Our solar system is a planetary system. Some planets and other smaller objects orbiting around one or more stars create a planetary system. Our system is centered (almost) around the Sun, our closest star.

The most prominent components of the solar system are: planets, dwarf planets, moons (natural satellites), asteroids and comets. As far as we know, there are 8 planets, 5 dwarf planets, more than 200 moons, more than a million asteroids, and more than three thousand comets in our solar system.

Explore the solar system using https://eyes.nasa.gov/apps/solar-system. The following video shows some key attractions from the simulation given in the website.

The first thing that strikes anyone watching the solar system from a god’s eye view is that the system is flat, almost all the objects orbit around the sun on a single plane. The reason will be clear when we discuss the birth of the solar system.

The sun accounts for almost 99.8% of the total mass of the solar system. The system was born from a nebula, and only 0.2% of the material in that nebula could give rise to everything other than the sun.

Watch the video and get your hands into the simulation link given above. However, a general overview of the solar system is better understood from the four panels shown below.

In the first panel (top left), you see the 4 inner rocky (terrestrial) planets—Mercury, Venus, Earth, Mars—, the asteroid belt containing more than a million asteroids, and the planet Jupiter.

Now zoom out and go to the second panel (top right). The enitre first panel fits into the smallest orange circle of the second panel. Here we begin from the orbit of Jupiter, and then you see the other three outer Jovian planets, or gas giants as we call them: Saturn, Uranus and Neptune. Outside Neptune, you also see the Kuiper belt (কাউপার বেল্ট), the dwelling place of many of our comets.

Zoom out to the third panel (bottom right) and the entire second panel fits into the small purple circle of the third panel. Here you see the extremely elliptical orbit of the dwarf planet Sedna (red ellipse).

Zoom out more and you see that the entire orbit of Sedna fits within the small red ellipse of the fourth panel (bottom left), and this entire region is extremely small compared to the Oort cloud.

Here you see the distances of various objects from the sun. The sizes and distances are not to scale. All distances are shown with respect to the distance of earth from the sun: 1 astronomical unit (au), or 150 million km.

Clearly, Saturn is 10 times as far from the sun as earth, or 10 au from the sun. Neptune is 30 au from the sun. The Kuiper belt (made of small icy objects) spans from 30 au to 50 au. The heliopause (more on this in the next section about the sun) is at 100 au. The human-made satellite Voyager 1 has already crossed the heliopause and entered interstellar space.

Oort cloud, the ice-cold home of trillions of potential comets, begins at around 1 thousand au and might as well end at 1 lakh au. But most of its mass is within 1–10 thousand au from the sun. The first comet captured by our Ashvin 2 telescope was C/2022 E3 (ZTF). Do you know where the comet came from? Oort cloud.

The outer edge of Oort cloud is almost 3 light years from the sun, very close to the nearest star to the sun, the famous $\alpha$-Centauri or Proxima Centauri.

All objects of the solar system actually do not orbit the sun, but orbit the center of mass of the solar system. However, as the sun accounts for 99.8% of all mass in the system, the center of mass is actually inside the sun. So for practical purposes, we can say that the sun is at the center of our system.

Ths Sun. A gigantic glowing ball of hydrogen and helium gas almost one and a half million km in diameter. Hydrogen accounts for almost 71% of the total mass of the sun, helium accounts for 27% and the remaining 2% comes from all the other elements.

The sun is very big. You have to put almost 100 earths side by side to get to its diameter. But even such a big star is pretty small compared to many other stars in our galaxy. Stars as much as 100 times heavier than the sun has been found.

The sun orbits around the Milky Way once in 230 million years. And just like earth, sun also rotates along its own axis. But because the sun is not solid, different parts rotate at different rates. The equatorial zone rotates once in 25 earth days whereas the polar zone rotates once in 35 earth days.

The sun is extremely hot. The temperature at its core (center) can reach as much as 15 million degrees celsius. The surface temperature is much colder, only around 5,500 degrees (compare with the boiling temperature of water, only 100 degrees). The surface is called the photosphere. There is a thin corona surrounding the photosphere which is something like the atmosphere of the sun. Temperature in the corona can be as high as 2 million degrees celsius.

The sun has spots. No, they are not holes on the surface. They look darker because the temperature in the sunspots is lower than the surroundings. The temperature decreases because at these places the magnetic field of the sun pokes out from the interior to the atmosphere.

Yes, the sun has very strong magnetic field which extends out into to the farthest reaches of the solar system. The magnetic field pervading through the solar system is created by solar wind, charged particles streaming out from the sun.

The region dominated by solar magnetic field and solar wind is called the heliosphere, the kingdom of the sun which ends at a boundary called the heliopause. NASA’s spacecrafts Voyager-1 and Voyager-2 both have crossed the heliopause and entered interstellar space as you see above.

The four inner planets (Mercury, Venus, Earth, Mars) are similar as they have rocky surfaces. They are called terrestrial because terra stands for ‘earth’ in Latin.

Mercury is very small, only slightly larger than the Moon. It is so close to the sun that its daytime temperature is a whopping 430 degrees, but the temperature can drop to as low as -290 degrees at night. If you stand on its moon-like scarred surface, the sun would look 3 times bigger. Its entire surface has been mapped by NASA spacecraft MESSENGER by 2015.

Venus is farther from the sun, but its temperature is higher than Mercury because of its dense temperature and extreme greenhouse effect. Venus is Earth’s twin sister: they are similar at heart but very different in appearance. Their size, mass and density are similar, but the surface, temperature and atmosphere are very different. Temperature can be as high as 475 degrees and the air pressure on the surface is 90 times that of earth. This is almost the same pressure you would feel a kilometer below the surface of earth’s oceans.

Earth has a very special natural satellite, the moon. No other planet has such a large moon compared to its own size. The earth-moon system is almost a twin-planet system orbiting around the sun. The earth is the fifth largest planet in the solar system and the only one with liquid water on the surface. A picture of earth was taken by NASA’s spacecraft Voyager 1in 1990 that revolutionized human culture because earth looked like a tiny pale blue dot in the picture taken from a distance of 6 billion km.

Mars is a cold and dusty planet with a thin atmosphere. It looks red in the nightsky because of its rusty dust. It has the largest volcano in the solar system, the Olympus Mons whose base is two times bigger than Bangladesh. In the past, Mars might have been a planet with liquid running water on the surface. There are traces of rivers, lakes and deltas on its surface. There could have been huge floods on the planet 3.5 billion years ago. Even today, there are a lot of ice on mars, especially in the poles. Mars is very cold, temperature as low as -60 degrees celsius. It has two moons: Phobos and Deimos, very small compared to our moon, the Moon.

The name ‘Jovian’ comes from Jove, another name of Jupiter, the largest planet in the solar system. Jupiter, Saturn, Uranus and Neptune are alike because they are all dominated by gas. They are giant spheres of gas similar to stars, but they are not stars because they cannot glow as bright as a star, their inner temperature is not high enough to make them shine or start nuclear reaction. They do not have solid surface.

Jupiter is twice as massive as all the other planets combined. The stripes and swirls on its surface are actually clouds of ammonia and water floating in its atmosphere dominated by hydrogen and helium. The Great Red Spot (visible above) is a storm bigger than the whole earth that has been raging for hundreds of years. Jupiter orbits the sun once in 12 earth-years, but rotates around its own axis in only 10 earth-days.

There are almost two million asteroids organized in a belt between Mars and Jupiter, but some asteroids are also found in the orbit of Jupiter itself. They are called the Greeks and the Trojans. Jupiter can influence the asteroids and sometimes push them toward other parts of the solar system.

Jupiter has 79 moons, but the 4 discovered by Galileo in 1610 (Io, Ganymede, Europa, Callisto) are the most massive and influential. Io is a fiery moon, the place with the highest volcanic eruptions in the solar system. Ganymede is the largest moon in the solar system and the only one that has a magnetic field. Europa is the only place besides earth in the solar system where some form of life might exist.

Saturn is famous for its rings made of billions of ice and rocks coated with dust. It is the farthest planet visible with naked eye. It is surrounded by almost 60 moons some of which have the most amazing landscapes in the solar system even though Saturn itself does not have any land. Water jets come out of the moon Enceladus. There are lakes of liquid methane in Titan. The rings are probably made of fragments of comets, asteroids and moons destroyed by Saturn’s powerful gravity before they could come closer to the planet.

Uranus also has rings, 13 rings (not as shiny as those of Saturn), and 27 small moons. It is the most peculiar planet as it rotates with respect to an axis that lies almost in the plane of its orbit; it is a sideways-world. The video above explains this tilt. If you remember the reason behind seasons in the first lectures, you will immediately realize that Uranus has some of the most extreme seasons, extreme heat and extreme cold. Because it has the largest tilt of the axis. Uranus is an ice giant; it has a small rocky core, but more than 80% of the planet is made of icy cold water, methane and ammonia.

Neptune is cold and windy like all other Jovian planets, but much more so. Discovered in 1846, this planet is 30 times farther from the sun than the earth. Sun appears very faint there, 900 times fainter compared to the sunlight on earth. It is an ice giant like Uranus and, hence, also has thick layers of icy things surrounding a solid core as big as earth.

The birth of a solar system is intimately linked with the birth of its parent star. The planets, dwarf planets, natural satellites and small solar system bodies are like the children and grandchildren of the star.

We will talk about the birth of a star in a later lecture. Now let us try to understand the birth of our solar system from our observations of the objects in the solar system.

What is the most striking feature of the solar system?

The most striking feature is that all the planets move around the sun in the same direction and in on the same plane. But why? The planets are far from each other; how could they group together and decide to orbit the sun in the same direction and on the same plane? Here lies the clue behind the formation of the solar system. The sun, all the planets, satellites, dwarf planets and small solar system bodies, everything formed from a single giant molecular cloud. The sun occupied the central position and everything else moved around it in the direction that the cloud was moving in the very beginning.

This is how the cloud surrounding our young sun might have looked like 4.5 billion years ago. All stars and planetary systems form from rotating and shrinking molecular clouds. Initially the cloud must have been extremely big, maybe even a few light years, trillions of kilometers. The cloud began to shrink when it became too heavy. Heavy things shrink and collapse toward their own center. Initially the cloud was big, spherical and fast rotating. As it shrank, it became more flat like a disk and its speed of rotation increased even more. The faster it rotated the more flat it became. This is the reason everything in the solar system is on a single plane. Because they formed from a single cloud of gas and dust that was shaped like a disk.

Okay. We get the point that most of the mass got to the center and created the sun. But why did the gas disk surrounding the center fragmented and coalesced into planets?

We do not know for sure. But the above picture shows a plausible mechanism of forming rings of materials in a disk. As the disk shrinks and rotates even faster, the outermost part of the disk expands itself into a separate ring because of too much outward push caused by the rotation. As the ring expands, it cools down and shrinks again due to inward gravity. A balance is established between the outward rotational push and the inward gravity but the ring does not merge into the disk again, it remains independent.

The disk is now surrounded by one ring. After some time, again the outer part of the disk separates itself inward of the first ring and a second ring is produced. In this way, the whole disk could be fragmented into several rings. The material in a particular ring could have condensed and coalesced into planets later.

Okay. But why are the inner planets rocky and the outer planets gassy? We do not have time to discuss this question this semester.