The Cosmos		$$
«Celestial Bodies & the Universe		The Solar System

The Universe - 02

The Solar System

The Solar System (Fig. ), also known as the Solar Family, is made up of the Sun, the planets and their satellites, and other celestial bodies like the asteroids and comets.

The Solar System has the Sun at its center. The Sun keeps its solar family together with its gravitational pull. The planets, asteroids and comets revolve around the Sun in different orbits (but in more or less the same plane) (Fig. ).

The Solar System is considered to be about 4.6 billion years old. It came into existence from dust and gas of the ancient Solar Nebula clumping together gravitationally and flattening over millions of years (Fig. ). The central core of the disk-shaped nebula finally compressed and heated enough to start nuclear fusion, and the Sun was born. Smaller clumps of material on the disk away from the central core led to the formation of planets.

The Sun

The Sun is an average sized star—a huge ball of burning hydrogen and helium gas—and contains 99% of the mass of the solar system. Like most stars, it is powered by nuclear fusion in its core which converts hydrogen to helium with release of vast amounts of energy.

The Sun rotates on its axis in about 27 days. The temperature of the Sun's surface is around 6000°C, while it is as hot as 15 million°C at its centre. Our Earth does not feel that much heat because it is very far from the Sun—about 150 million kilometers away.

The distance between the Sun and the Earth—150 million kilometers—is known as an Astronomical Unit (1AU).

1AU = 150,000,000km

The AU is used by astronomers for measuring inter-planetary distances within the Solar System.

The Sun emits a continuous stream of electrically charged particles—mostly electrons and protons—called the solar wind, which flow outward and through the solar system.

The Planets and the Asteroid Belt

Till 2006, the solar system was defined to contain nine planets. In order of their distances from the Sun, the planets were: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (Fig. ). All of them revolve around the Sun in the same direction. The orbits of the planets lie roughly on the same plane, called the ecliptic (the plane of the Earth's orbit, actually). The planetary orbits are almost circular, except those of Mercury and Pluto, which are elongated.

The planets are classified into two groups – the inner planets (Mercury, Venus, Earth and Mars) and the outer planets (Jupiter, Saturn, Uranus, Neptune and Pluto). Another classification divides them into inferior planets—those whose orbits lie between the Sun and the Earth—and superior planets—those whose orbits lie beyond that of Earth.

In 2006, astronomers decided that Pluto cannot be classified as a planet because of its much smaller size, and it was moved out of the list of planets into another group called the dwarf planets.

Yet another classification of planets divides them into the terrestrial planets and the jovian planets (gas giants). The terrestrial planets (Mercury, Venus, Earth and Mars) are more or less the same size as Earth and are made up of similar material. The jovian planets (Jupiter, Saturn, Uranus and Neptune) are much larger than the Earth, and are mostly composed of hydrogen and helium gas.

Most planets can be seen in the night (or during dawn or dusk) appearing as stars. However, one feature which visually distinguishes a planet from a star in the sky is that unlike a star, a planet does not twinkle.

Mercury

Named after the ancient Roman 'Messenger of the Gods,' Mercury is the nearest planet to the Sun, and the smallest planet in the Solar System (Pluto was the smallest before it was reclassified as a dwarf planet in 2006). From Earth, it can sometimes be seen like a star close to the Sun at dawn or dusk. Mercury is about 1.5 times larger than our moon, and has a very weak magnetic field. It looks very similar to the moon. It is a heavily cratered planet, and has a very thin layer of atmosphere. Daytime temperatures can be as high as 400°C, while at night it can be as low as −170°C.

Venus

Named after the ancient Roman 'Goddess of Love and Beauty,' Venus, the second planet from the Sun, is the brightest planet in the sky. It can be seen as a bright star near the Sun at dawn or dusk. When seen at dawn, it is called the Morning Star, and when seen at dusk, it is called the Evening Star. Because it is of almost the same size as the Earth, Venus is referred to as the Earth’s twin.

The planet has a very dense and poisonous atmosphere, composed mostly of carbon-dioxide, while its clouds are mostly sulphuric acid. The atmospheric pressure on the surface is about 90 times that of Earth. Its surface temperatures is around 450°C, hot enough to melt lead. It is hotter than Mercury even though it is further from the Sun as its dense atmosphere and clouds trap the Sun's heat, creating what is known as the greenhouse effect. Venus lacks a magnetic field.

The thick layers of cloud hides the surface of Venus from telescopic observations. Astronomers have to rely on radar to get information about the surface features.

Venus has a retrograde rotation (opposite to the direction of rotation of the other planets). If you were on Venus, the Sun would rise in the west and set in the east. Of course, it will take about 120 days from sunrise to sunset!

Earth

Earth, the third planet, is the fifth largest of all the planets. Earth is the only planet in the solar system which can support life as we know it. It has liquid water and air (containing oxygen, carbon-dioxide and nitrogen), both so important for living things. Also, at a comfortable distance from the Sun, and some help from its atmosphere in heat regulation, the Earth enjoys moderate temperatures. The average temperature of the Earth is about 16°C.

From outer space, the Earth appears blue because about 70% of its surface is covered with water. For this reason, it is also known as the Blue Planet (Fig. ). Most of the water is in the form of water bodies like oceans, seas, lakes, rivers and groundwater. Some of the water is frozen as snow, ice and glaciers in the mountains and as ice-sheets around the north and south poles. The rest of the water is found in the air as water vapour, from which clouds form.

The land surface of the earth is made up of stone, rock and soil. The surface has features like plains, mountains, valleys and deserts.

The Earth is not perfectly round—it is a sphere which is slightly flattened at the poles and bulges slightly near the equator. Such a shape is called a geoid. Earth possesses a magnetic field, which protects life from the dangerous effects of the solar wind streaming out from the Sun.

The Earth's axis is titled at an angle of 23½° to the ecliptic, always pointing in the same direction in space. This results in the phenomena of seasons as the Earth orbits the Sun (see Fig. ). The Northern Hemisphere gets more direct sunlight at certain times of the year (around June) and has its summer, while at the same time, the Southern Hemisphere gets more slanting rays and has its winter. Half a revolution later (around December), the situation gets reversed as the Southern Hemisphere gets its summer, while the Northern Hemisphere experiences winter.

Earth's Satellite - The Moon

The Moon, Earth’s only satellite, has a diameter only ¼ that of the Earth. It appears big because it is only 384000km away. The moon is made up of solid rock. The surface consists of mountains, peaks and craters, deep valleys and plains. The plains of the moon are also called maria.

The moon does not have water or atmosphere, and is not suitable for existence of life. Although seen at night, the moon may also be present in the sky during the daytime, but the Sun is generally bright enough to hide it.

The moon has extremes of temperatures. Its daytime temperatures are higher than 100°C, while at night it can be as cool as -150°C. These extremes, even though the Moon is about the same distance from the Sun as the Earth is, is because the Moon lacks atmosphere to help in temperature regulation.

The Moon takes about 27 days to rotate once on its axis, and about the same time to revolve around the Earth. As a result, the same side of the moon always faces the Earth.

The Moon is the only celestial body ever visited by humans. In July 1969, the American Astronauts Neil Armstrong and Edwin Aldrin became the first men to land on the surface of a celestial body. As their spacecraft Apollo 11 descended onto the Moon's surface, they saw the beautiful sight of the Earth rising above the horizon of the moon (Fig. ). Neil Armstrong became the first man to step onto the surface of the moon.

Phases of the Moon

We see the moon at night because it reflects the light from the Sun. However, it appears to go through different phases—sometimes it is seen as full circle, while at other times it is just a thin crescent. This is because of the moon's revolution around the Earth, while being illuminated by the Sun from one side (see Fig. ).

When, in its orbit, the moon comes between the Sun and the Earth (position 1, Fig. ), its lighted side is facing away from us and so it is dark and cannot be seen, as indicated as a dark circle at the bottom of Fig. for position 1. This phase is called the New Moon. As it revolves around the Earth, the moon successively goes through positions 2, 3 and 4. The view of the moon, as seen by us in these positions, is shown at the bottom of Fig. as crescent moon (position 2), half moon (position 3) and gibbous moon (position 4). When the moon positions itself at position 5 so that the Earth lies in between the moon and the Sun, we can see the full lighted face of the moon. This phase is called the Full Moon. The sequence of phases from the New Moon to the Full Moon is called the Waxing Phase.

From position 5 through 8, and finally 1, the sequence reverses as the moon goes successively through the gibbous, half moon and crescent phases to finally end as the New Moon. This sequence of phases from the Full Moon to the New Moon is called the Waning Phase.

Mars

Named after the ancient Roman 'God of War,' Mars, the fourth planet, is known as the Red Planet. It looks like a reddish star in the sky.

Mars is half the size of Earth. The land surface is reddish coloured because of the presence of iron oxides. Mars has Olympus Mons, the largest known volcano in the solar system. It is 27km high, three times higher than Mt. Everest, the highest mountain peak on Earth.

Mars has a thin atmosphere, mainly made up of carbon-dioxide. Scientists say that water exists in Mars in the form of ice at the poles and as water vapour in the atmosphere. Mars is much colder than Earth. The temperature on the surface ranges from a low of about −80°C to a high of −30°C.

Mars has no magnetic field.

Satellites of Mars

Phobos and Deimos

The two (very small) satellites of Mars are Phobos (roughly 22km across) and Deimos (roughly 15km across). They are irregular shaped, and believed to be captured asteroids (small rocky bodies orbiting the Sun, see the writeup: Asteroid Belt ). Phobos is in a lower orbit, and hence orbits faster. As the rotation speed of Mars lies between the orbital speeds of the two moons, they appear to move in opposite direction when viewed from the surface of Mars.

Jupiter

Named after the ancient Roman 'Ruler of the Gods,' Jupiter, the fifth planet, is the largest in size. It is so huge that more than a 1000 earths can fit inside it if it were hollow. It is more massive than all the planets of the solar system put together.

Jupiter is essentially composed entirely of gas, just like the other three jovian planets. The atmosphere is similar to the Sun's—it is made up of mostly hydrogen and some helium gases. Had the planet been larger, its gravity would have been sufficient to kick-start nuclear fusion at its core, and Jupiter could have become the second Sun of the solar system. The planet is frequently referred to as "the star that failed."

The planet is covered with clouds which often have violent storms lasting a long time. The most famous example of one such huge storm is the Great Red Spot. It is so huge that two Earths can fit into it side by side. The Great Red Spot has been seen for more than 300 years. The clouds of Jupiter extend for thousand of kilometers. At the base of the clouds, the pressure is so strong that the gases are compressed into liquid form.

Jupiter has a narrow system of rings made up of tiny rocks and dust particles which revolve around the planet. Jupiter has a powerful magnetic field.

Satellites of Jupiter

Jupiter has more than 60 satellites. Four moons of Jupiter are large and are known as the Galilean satellites, since they were discovered by the Italian astronomer Galileo while observing through his primitive telescope in 1610. The Galilean satellites, from Jupiter outwards, are Io, Europa, Ganymede and Callisto.

Io

Io is rocky and a little larger than our Moon. It is actively volcanic, its surface mostly covered with solidified lava flows.

Europa

Unlike Io, volcanism is absent from Europa. It is covered in a thin layer of ice, and astronomers believe that the ice layer hides an underground ocean of liquid water.

Ganymede

Ganymede is the largest moon in the solar system, bigger than Mercury. It is also covered in ice, with rocky surface below.

Callisto

Callisto is also an ice covered satellite. It is more heavily cratered that even Mercury.

Saturn

Named after the ancient Roman 'God of Agriculture,' Saturn, the sixth planet, is the second largest planet in the solar system. It is the most beautiful of all planets, having a magnificent system of rings made up of water ice and ice covered particles revolving around the planet.

Saturn's density is less than that of water. If there was so much water available, Saturn could float on it!

Like Jupiter, Saturn is also made up of mostly hydrogen and some helium gases. The planet is covered with clouds which, however, have less violent storms than Jupiter. Just like Jupiter, the clouds extend for thousand of kilometers, and at the base of the clouds the pressure is strong enough to compress the gases into liquid form. Saturn has a weaker magnetic field than Jupiter.

Satellites of Saturn

Saturn also has more than 60 satellites. Prominent ones are Titan, Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas.

Titan

Titan is the largest satellite of Saturn, the second largest in the Solar System, and slightly larger than Mercury. It has a very thick atmosphere comprising mostly nitrogen.

Uranus

Named after the ancient Greek and Roman 'God of the Heavens,' Uranus, the seventh planet, is the third largest. It was first seen using a telescope in 1781. Just like Jupiter and Saturn, Uranus is made up of mostly hydrogen and some helium gases. The blue-green colour of Uranus is because of the presence of methane ice in the clouds.

The rotation axis of Uranus is tilted at an angle of 98° to the ecliptic, such that the planet's rotation appears to make it roll on its side as it orbits the Sun. Uranus, too, has a system of rings, but they are quite narrow.

Satellites of Uranus

Uranus has more than 25 satellites. Prominent ones are Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Named after the ancient Roman 'God of the Sea,' Neptune, the eighth planet, is the fourth largest. It was discovered telescopically in 1846. Like the other gas giants, it is also made up of gases, mostly hydrogen and some helium. It looks bluish in colour because of the presence of methane in the atmosphere.

Satellites of Neptune

Neptune has more than 10 satellites, of which only Triton is of respectable size. It's diameter is about 80% of that of our Moon.

Pluto

Named after the ancient Roman 'God of the Underworld,' Pluto, the erstwhile ninth planet, was the smallest—its diameter is less than half the size of Mercury. It was discovered in 1930. Pluto's orbit is so elongated that for a part of its journey around the Sun, it falls within the orbit of Neptune. During those times, Neptune becomes the most distant planet of the pre-2006 planets.

Pluto is now classified as a dwarf planet.

Satellites of Pluto

Charon is the largest moon of Pluto, about half the size of Pluto itself. Because of its impressive size in comparison to Pluto, the pair has been called a binary planet system.

Other Solar System Bodies

Apart from the Sun and the planets, the other important celestial bodies in our solar system are the asteroids, dwarf planets, meteors/meteorites and the comets.

The Asteroid Belt

Asteroids are small rocky bodies that orbit the Sun. They are also known as minor planets or planetoids. They are much smaller than planets, and can be of any shape. A few are nearly round while most are irregular.

Most of the asteroids are huddled together between the orbits of Mars and Jupiter to form what is known as the Asteroid Belt. The distribution of the asteroids with the belt is not uniform. Ceres is the largest asteroid in this belt, is spherical in shape, and has a diameter of about 930 kilometers. Since 2006, Ceres is now classified as a dwarf planet, of which Pluto is a member.

Dwarf Planets

Dwarf planets are round like normal planets, but are smaller in size. Pluto, Ceres, Eris, Makemake and Haumea belong to the group. Of these, only Ceres belongs to the asteroid belt, while the others lie beyond the orbit of Neptune.

A round body orbiting the Sun is classified as a dwarf planet only if it has not been able to clear the region around its orbit of smaller bodies with its gravity. For practical purposes, round objects are classified as dwarf planets if they are smaller in size than planet Mercury.

Meteors and Meteorites

Some asteroids follow a wide range of orbits around the Sun. Many of small ones enter the Earth's atmosphere each day and burn up before reaching the surface due to the heat generated by friction with air at their extremely high speeds. These asteroids are called meteors. They are also know as shooting stars, as they can be sometimes seen at night as sudden streaks of light travelling across the sky, before they burn up.

Those meteors which are large enough to survive the burning, as they pass through the Earth's atmosphere, reach the surface and are called meteorites. Most of them, again, are small in size. However, large ones can create great damage due to their high speeds.

Some scientists believe that one large meteorite, around 10 to 20 km across, hit Earth 65 million years ago. This would have created a tremendous dust cloud which spread into the atmosphere and prevented the Sun rays from properly reaching the Earth for some years. Many plants all over the world would have died, and this would have led to the final extinction of the dinosaurs.

Comets

Comets are small bodies of rock, ice and gas which orbit the Sun in paths which are very much elongated as compared to the planetary orbits. One side of the orbit is very close to the Sun, while the other side may be so far as to be beyond the orbit of the most distant planet in the solar system. Some comets with long orbits may take hundreds, or even thousands, of years to make a complete trip.

As a comet nears the Sun, it begins to give off gas and dust because of the increasing heat. The pressure of the solar wind then creates a long tail for the comet which can be observed by people on Earth. Since the tail is created by the pressure of the solar wind, it always aligns itself away from the Sun. As the comet rounds the Sun and moves away, the tail gradually disappears.

«Celestial Bodies & the Universe		The Solar System
The Cosmos		$$