THE SOLAR SYSTEM ‘The Solar System’ is the name given to the planetary system of which the Earth is part. It comprises 8 planets, moons, comets, meteors, asteroids & dwarf planets which are all held together by the gravitational pull of a star, named either the Sun or ‘Sol’. Formation of the solar system: The formation of the Solar System is estimated to have begun 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a proto planetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed. The Planets: The known planets in the solar system can be divided into two groups. The four planets closest to the sun: Mercury, Venus, Earth & Mars are called the ‘terrestrial planets’. The outer four planets are called the ‘gaseous giants’ or ‘Jovian planets’. The terrestrial & Jovian 18 planets are divided by a belt of asteroids between Mars & Jupiter. Pluto was demoted to the status of ‘Dwarf planet’ giving it home among similar sized objects & asteroids which make up the Kuiper Belt. End of the solar system: It is difficult to calculate exactly where our solar system ends. It ends at a point at which objects are no longer affected by the sun’s gravitational pull. The farthest reaches of the solar system are thought to be surrounded by a great halo: Oort cloud; home to millions of comet nuclei & small icy rocks. Voyager 1 & 2 are the farthest reaching man-made objects in the solar system. In 2004, Voyager 1 cleared the termination shock, the area where solar winds begin to slow down & increase in temperature as they come up against interstellar winds. Voyager probes will continue transmitting data for several more decades, but it would take thousands of years for them to reach the nearest star, Alpha Centauri. 19 4.1 THE SUN: In the grand scheme of the universe, the sun in just another middle aged star. Its stellar classification, G2, denoting its yellowish colouring & its surface temperature (5000°-6000° kelvin) is not extraordinary either; there are countless G2-type stars. Fusion: In the sun, Hydrogen nuclei collide with one another to form Helium atoms. In this process, called ‘fusion’, mass is transferred into energy, which is explained by Einstein’s famous equation: 퐸 = 푚푐2. The period during which sun generates energy through fusion is called ‘Main sequence’. Our sun is estimated to be 5 billion years old & its main sequence will last another 5 billion years. Solar Layers: The atmosphere of the sun is composed of several layers, mainly the photosphere, the chromosphere and the corona. It's in these outer layers that the sun's energy, which has bubbled up from the sun's interior layers, is detected as sunlight. Sunspots: These are minor regions where the temperature of the photosphere is cooler than its surroundings. Sunspots are the result of string localised magnetic field & follow a cycle of 11 years. 20 4.2 MERCURY Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Seen from Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. It has no known natural satellites. The planet is named after the Roman deity Mercury; the messenger to the gods Mercury has almost no atmosphere. Its surface is heavily cratered due to impacts from meteorites. 4.3 VENUS Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has no natural satellite. It is named after the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, bright enough to cast shadows. Venus is a terrestrial planet and is sometimes called Earth's "sister planet" because of their similar size, gravity, and bulk composition. It is the hottest planet in solar system. It rotates on its axis from east to west. Therefore, on Venus the sun rises in west & sets in east. 21 4.4 EARTH Earth, also known as Terra, is the third planet from the Sun, the densest planet in the solar system, the largest of the Solar System's for terrestrial planets, and the only celestial body to accommodate life. It is home to millions of species, including billions of humans. The Earth takes 365.26 days to orbit the sun & 23.93 hours revolve once around its axis which is tilted 23.5° to the planet’s orbit. 4.5 THE MOON The Moon (Latin: Luna) is the Earth's only natural satellite. Although not the largest natural satellite in the Solar System, it is, among the satellites of major planets, the largest relative to the size of the object it orbits and, after Jupiter's satellite Io, it is the second densest satellite among those whose densities are known. The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters . It is the most luminous object in the sky after the Sun. It takes moon 27.3 days to revolve around earth. 22 4.6 MARS Mars is the fourth planet from the Sun and the second smallest planet in the solar system, after Mercury. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars’s atmosphere is consists of 95% of carbon dioxide & remaining 5% of Argon & Nitrogen. Average surface temperature is 60° Celsius. Because of this water exist on mars in frozen & vapour form. 4.7 JUPITER Jupiter is the fifth planet from the sun and the largest planet in the solar system. It is a gas giant with mass one thousandth of that of the sun but is two and a half times the mass of all the other planets in the Solar System combined. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium. There are also at least 79 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury. 23 The Giant Red Spot is its most notable feature. It is a giant storm which flows anti-clockwise across in area of 24,000 km in length & 12,000 km in width. 4.8 SATURN - 82 Moon Saturn is the sixth planet from the sun and the second largest planet in the Solar System, after Jupiter. It is named after the Roman god of agriculture. Saturn is gas giant with an average radius about nine times that of Earth. While only one-eighth the average density of Earth, with its larger volume Saturn is just over 95 times more massive. One of the greatest fascinations of the solar system is the Rings of Saturn. They have often earned Saturn the title of the most attractive planet in solar system. Titan, Saturn's largest and the Solar System's second largest moon larger than the planet Mercury and is the only moon in the Solar System to retain a substantial atmosphere. 4.9 URANUS Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both are of different chemical composition to the larger gas giants Jupiter and Saturn. For this reason, astronomers sometimes place them in a 24 separate category called "ice giants". It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of −224.2 °C and has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane the uppermost layer of clouds. In contrast, the interior of Uranus is mainly composed of ices and rock. 4.10 NEPTUNE Neptune is the eighth and farthest planet from the sun in Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Among the gaseous planets, Neptune is the densest. Neptune is 17 times the mass of Earth and is slightly more massive than its near- twin Uranus, which is 15 times the mass of Earth but not as dense. Neptune orbits the Sun at an average distance of 30.1 astronomical units. It is named after the Roman god of the sea. Neptune's atmosphere is notable for its active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. 4.11 PLUTO & THE KUIPER BELT Discovered in 1930, Pluto was originally classified as the ninth planet from the Sun. Its status as a major planet fell into question following further study of it and the outer Solar System over the ensuing 75 years. Starting in 1977 with the discovery of the minor planet 2060 25 Chiron, numerous icy objects similar to Pluto with eccentric orbits were found. The most notable of these was thes cattered disc object Eris, discovered in 2005, which is 27% more massive than Pluto. Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Nix, Hydra, Kerberos & Styx.