the Sun and its enormous gravity.