Chapter 1
Our Universe
The universe is a huge wide-open space that holds everything from the smallest particle to the biggest galaxy. No one knows just how big the Universe is. Astronomers try to measure it all the time. They use a special instrument called a spectroscope to tell whether an object is moving away from Earth or toward Earth. Based on the information from this instrument, scientists have learned that the universe is still growing outward in every direction. Scientists believe that about 13.7 billion years ago, a powerful explosion called the Big Bang happened. This powerful explosion set the universe into motion and this motion continues today. Scientists are not yet sure if the motion will stop, change direction, or keep going forever.
The universe (Latin: universus) is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire universe is unknown, it is possible to measure the size of the observable universe, which is currently estimated to be 93 billion light-years in diameter. In various multiverse hypotheses, a universe is one of many causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and its contents.
The earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe.
Further observational improvements led to the realization that the Sun is one of hundreds of billions of stars in the Milky Way, which is one of at least two trillion galaxies in the universe. Many of the stars in our galaxy have planets. At the largest scale, galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the universe had a beginning and that space has been expanding, since then, and is currently still expanding at an increasing rate.
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The Big Bang theory is the prevailing cosmological description of the development of the universe. According to estimation of this theory, space and time emerged together 13.799±0.021 billion years ago and the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, and the separation of the four known fundamental forces, the universe gradually cooled and continued to expand, allowing the first subatomic particles and simple atoms to form. Dark matter gradually gathered, forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were gradually drawn to the places where dark matter was most dense, forming the first galaxies, stars, and everything else seen today. It is possible to see objects that are now further away than 13.799 billion light-years because space itself has expanded, and it is still expanding today. This means that objects which are now up to 46.5 billion light-years away can still be seen in their distant past, because in the past, when their light was emitted, they were much closer to Earth.
From studying the movement of galaxies, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly). The ΛCDM model is the most widely accepted model of our universe. It suggests that about 69.2%±1.2% [2015] of the mass and energy in the universe is a cosmological constant (or, in extensions to ΛCDM, other forms of dark energy, such as a scalar field) which is responsible for the current expansion of space, and about 25.8%±1.1% [2015] is dark matter. Ordinary ('baryonic') matter is therefore only 4.84%±0.1% [2015] of the physical universe. Stars, planets, and visible gas clouds only form about 6% of ordinary matter, or about 0.29% of the entire universe.
There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that likewise exist
Evolution Of Universe
The three main theories put forward to explain the origin and evolution of the universe are:
- The Big Bang Theory
- The Steady State Theory
- The Pulsating Theory
The Big Bang Theory: Le Maitre and Gammow proposed this theory. According to this theory, at the beginning of the universe, the whole matter of the universe was once concentrated in an extremely dense and hot (~10 12K) fireball. Then about 20 billion years ago a vast explosion (big bang) occurred. The matter was broken into pieces, which were thrown out with high speed in all directions forming stars and galaxies; which are still moving way from one another. According to Hubble’s law, the velocity of recession of a galaxy becomes equal to the velocity of light at a distance equal of 20 billing light years. It means, the light rays from stars and galaxies, which are situated at a distance of 20 billion light years or more, can never reach us. Thus this distance becomes the boundary of observable universe. On account of continuous recession, more and more galaxies will go beyond this boundary and they will be lost. As a result of this, the number of galaxies per unit volume will go on decreasing and ultimately a time may come when we may have empty universe
Steady State Theory: Bondi, Gold and Fred Hoyle developed this theory. According to this theory, the number of galaxies in the observable universe is constant and new galaxies are continuously being created out of empty space, which fill up the gaps caused by those galaxies, which have crossed the boundary of the observable universe. As a result of it, the overall size of mass of the observable universe remains constant. Thus a steady state of the universe is not disturbed at all.
Pulsating Theory: According to this theory, the universe is supposed to be expanding and contracting alternately i.e. pulsating. At present, the universe is expanding. According to pulsating theory, it is possible that at a certain time, the expansion of the universe may be stopped by the gravitational pull and they may contract again. After it has been contracted to a certain size, explosion again occurs and the universe will start expanding. The alternate expansion and contraction of the universe give rise to pulsating universe.
Galaxies A galaxy contains stars, gas, and dust which are held together as a group by gravity. There may be millions, or even billions, of stars in one galaxy. There are billions of galaxies in the universe. Galaxies are labeled according to their shape. Some galaxies are called “spiral”, because they look like giant pinwheels in the sky. The galaxy we live in, the Milky Way, is a spiral galaxy. Some galaxies are called “elliptical”, because they look like flat balls. A galaxy may be called “irregular” if it doesn’t really have a shape. A new type of galaxy was discovered recently, called a “starburst” galaxy. In this type of galaxy, new stars just seem to ‘burst out’ very quickly.
(I) The Milky Way The Milky Way is over 100,000 light-years wide. It is called a spiral galaxy because it has long arms which spin around like a giant pinwheel. Our Sun is a star in one of the arms. When you look up at the night sky, most of the stars you see are in one of the Milky Way arms.
Ø Before we had telescopes, people could not see many of the stars very clearly. They blurred together in a white streak across the sky. A myth by the ancient Greeks said this white streak was a “river of milk”. The ancient Romans called it the Via Galactica, or “road made of milk”. This is how our galaxy became known as the Milky Way.
Ø A light-year is the distance light travels in one year. It is 9.5 trillion (9,500,000,000,000) kilometers. The size of a galaxy may be as little as a thousand light-years across or as much as a million light-years across.
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STARS · A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light — except for the sun. The sun looks like a ball because it is much closer to Earth than any other stars. Stars are formed initially from gas and dust. They are composed mainly of the hydrogen gas. Gas are very hot and give off huge amounts of energy in the form of heat and light.Our Sun is a medium sized star. Stars have a life-span of about 10 billion years, after which they will cease to exist. Stars are very far away from Earth. The closest Star is about 23.5 trillion miles away. |
Quasars
Quasars are farther away from Earth than any other known object in the universe. Because they are so far away from us, it takes billions of years for the light they give off to reach Earth. The light stays the same, it just has to travel a long time to get to us. When we look at a quasar, it is like we are looking back in time. The light we see today is what the quasar looked like billions of years ago. Some scientists think that when they study quasars they are studying the beginning of the universe.
Quasars give off huge amounts of energy. They can be a trillion times brighter than the Sun! Astronomers think that quasars are located in galaxies which have black holes at their centers. The black holes may provide quasars with their energy. Quasars are so bright that they drown out the light from all other stars in the same galaxy. The word quasar is short for quasi-stellar radio source. Quasars give off radio waves, X-rays, gamma-rays, ultraviolet rays, and visible light. Most of them are larger than our solar system.
Quasars give off more energy than 100 normal galaxies combined
Dark Matter
Matter is anything that takes up space and has mass. We are used to matter which we will call visible matter. Visible matter can be seen because it gives off light or reflects light given off by another object. Dark matter cannot be seen. It does not give off light or reflect light.
Scientists believe that over ninety-percent of the matter in the universe is dark matter. They also believe that by studying dark matter they will gain new information about the universe. Some of the information they hope to discover is the size, shape and future of the universe. Scientists also hope to learn about how galaxies formed by studying dark matter.
Scientists cannot see dark matter, so they have a special way of studying it. Scientists study dark matter by looking at how it affects visible matter. Scientists use computers and satellites to study dark matter. The Hubble Space Telescope has taken pictures that have helped scientists discover where dark matter can be found.
Dark matter was once called “missing matter”. It was called this because scientists looking at the sky could not find it.
Solar System The word “solar” refers to the sun; the sun is one of the 150 billion stars of the Milky Way. It moves through space taking with it a larger family of objects. The whole group is called the solar system. Our solar system is elliptical in shape. The sun is the center of the solar system. Solar system is always in motion. Its largest and most important members are the nine known planets and their moons, along with smaller objects called comets, asteroids, and meteoroids that orbit the sun. The sun is the biggest object in our solar system. It contains 99.8% of the solar system’s mass. Many scientists believe that our Solar System is over 4.6 billion years old.
Scientists believe that the solar system was formed when a cloud of gas and dust in space was disturbed, may be by the explosion of a nearby star called Supernova. This explosion made waves in space that squeezed the cloud of gas and dust. Squeezing made the cloud start to collapse, as gravity pulled the gas and dust together, forming a solar nebula. The sun’s nuclear fires, ignited at the dense center of this nebula. The planets were born in the swirling currents of the great cloud.
The planets Mercury, Venus, Earth, and Mars, evolved as globes of rock that are present near the Sun. They were too small and their gravitational fields too weak to capture. However, far from the sun, the massive planets Jupiter and Saturn, with powerful gravitational fields, did attract and hold thick gaseous atmospheres of Hydrogen and Helium.
The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, the dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.
The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest planets, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System also contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be. Such objects are categorized as dwarf planets. The only certain dwarf planet is Pluto, with another trans-Neptunian object, Eris, expected to be, and the asteroid Ceres at least close to being a dwarf planet. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy.
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Comprehensive overview of the Solar System. The Sun, planets, dwarf planets and moons are at scale for their relative sizes, not for distances. A separate distance scale is at the bottom. Moons are listed near their planets by proximity of their orbits; only the largest moons are shown.
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WHY ARE STARS HOT AND BRIGHT? Nuclear Fusion and Nucleosynthesis Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright. In most stars, the primary reaction converts hydrogen atoms into helium atoms, releasing an enormous amount of energy. This reaction is called nuclear fusion because it fused the nuclei (center) of atoms together, forming a new nucleus. The process of forming a new nucleus (and element) is nucleosynthesis. |
The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium. Jupiter and Saturn, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium. A composition gradient exists in the Solar System, created by heat and light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points. The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 AU from the Sun.
The objects of the inner Solar System are composed mostly of rock, the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula. Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula. Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide, have melting points up to a few hundred kelvins. They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase. Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit. Together, gases and ices are referred to as volatiles.
Distances and scales
The distance from Earth to the Sun is 1 astronomical unit [AU] (150,000,000 km; 93,000,000 mi). For comparison, the radius of the Sun is 0.0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10−6) that of the Sun.
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The closest star to us is the sun! Other than that, the closest star is Proxima Centauri, aka Alpha Centauri C (the dimmest star in the Alpha Centauri system). Proxima Centauri is 4.3 light-years from the Sun. |
The distance from Earth to the Sun is 1 astronomical unit [AU] (150,000,000 km; 93,000,000 mi). For comparison, the radius of the Sun is 0.0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10−6) that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km) from the Sun.
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WHY DO STARS TWINKLE? The scientific name for the twinkling of stars is stellar scintillation (or astronomical scintillation). Stars twinkle when we see them from the Earth’s surface because we are viewing them through thick layers of turbulent (moving) air in the Earth’s atmosphere. Stars closer to the horizon appear to twinkle more than stars that are overhead - this is because the light of stars near the horizon has to travel through more air than stars overhead and subject to more refraction. Also, planets do not usually twinkle - they are big enough that this effect is not noticeable (except when the air is extremely turbulent). Stars would not appear to twinkle if we viewed them from outer space (or from a planet/moon that didn’t have an atmosphere). |
With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), but no such theory has been accepted. The images at the beginning of this section show the orbits of the various constituents of the Solar System on different scales.
Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas. The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Ericsson Globe in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away.
If the Sun–Neptune distance is scaled to 100 metres, then the Sun would be about 3 cm in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm, and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm) at this scale.
Celestial Bodies
Ø Celestial bodies are objects like Sun, moon, stars and others that shine in the night sky.
Ø Some celestial bodies are very big and are made up of gases and heat. They have their heat and light which is emitted in large amounts. These celestial bodies are called stars and our Sun is a star.
Ø The sun, the moon, and all those glittering objects in the night sky are called celestial bodies.
Constellations
Ø Different groups of stars form various patterns and they are called constellations. Saptarshi is an example of constellations.
Ø In ancient times, with the help of stars, directions were determined during night time. The North Star (Pole Star) indicates the north direction and it remains in the same position in the sky.
Ø Celestial bodies that do not have their own heat and light and lit by the light of the stars are called planets.
The Sun
The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses), which comprises 99.86% of all the mass in the Solar System, produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium, making it a main-sequence star. This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.
The Sun is a G2-type main-sequence star. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up 85% of the stars in the Milky Way.
The Sun is a population I star; it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars. Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the Universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This high metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals.
The Sun
The Sun is our closest star. It is a member of the Milky Way galaxy. The diameter of the Sun is 1,392,000 kilometers. It is believed to be over 4 billion years old. The Sun is a medium sized star known as a yellow dwarf. The Sun spins slowly on its axis as it revolves around the galaxy. The Sun is a large ball of gas consisting mostly of hydrogen and helium. The Sun is about 109 times larger than Earth.
The center, or core, of the Sun is very hot. The temperature in its core is estimated to be over 15,000,000 degrees Celsius. A process called “nuclear fusion” takes place there. Nuclear fusion produces a lot of energy. Some of this energy travels out into space as heat and light. Some of it reaches the Earth! We can see storms on the Sun’s surface called as “sunspots” because they look like dark spots on the Sun’s surface. The Sun also produces big explosions of energy called solar flares. These flares shoot fast moving particles off the Sun’s surface. These particles can hit the Earth’s atmosphere and cause a glow called an Aurora.
The Sun has several layers: the core, the radiation zone, the convection zone, and the photosphere (which is the surface of the Sun). In addition, there are two layers of gas above the photosphere called the chromosphere and the corona. The following are the events that occur on the Sun frequently: sunspots, solar flares, solar wind, and solar prominences.
Without the Sun, the Earth would be a lifeless ball of rock and ice. The Sun warms our planet, creates our weather, and gives energy to plants providing food and energy to support life on Earth. The Sun is a large ball of gas consisting mostly of hydrogen and helium. The Sun is about 109 times larger than the Earth. Scientists estimate that the temperature at the center of the Sun is about 15 million degrees Celsius. This is similar to exploding a hydrogen, or nuclear, bomb. Large explosions on the Sun’s surface cause solar flares that shoot up high into space. The surface temperature is about 4000 degrees Celsius. Energy released from the Sun radiates in all directions, reaching the Earth and other planets. The further the planet is from the Sun, the less energy it receives.
Ø The sun is in the center of the solar system.
Ø It is huge and made up of extremely hot gases.
Ø It provides the pulling force that binds the solar system.
Other Objects in the Solar System
Asteroids: Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered as planets. They are known as minor planets. Most of the asteroids in our solar system can be found orbiting the Sun between the orbits of Mars and Jupiter. This area is sometimes called the “asteroid belt”. A few asteroids approach the Sun more closely.
Asteroid belt: The asteroid belt is a doughnut shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars. Comets: A comet is made of dirty ice, dust, and gas. Scientists believe that comets are made up of material left over when the Sun and the planets were formed. When a comet gets close to the Sun, part of the ice starts to melt. Scientists think about 100,000 million comets orbit the Sun. There are some comets orbiting the Sun like planets. Their orbits take them very close to and very far away from the Sun. Comet can be seen only when it comes close to the Sun. The Sun’s heat melts the comet’s ice to form glowing gases. The gases stream out into a long tail that can extend to millions of kilometers.
Meteorites: Besides asteroids some smaller pieces of rocks and dust also orbit the Sun. These pieces of rock or dust enter the Earth’s atmosphere. As they pass they encounter great friction, which causes them to heat up and burn out. These burning pieces of rock or dust are called as meteors. Although they are not stars, people call them as shooting stars, because they flash light across the sky. Most of the meteors burn up before they reach the Earth. Some are so large that a part of it reaches the ground as a meteorite. Any leftover part that does strike the Earth is called a meteorite. A meteorite can make a hole or crater in the ground when it hits it. The larger the meteorite, the bigger the hole.
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Sun Reference Data
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Diameter: |
1.4 million km (870,000 miles) |
Age: |
4.5 billion years |
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Mass: (93 million miles) |
330,000 x Earth |
Distance from Earth: |
149.6 million km |
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Density: |
1.41 (water=1) |
Distance to Nearest Star: |
4.3 light years |
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Solar Wind |
3 million km/hr. |
Luminosity: |
390 billion billion |
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Solar Cycle:atsurface: |
8 - 11 years |
Temperature |
5,500oC (9,932oF) |
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Temperature |
14 millionoC at Core: |
Temperature of |
4,000oC (7,232oF) Sunspots: |
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Rotation Period at Equator: |
25 Earth days |
Rotation Period at Poles: |
35 Earth days |
Planets By the current count of astronomers, our solar system includes 8 planets and 5 dwarf planets. The planets were formed during the process of solar system formation, when clumps began to form in the disk of gas and dusk rotating about our young Sun. Eventually, only the planets and other small bodies in the solar system remained. The four rocky planets at the center of the solar system Mercury, Venus, Earth, Mars, are known as the inner planets. Jupiter, Saturn, Uranus, and Neptune are all composed primarily of gas and are known as the outer planets.
There are eight planets in our solar system.
In order of their distance from the sun, they are:
1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus and
8. Neptune
Inner Solar System
The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the frost line, which is a little less than 5 AU (about 700 million km) from the Sun.
Inner planets
The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as the silicates—which form their crusts and mantles—and metals, such as iron and nickel, which form their cores.
Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
Inner Planets could be summed up as:
Ø These planets are very close to the sun.
Ø They are made up of rocks.
Ø Inner Planets are:
Mercury- One orbit around sun – 88 days, One spin on axis – 59 days.
Venus – One orbit around the sun – 255 days. One spin on axis – 243 days
Earth – One orbit around the sun – 365 days. One spin on axis – 1 day Number of moons – 1
Mars – One orbit around the sun – 687 days. One spin on axis – 1 day, number of moons – 02
Ø The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids.
Ø They are composed mainly of silicates and metals.
Ø The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems.
Ø They are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals, such as iron and nickel, which form their cores.
Ø Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.
Ø The term inner planet should not be confused with the inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
Ø The term superior planet designates planets outside Earth’s orbit and thus includes both the outer planets and Mars.
Mercury
Mercury (0.4 AU from the Sun) is the closest planet to the Sun and on average, all seven other planets. The smallest planet in the Solar System (0.055 M⊕), Mercury has no natural satellites. Besides impact craters, its only known geological features are lobed ridges or rupees that were probably produced by a period of contraction early in its history. Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy.
Ø Mercury’s surface appears heavily cratered and is similar in appearance to the Moon’s, indicating that it has been geologically inactive for billions of years (because there is no atmosphere on Mercury).
Ø When viewed from Earth, the planet can only be seen near the western or eastern horizon during the early evening or early morning.
Ø It may appear as a bright star-like object but is less bright than Venus.
Ø Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System (−173 °C at night to 427 °C during the day).
Ø Mercury is smaller than the largest natural satellites in the Solar System, Ganymede (largest moon of Jupiter) and Titan (largest moon of Saturn).
Ø However, Mercury is massive (has more mass) than Ganymede and Titan.
Ø Images obtained by Messenger spacecraft in 2004 have revealed evidence for pyroclastic flows (vulcanicity) and water ice at Mercury’s poles.
Venus
Venus (0.7 AU from the Sun) is close in size to Earth (0.815 M⊕) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere. No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.
Ø Venus is the brightest planet in the solar system and is the third brightest object visible from earth after the sun and the moon.
Ø It is the brightest among planets because it has the highest albedo due to the highly reflective sulfuric acid that covers its atmosphere. It is sometimes visible to the naked eye in broad daylight.
Ø Venus is sometimes called Earth’s sister planet or Earth’s twin because of their similar size, mass, proximity to the Sun, bulk composition and presence of similar physical features such as high plateaus, folded mountain belts, numerous volcanoes, etc.
Ø It is radically different from Earth in other respects. The surface of Venus is totally obscured by a thick atmosphere composed of about 96% carbon dioxide, covered with clouds of highly reflective sulfuric acid.
Ø It has the densest atmosphere of the four terrestrial planets. The atmospheric pressure at the planet’s surface is 92 times that of Earth, or roughly the pressure found 900 m (3,000 ft) underwater on Earth.
Ø Venus is by far the hottest planet in the Solar System, even though Mercury is closer to the Sun.
Ø This is because of the greenhouse effect arising from high concentrations of CO2 and thick atmosphere.
Ø A day on Venus is equivalent to 243 earth days and lasts longer than its year (224 days).
Ø It rotates in the opposite direction (clockwise) to most other planets.
Ø In the ancient literature, Venus was often referred to as the morning star and evening star.
Earth
Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist. Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.
Mars
Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 M⊕). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of Earth). Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago. Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids, or ejected debris from a massive impact early in Mars's history.
Ø Mars is often referred to as the “Red Planet” because of the reddish iron oxide prevalent on its surface.
Ø Mars has a thin atmosphere and has surface features ranging from impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.
Ø Mars is the site of Olympus Mons (shield volcano), the largest volcano and the highest known mountain (24 km) in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System.
Ø Mars has two irregularly shaped moons, Phobos and Deimos, which are thought to be captured asteroids.
Ø Liquid water cannot exist on the surface of Mars due to low atmospheric pressure (less than 1% of the Earth’s).
Ø The two polar ice caps appear to be made largely of water.
Ø Mars can easily be seen from Earth with the naked eye.
Ø Mars is less dense than Earth, having about 15% of Earth’s volume and 11% of Earth’s mass.
Ø Landforms visible on Mars strongly suggest that liquid water has existed on the planet’s surface.
Ø Mars lost its magnetosphere 4 billion years ago, possibly because of numerous asteroid strikes, so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density.
Ø The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.
Ø Methane has been detected in the Martian atmosphere (may indicate the existence of life).
Ø Methane can exist in the Martian atmosphere for only a limited period before it is destroyed — estimates of its lifetime range from 0.6-4 years.
Ø Its presence despite this short lifetime indicates that an active source of the gas must be present.
Ø Geological means such as serpentinization, volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources.
Ø Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets’ rotational axes.
Ø The lack of a magnetosphere and the extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against the bombardment of the solar wind.
Ø Mars is nearly geologically dead; the end of volcanic activity has stopped the recycling of chemicals and minerals between the surface and interior of the planet.
Asteroid belt
Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of refractory rocky and metallic minerals, with some ice. They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions.
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth. The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.
Ceres
Ceres (2.77 AU) is the largest asteroid, a protoplanet, and a dwarf planet. It has a diameter of slightly under 1000 km, and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801, and was reclassified to asteroid in the 1850s as further observations revealed additional asteroids. It was classified as a dwarf planet in 2006 when the definition of a planet was created.
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Asteroid groups
Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water.
Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term trojan is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.
The inner Solar System also contains near-Earth asteroids, many of which cross the orbits of the inner planets. Some of them are potentially hazardous objects.
Outer Planets
Very far from the sun are huge planets made up of gases and liquids
Ø Jupiter – One orbit around the sun – 11 years, 11 months about 12 years. One spin on axis – 9 hours, 56 minutes, number of moons – 16
Ø Saturn – One orbit around sun – 29 years, 5 months. One spin on axis – 10 hours 40 minutes, number of moons – about 18.
Ø Uranus – One orbit around the sun – 84 years. One spin around an axis – 17 hours 14 minutes, number of moons – about 17.
Ø Neptune – One orbit around the sun – 164 years. One spin on axis-16 hours 7 minutes, number of moons – 8
Planets in Detail
A celestial body moving in an elliptical orbit around a star is known as a planet. The planets of our solar system are divisible in two groups:
1. the planets of the inner circle (as they lie between the sun and the belt of asteroids) or the inner planets or the ‘terrestrial planets’ (meaning earth-like as they are made up of rock and metals, and have relatively high densities) and
2. the planets of the outer circle or outer planets or the ‘gas giant planets’ or the Jovian planets – meaning Jupiter-like.
Ø The inner circle consists of four planets (Mercury, Venus, Earth and Mars) having smaller and denser bodies while the outer circle comprises four planets (Jupiter, Saturn, Uranus, and Neptune) having a larger size and less dense materials and have a thick atmosphere, mostly of helium and hydrogen
Ø Jovian planets are more like the sun than like the terrestrial planets.
Ø If we take Jupiter, the biggest planet, as the centre of the planets of our solar system, the size of the planets becomes smaller as we go away from either side of Jupiter (Mars being the exception).
Ø The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits.
An Astronomical Unit (AU) is the average distance between Earth and the Sun, which is about 150 million km.
|
Planet |
Surface Temp in ֯C |
Period of Rotation |
Period of Revolution |
Distance from Sun (AU) |
|
|
1. Mercury |
+427 |
58 days |
87 days |
0.4 |
0 |
|
1. Venus |
+480 |
243 days |
224 days |
0.7 |
0 |
|
1. Earth |
+22 |
23:56 hrs |
365 days |
1 |
1 |
|
1. Mars |
-23 |
1.025 days |
687 days |
1.5 |
2 |
|
1. Jupiter |
-150 |
9.9 hrs |
11.9 years |
5.2 |
79 |
|
1. Saturn |
-180 |
10.7 hrs |
29 years |
9.6 |
62 |
|
1. Uranus |
-214 |
17 hrs |
84 years |
19.2 |
27 |
|
1. Neptune |
-220 |
16 hrs |
164 years |
30.0 |
13 |
|
Pluto (dwarf) |
-223 |
6.39 days |
248 years |
39.5 |
5 |
Major Moons of Various Planets (Source)
Size comparison of largest moons in the Solar System (User:primefac, via Wikimedia Commons)
Size comparison of the largest moons with earth
Dwarf Planets
In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System. Their scheme includes three classes of objects: “small solar system bodies” (including most asteroids and comets), the much larger planets (including Earth, Jupiter, and so on), and the new category of in-between sized “dwar+f planets”.
There are currently five official dwarf planets. Pluto, formerly the smallest of the nine “traditional” planets, was demoted to dwarf planet status. Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The three other (for now!) dwarf planets are Eris, Makemake, and Haumea. Pluto, Makemake, and Haumea orbit the Sun on the frozen fringes of our Solar System in the Kuiper Belt. Eris, also a Trans-Neptunian Object, is even further from the Sun.
What’s the difference between regular planets and dwarf planets?
It’s partly an issue of size, with dwarf planets being smaller. But just how big does a planet need to be to become a full-fledged planet instead of a dwarf? You might think the minimum size requirement is arbitrary, but the size cutoff is actually based on other properties of the object and its history in the Solar System. Both planets and dwarf planets orbit the Sun, not other planets (in which case we call them moons). Both must be large enough that their own gravity pulls them into the shapes of spheres; this rules out numerous smaller bodies like most asteroids, many of which have irregular shapes. Planets clear smaller objects out of their orbits by sucking the small bodies into themselves or flinging them out of orbit. Dwarf planets, with their weaker gravities, are unable to clear out their orbits.
Though there are just five dwarf planets now, their number is expected to grow. Scientists estimate there may be 70 dwarf planets amongst outer solar system objects that have been discovered already. Since we don’t know the actual sizes or shapes of many of the objects we’ve found (because they are so far away), we can’t yet determine whether they are actually dwarf planets or not. More observations and better telescopes will help us determine which other objects are dwarf planets.
Satellites
Moon
Ø Its diameter is only one-quarter that of the earth.
Ø It is about 3,84,400 km away from us.
Ø A ray of light from the sun takes about eight minutes to reach the earth. Light takes only a second to reach us from the moon.
Ø The moon is tidally locked to the earth, meaning that the moon revolves around the earth in about 27 days which is the same time it takes to complete one spin.
Ø Tidal locking is the name given to the situation when an object’s orbital period matches its rotational period.
Ø As a result of tidal locking, only one side of the moon is visible to us on the earth.
Ø The moon is a significant stabiliser of Earth’s orbital axis. Without it, Earth’s tilt could vary as much as 85 degrees (at present the Earth’s axis of rotation is tilted at an angle of 23.5֯ relative to our orbital plane).
Ø Neil Armstrong was the first, and Buzz Aldrin was the second to step on the surface of the moon on 29 July 1969 (Apollo 11 mission).
Ø Till date, only Twelve astronauts walked on the Moon’s surface.
Formation of the moon
Ø It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of ‘giant impact’ or what is described as ‘the big splat’.
Ø A body of the size of one to three times that of mars collided into the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space.
Ø This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.
|
|
Ø Scientists estimate that a day in the life of early Earth was only about 6 hours long.
Ø The Moon formed much closer to Earth than it is today.
Ø As Earth rotates, the Moon’s gravity causes the oceans to seem to rise and fall. There is a little bit of friction between the tides and the turning Earth, causing the earth’s rotation to slow down just a little (1.4 milliseconds in 100 years).
Ø As Earth slows, it lets the Moon move away by a little (four centimeters per year).
Colonizing the moon
Ø Discovery of lunar water at the lunar poles by Chandrayaan-1 has renewed interest in the Moon.
Ø Locations on the Lunar poles avoid the problem of long lunar nights (350+ hours).
Ø Exploration of the lunar surface by spacecraft began in 1959 with the Soviet Union’s Luna program.
Ø Luna 2 made a hard landing (impact) into its surface and became the first artificial object on the moon.
Ø Crewed exploration of the lunar surface began in 1968 when the Apollo 8 spacecraft orbited the Moon.
Ø The following year, the Apollo 11 Apollo Lunar Module landed two astronauts on the Moon.
Ø In 2009, the Chandrayaan probe discovered that the lunar soil contains 0.1% water by weight.
Advantages of colonising the moon
A lunar base could be a site for launching rockets with locally manufactured fuel to distant planets.
There are several disadvantages to the Moon as a colony site
Ø The long lunar night would impede reliance on solar power.
Ø The Moon is highly depleted in carbon and volatile elements, such as nitrogen and hydrogen.
Ø The low gravity on the Moon will have adverse effects on human health in the long term.
Ø The lack of a substantial atmosphere results in temperature extremes, harmful radiation reaching the surface and increased chances of the colony’s being hit by meteors.
Ø Growing crops on the Moon is difficult due to the long lunar night, extreme variation in surface temperature, exposure to solar flares, nitrogen-poor soil, and lack of insects for pollination.
Mars
Ø Mars is often referred to as the “Red Planet” because of the reddish iron oxide prevalent on its surface.
Ø Mars has a thin atmosphere and has surface features ranging from impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.
Ø Mars is the site of Olympus Mons (shield volcano), the largest volcano and the highest known mountain (24 km) in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System.
Ø Mars has two irregularly shaped moons, Phobos and Deimos, which are thought to be captured asteroids.
Ø Liquid water cannot exist on the surface of Mars due to low atmospheric pressure (less than 1% of the Earth’s).
Ø The two polar ice caps appear to be made largely of water.
Ø Mars can easily be seen from Earth with the naked eye.
Ø Mars is less dense than Earth, having about 15% of Earth’s volume and 11% of Earth’s mass.
Ø Landforms visible on Mars strongly suggest that liquid water has existed on the planet’s surface.
Ø Mars lost its magnetosphere 4 billion years ago, possibly because of numerous asteroid strikes, so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density.
Ø The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water.
Ø Methane has been detected in the Martian atmosphere (may indicate the existence of life).
Ø Methane can exist in the Martian atmosphere for only a limited period before it is destroyed — estimates of its lifetime range from 0.6-4 years.
Ø Its presence despite this short lifetime indicates that an active source of the gas must be present.
Ø Geological means such as serpentinization, volcanic activity, cometary impacts, and the presence of methanogenic microbial life forms are among possible sources.
Ø Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets’ rotational axes.
Ø The lack of a magnetosphere and the extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against the bombardment of the solar wind.
Ø Mars is nearly geologically dead; the end of volcanic activity has stopped the recycling of chemicals and minerals between the surface and interior of the planet.
Mars Compared to Earth
Ø 53% the diameter of Earth.
Ø 10% the mass of Earth.
Ø Surface gravity on Mars is only 38% the gravity on Earth.
Ø A day on Mars lasts 1.03 Earth days.
Ø Axial tilt on Mars is 25.19 degrees. Very close to Earth’s 23.5֯ tilt.
Ø A year on Mars lasts about twice as long as an Earth year; the seasons are twice as long.
Ø The atmosphere of Mars (95% carbon dioxide) is less than 1% the thickness of Earth’s atmosphere.
Outer Planets
Ø Outer Planets are Jupiter, Saturn, Uranus, Neptune and the dwarf planet – Pluto.
Ø The four outer planets, called the gas giants, collectively make up 99% of the mass known to orbit the Sun.
Ø They are composed mainly of hydrogen & helium & lack a solid surface. Their moons are, however, solid.
Ø The two outermost planets, Uranus and Neptune, are composed of substances called ices, such as water, ammonia and methane, and are often referred to separately as “ice giants”.
Ø All four gas giants have rings, although only Saturn’s ring system is easily observed from Earth.
Ø The gas giants have a magnetosphere, numerous moons and significant atmospheric activity.
Ø Neptune has the strongest wind speed (2,100 km/h) followed by Saturn (1,800 km/h).
Jupiter
Ø It is composed mostly of gas and liquid swirling in complex patterns with no solid surface.
Ø Jupiter’s four large moons (Io, Europa, Ganymede, and Callisto), called the Galilean satellites because Galileo discovered them.
Ø Ganymede is the largest natural satellite (5,268 km in diameter) in this solar system and is larger than Mercury, and three times larger than the earth’s Moon (3,474 km in diameter, the fifth largest moon).
Ø It is the third-brightest natural object in the night sky after the Moon and Venus and the fourth brightest object in the sky after the Sun, the Moon and Venus.
Ø Because of its rapid rotation (once every 10 hours), the planet’s shape is that of an oblate spheroid (slight bulge at the equator).
Ø The outer atmosphere is visibly segregated into several bands, resulting in turbulence and storms.
Ø The latest probe to visit Jupiter is Juno.
Saturn
Ø Saturn’s rings are probably made up of billions of particles of ice and ice-covered rocks.
Ø Titan is the second-largest moon in the Solar System (larger than Mercury) and it is the only satellite in the Solar System with a substantial atmosphere (nitrogen-rich).
Ø Saturn is the sixth planet from the sun, with the largest planetary rings in the Solar System.
Ø It is the second-largest planet after Jupiter.
Ø It has a diameter of 120.536 km or 74.897 mi, almost 9.5 times bigger than the diameter of Earth and a surface area about 83 times greater.
Ø The rings of Saturn are the most extensive of any other planet. This rings can’t be seen with the unaided eye. Generally, only 3 rings can be seen by regular telescopes, powerful telescopes can view 8 rings and when the spacecraft Cassini orbited Saturn, it counted well above 30 rings. It is important to understand that these rings are actually countless since they are comprised of millions of generally small rocks creating illusions of ring systems.
Ø Saturn has the lowest density of all the planets.
Ø Though it doesn’t have a solid surface, being enveloped in swirling gases and liquids deeper down, it is believed that Saturn has a core much smaller than Jupiter, almost twice the size of Earth – comprised mostly of metals like iron and nickel surrounded by rocky material, and other compounds, solidified by the intense pressure and heat.
Ø The largest moon of Saturn is named Titan. It is the second-largest moon in the Solar System after the Ganymede satellite of Jupiter. Titan is even larger than Mercury, a planet.
Ø Though the potential for life is zero for Saturn, many of its moons such as Titan or Enceladus, have internal oceans that could possibly hold life.
Ø Saturn – also nicknamed the “Ringed Planet”
Ø Recently the discovery of 20 new moons of Saturn has made Saturn the planet with the highest number of moons (82) against 79 moons of Jupiter.
Ø The discovered moons of planet Saturn may have once comprised a larger moon that was broken apart in the distant past.
Ø The newly discovered moons are able to continue orbiting Saturn after their parent moons broke apart indicates that these collisions occurred after the planet-formation process was mostly complete.
Ø These moons are the remnants of the objects that helped for planet formations thus studying the remnants might reveal the origin of the planet.
Ø Also, studying the orbits of these moons can reveal their origins, as well as information about the conditions surrounding Saturn at the time of its formation.
Uranus
Ø In contrast to all other planets, it is tipped and spin on its sides, that is its axis of rotation lies in nearly the plane of its orbit. (The poles of Uranus lie in a plane where equators of other planets lie)
Ø All eight planets in the Solar System orbit (revolve) the Sun in the direction of the Sun’s rotation, which is counterclockwise when viewed from above the Sun’s north pole.
Ø Six of the eight planets also rotate about their axis in this same direction (counterclockwise).
Ø Venus and Uranus have a strange retrograde rotation (clockwise), i.e., opposite of sun’s rotation.
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Neptune
Ø Uranus and Neptune (the ice giants) are called the twins of the outer solar system.
Ø They are surrounded by a thick atmosphere of hydrogen and helium and contains a higher proportion of “ices” such as water, ammonia, and methane ice giants” to emphasise this distinction.
Ø Neptune has the strongest sustained winds (2,100 km/h) of any planet in the Solar System.
Why are the inner planets rocky while the outer planets are mostly in gaseous form?
Ø The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases on the surface to condense to solid particles.
Ø The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the surface of the terrestrial planets.
Ø The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.
Ø The terrestrial planets are smaller, and their lower gravity could not hold the escaping gases.
Other Solar System Objects
Asteroid belt
Ø Asteroids are remnants of planetary formation that circle the Sun in a zone lying between Mars and Jupiter. The circular chain of asteroids is called the asteroid belt.
Ø The remnants of planetary formation failed to coalesce because of the gravitational interference of Jupiter.
Ø The asteroid belt lies between 2.3 and 3.3 AU from the Sun.
Ø Asteroids (planetoids ― another term for an asteroid) are composed mainly of refractory rocky and metallic minerals, with some ice.
Ø Asteroids range in size from hundreds of kilometres across to microscopic.
Ø All asteroids except the largest, Ceres, are classified as small Solar System bodies.
Ø Fragments of asteroids break off to form meteoroids, which can reach the Earth’s surface.
Ceres
Ø Ceres (2.77 AU) is the largest asteroid (946 km in diameter), a protoplanet, and a dwarf planet.
Ø Ceres has a mass large enough for its own gravity to pull it into a spherical shape.
Pluto and Charon
Ø Pluto (39 AU) is the largest known object in the Kuiper belt. Charon is Pluto’s largest moon.
Ø When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of a planet.
International Astronomical Union’s definition of a Planet: a Planet is an object that:
1. orbits the sun;
2. has sufficient mass to assume hydrostatic equilibrium ― a nearly round shape;
3. is not a satellite (moon) of another object, and
4. has removed debris and small objects from the area around its orbit
IAU’s definition of Dwarf planet: Dwarf planet is an object that meets planetary criteria except that it has not cleared debris from its orbital neighbourhood.
Ø Pluto is a part of the Kuiper belt that contains millions of rocky and icy objects. Also, there are numerous other objects in the Kuiper belt which are of similar size to Pluto. E.g. Eris (diameter: 2,326 km).
Ø So, if Pluto is considered a plant, then many other objects like Eris will also have to be considered as planets.
Ø Hence, Pluto (diameter: 2,377 km) (Kuiper belt) was voted by IAU as a dwarf planet just like Ceres (asteroid belt) and Eris (diameter: 2,326 km) (Kuiper belt).
Kuiper belt
Ø The Kuiper belt is a great ring of debris similar to the asteroid belt but consisting mainly of objects composed primarily of ice.
Ø It extends between 30 and 50 AU from the Sun.
Comets
Ø A comet is an icy small Solar System body that, when passing close to the Sun, heats up due to the effects of solar radiation and the solar wind upon the nucleus and begins to outgas, displaying a visible atmosphere or coma, and sometimes also a tail.
Ø Comets have highly elliptical orbits, unlike the planets which have near-circular orbits.
Ø They are made of frozen gases (water, ammonia, methane and carbon dioxide) which hold together small pieces of rocky and metallic minerals.
Ø Short-period comets (orbital period of a few hundred years) originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune.
Ø Longer period comets, with orbits of thousands of years, come from the more distant Oort Cloud.
Ø Oort cloud is a giant shell of icy bodies that encircle the solar system occupying space at a distance between 5,000 and 100,000 AU.
Ø One of the larger comets is the Halley’s Comet. The orbit of Halley’s Comet brings it close to the Earth every 76 years. It last visited in 1986.
Meteoroid, Meteor and Meteorite
Ø A meteoroid is any solid debris originating from asteroids, comets or other celestial object and floats through interplanetary space.
Ø A meteor is the streak of light that appears in the sky when a meteoroid enters the atmosphere (mesosphere) at about 200 km at high speed and burns up because of the friction.
Ø A meteor is popularly termed a ‘shooting star’ or ‘falling star’.
Ø In some cases, the meteoroid does not burn up completely and makes its way to the Earth’s surface. The surviving chunk is called a meteorite.
Ø The circular depression created on the earth’s surface after the meteorite’s impact is called as a meteorite crater.
Ø Meteorite impacts are common on all planets and moons in the solar system.
Ø The most conspicuous meteorite craters can be found on the surfaces of the Moon and Mercury (because they are geologically inactive due to negligible atmosphere).
Ø Largest Meteor Crater: A meteor crater in Arizona (USA) is 1,300 m deep is the largest meteor crater in the world. It was formed over 10,000 years ago.
Ø Chicxulub crater (Mexico’s Yucatan Peninsula) was caused by a meteor impact that is believed to have wiped out the dinosaurs (mass extinction at the end of the Cretaceous 65 million years ago).
Meteorite craters in India
Ø Lonar Lake (1.8 km in diameter) in Buldhana District of Maharashtra
Ø Dhala crater (14 km in diameter) in Shivpuri district, Madhya Pradesh and
Ø Ramgarh crater (3.5 km in diameter) is a potential meteorite crater in Kota plateau in Rajasthan.
The origin and evolution of the earth
Early Theories
1. Nebular Hypothesis
Ø Immanuel Kant, a German philosopher, gave this theory.
Ø In 1796, a mathematician Pierre-Simon Laplace reexamined it.
Ø According to this hypothesis, the planets were moulded out of a cloud of material associated with a young Sun, which was rotating slowly.
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Binary theories
Ø As per these theories, the sun had a companion.
3. Revised Nebular Hypothesis
Ø Revised Nebular Hypothesis was propounded by Carl Weizascar in Germany and Otto Schmidt in Russia.
Ø They regarded that a solar nebula surrounded the sun and that the nebula comprised of chiefly hydrogen, helium and something called dust.
Ø The collision of particles and the friction caused a disk-shaped cloud to be formed and then the planets were created via the accretion process.
Modern theories
1. Big Bang Theory
Ø Alternatively called the expanding universe hypothesis.
Ø As per this theory, in the beginning, all matter or substance forming this universe existed at one place as a tiny ball. This tiny ball had an extremely small volume, infinite density and temperature.
Ø At the Big Bang, this ball blasted fiercely and forcefully and started a substantial process of expansion which continues to this day.
Ø Now it is accepted that this event took place 13.7 billion years ago.
Origin of Earth
Formation of Planets
The following are regarded as the stages in the planet’s development:
Ø The stars are localised gas lumps inside a nebula.
Ø A core to the gas cloud as well as a spinning disc of dust and gas are created because of the gravitational force within the lumps.
Ø After this, the cloud of the gas condenses and the matter over the core is changed into tiny rounded objects.
Ø These small round objects develop into what are called planetesimals by a cohesion process.
Ø The smaller objects start forming larger bodies by colliding with one another and they stick together because of gravitational force.
Ø In the last stage, these large number of small planetesimals aggregate to develop into a smaller number of large bodies called planets.
Earth
The Earth is the third planet from the Sun in our Solar System. It is the planet, we evolved on and the only planet in our Solar System that is known to support life. It is 149.6 million kilometers away from the Sun. It has one moon. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune)
The Earth’s atmosphere is ideal for supporting life. Most of the Earth’s surface is covered with water. From the sky, the Earth appears blue. Earth is the only inner planet in our solar system that has liquid water on its surface. Seventy percent of the Earth’s surface is covered with water. Mountains, volcanoes, deserts, plains, and valleys cover the remaining 30 percent. Earth has an atmosphere made up of many different gases. The atmosphere gives us air to breathe. We live on the planet Earth.
Each day on the Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once). Each year on the Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once).
i Rotation and Revolution of the Earth
a. Rotation of the Earth: The Earth orbits around the Sun. It takes one year to go around the Sun one complete time. The Earth also rotates, or spins, on its axis. It takes one day to spin around itself one complete time. The Earth’s axis is not straight up and down, but tilted at an angle of 23.5 degrees. The rotation is what causes the change from day to night. This tilt is responsible for having seasons. If the Earth were not tilted, we would have the same season all year long.
b. Revolution of the Earth: The movement of the Earth around the Sun in a fixed orbit is called as revolving. One full orbit around the sun is one revolution. The Earth takes 365 days or 1 year to complete one revolution. The Earth revolves around the Sun because of gravity. The Earth really rotates 365 ¼ times during each revolution. The calendar always has 365 days, after every 4 years, the earth has made one extra rotation. How ever, one extra day is added to the month of February once in every four years forming a leap year with 366 days.
ii Day and Night
The Earth exhibits two different kinds of motion. One is rotation around its own axis and another movement is it revolves around the sun in a fixed orbit. The spinning of the Earth around its own axis causes day and night. It takes 24 hours to complete one rotation. However, sunlight shines only on the half of the Earth facing the Sun. That half has day the other half is dark and has a night.
The Moon
The moon is a satellite that revolves around the Earth in an oval shaped orbit. Earth’s moon is smaller than the Earth. It is smaller than the sun. It looks bigger because it is much closer to the Earth. It is 240,000 miles (384,000 kilometers) away.
Moon appears in the nighttime. The Moon turns so slowly that a moon’s day is two Earth-weeks long. Then it is night for the next two weeks. The moon is very hot during daytime and gets very cold at night. It appears hotter than boiling water because there is no air to protect the moon from the hot sunlight. It gets very, very cold during night because there is no air to hold heat on the moon. The moon has no atmosphere, air or water, to support life. Moon can be seen clearly with your eyes, binoculars, or a telescope.
Many manned and unmanned spacecrafts from the United States and Russia, Japan, China, India, have landed on the moon to study its surface. The surface of the moon has many craters caused by being hit by large meteoroids and asteroids. The moon is much like Earth in some ways. Its rocks are similar to Earth rocks. Lava rock from underground volcanic eruptions look like the lava rocks found on Earth.
i. Rotation of the Moon
ii. The Moon takes 29 1/3 days to make one orbit around the Earth. It also takes 29 1/3 days to complete one rotation on its axis.
iii. The phases, or changing appearance, of the Moon depends on its position in relation to the position of the Sun.
iv. Atmosphere of the Moon The moon is much like the Earth in some ways. Its rocks are similar to Earth rocks. The moon has no atmosphere, air or water, to support life. There are tall mountains and flat, dusty plains on the moon. The big holes on the moon are called Craters, which are made when space rocks hit the moon.
(a) Light from the Moon
We always see the one side of the Moon from the Earth. You have to go into space to see the other side. Moon does not make its own light. We can see it because it reflects light from the sun.
(b) Eclipse of the Moon
Every object, the Sun shines on casts its shadow. The Earth and moon also cast their shadows on each other. Most of the times these shadows fall on empty space. But sometimes we can see them from the Earth. As the moon revolves around the Earth, at times the Sun, the Earth, and the Moon are in a straight line.
Lunar Eclipse
The Earth blocks the sun’s light falling on the Moon. The Moon appears to be dark for a brief period, after which you would see a full moon. This is called as “Lunar Eclipse”.It occurs when the shadow of the Earth falls on the Moon. It occurs only on a full moon day, but not on all full moon days.
A penumbral lunar eclipse will be observed today midnight. The Earth will imperfectly align itself between the Sun and the moon, casting a shadow on the latter, marking the second lunar eclipse of the year.
Solar and Lunar eclipse has been quite frequent this year. Mark the major differences between them.
Lunar Eclipse
Ø A lunar eclipse occurs when the Moon moves into the Earth’s shadow.
Ø This can occur only when the Sun, Earth, and Moon are exactly or very closely aligned with Earth between the other two.
Ø A lunar eclipse can occur only on the night of a full moon. The type and length of a lunar eclipse depend on the Moon’s proximity to either node of its orbit.
Ø Any object that obstructs light will produce two shadows: one which will be dark and dense, is called the umbra; and the other which is light and diffused is called the penumbra.
Ø The only light reflected from the lunar surface has been refracted by Earth’s atmosphere.
Ø This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of bluer light. Due to this reddish colour, a totally eclipsed Moon is sometimes called a blood moon.
Types
In a total eclipse of the moon, the inner part of Earth’s shadow, called the umbra, falls on the moon’s face. At mid-eclipse, the entire moon is in shadow, which may appear blood red.
In a partial lunar eclipse, the umbra takes a bite out of only a fraction of the moon. The dark bite grows larger and then recedes, never reaching the total phase.
In a penumbral lunar eclipse, only the more diffuse outer shadow of Earth – the penumbra – falls on the moon’s face. This third kind of lunar eclipse is much more subtle and much more difficult to observe than either a total or partial eclipse of the moon.
How it is different from Solar Eclipse?
Ø A solar eclipse happens when the moon passes in between the earth and the sun. A lunar eclipse happens when the earth passes in between the moon and the sun.
Ø During a solar eclipse, the moon partially or fully hides the sun’s rays for a few minutes.
Ø Unlike a solar eclipse, which can only be viewed from a relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of Earth.
Ø Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than the full Moon.
What’s special this time?
Ø This eclipse is also called a strawberry moon eclipse — the term, interestingly, originates from an American concept and has little to do with the Euro-Asia region.
Ø June’s full moon usually coincides with the harvesting season of wild strawberries in America and the phenomenon was often addressed in reference to that.
Ø India had already witnessed an eclipse earlier this year, in January.
Ø The strawberry moon eclipse is going to be its second and probably the last visible lunar one in 2020.
Solar Eclipse
When the moon casts its shadow on the Earth it is called as “Solar Eclipse”. It occurs only on a new moon day, but not on all new moon days.
What is a Solar Eclipse?
It is a natural event that takes place on Earth when the Moon moves in its orbit between Earth and the Sun (this is also known as an occultation).
It happens at New Moon, when the Sun and Moon are in conjunction with each other.
During an eclipse, the Moon’s shadow (which is divided into two parts: the dark umbra and the lighter penumbra) moves across Earth’s surface.
Then, why isn’t there a solar eclipse every month?
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Country witnessed an annular solar eclipse on 21th June, 2020. This eclipse is a rare annular eclipse that occurs once in every one or two years, and coincides with the northern hemisphere’s longest day of the year, called the summer solstice. |
If the Moon was only slightly closer to Earth, and orbited in the same plane and its orbit was circular, we would see eclipses each month. The lunar orbit is elliptical and tilted with respect to Earth’s orbit, so we can only see up to 5 eclipses per year. Depending on the geometry of the Sun, Moon and Earth, the Sun can be totally blocked, or it can be partially blocked.
Solar Eclipse Types:
Total Solar Eclipse: It occurs when the Moon completely blocks the solar disk. In a total solar eclipse, the narrowest part of the path (where the Sun is completely blocked and the Moon casts its darkest shadow (called the umbra)) is called the “zone of totality”.
A phenomenon called “Bailey’s Beads” often appears as sunlight shines out through valleys on the lunar surface.
2. Annular Solar Eclipse:
When the Moon is farther away in its orbit than usual, it appears too small to completely cover the Sun’s disk. During such an event, a bright ring of sunlight shines around the Moon. This type of eclipse is a called an “annular” eclipse.
3. Partial Solar Eclipse:
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Eclipse Magnitude Ø Eclipse magnitude is the fraction of the Sun’s diameter which is covered by the Moon. Ø It is strictly a ratio of diameters and should not be confused with eclipse obscuration, which is a measure of the Sun’s surface area covered by the Moon. Ø Eclipse magnitude may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50). Ø By convention, its value is given at the instant of greatest eclipse. |
It occurs when Earth moves through the lunar penumbra (the lighter part of the Moon’s shadow) as the Moon moves between Earth and the Sun. The Moon does not block the entire solar disk, as seen from Earth. Depending on your location during a partial eclipse, you might see anything from a small sliver of the Sun being blotted out to a nearly total eclipse.
Seasons
You might have observed that the seasons change as a year progresses. You might have felt the heat of the summer and the chill in the winter. What causes these seasonal changes?
The Earth takes about one year to revolve around the Sun, and also it rotates on its axis. The axis on which the Earth rotates is slightly tilted, and this causes the cycle of seasons. While the Earth is revolving, if the Northern Hemisphere gets the direct sunlight, it has summer and on the other side, the Southern Hemisphere, will have winter. When the Southern Hemisphere faces direct sunlight it will have summer and it will be winter in the Northern Hemisphere.
Dark energy
Dark energy is an unknown form of energy which is hypothesised to permeate (spread throughout) all of space, tending to accelerate the expansion of the universe.
Dark matter
Ø The velocity of rotation for spiral galaxies depends on the amount of mass contained in them.
Ø But the outer arms of the Milky Way are rotating much too fast to be consistent with the amount of matter that we know exists in them.
Ø Such fast rotation is possible only when there is more mass, and that extra mass is believed to come from the dark matter.
Ø Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe.
Ø Dark energy plus dark matter constitutes 95.1% of the total content of the universe (the rest in the normal matter). In short, we are unsure about what’s there in 95% of the universe!
Ø The majority of dark matter is thought to be composed of some as-yet-undiscovered subatomic particles.
Ø The name dark matter refers to the fact that it does not appear to interact with observable electromagnetic radiation, such as light.
Ø It is thus invisible (or ‘dark’) to the entire electromagnetic spectrum, making it extremely difficult to detect.
Ø Dark matter interacts with the rest of the universe only through its gravity (that’s how we know it exists).
Anti-Matter
Ø It is hypothesized that every elementary particle in the Universe has a partner particle, known as an ‘antiparticle’.
Ø The particle and its antiparticle share many similar characteristics, but many other properties are the exact opposite.
Ø The electron, for example, has as its antiparticle the antielectron. They both have the same masses, but they have exactly opposite electrical charges.
Ø Most of the human understanding of the antimatter comes from high energy accelerator experiments.
Ø When a matter particle meets its antimatter particle, they destroy each other completely (i.e. annihilation), releasing the energy equivalent of their rest masses (following Einstein’s E = mc2).
For instance, when an electron meets an antielectron, the two annihilate and produce a burst of light which produces a corresponding energy level equivalent to the masses of the two particles.
Gravitational waves
Ø Gravitational waves are ‘ripples’ in the fabric of space-time caused by some of the most violent and energetic processes in the Universe.
Ø These ripples travel at the speed of light through the Universe, carrying with them information about their origins.
Ø Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity.
Ø He observed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that ‘waves’ of distorted space would radiate from the source (like the movement of waves away from a stone thrown into a pond).
Ø In 2015, LIGO (The Laser Interferometer Gravitational-Wave Observatory, USA) physically sensed the distortions in spacetime caused by passing gravitational waves generated by two colliding black holes nearly 1.3 billion light-years away!
While the processes that generate gravitational waves can be extremely violent and destructive, by the time the waves reach Earth, they are billions of times smaller.
Einstein’s theory of general relativity
Ø In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers and that the speed of light in a vacuum was independent of the motion of all observers.
Ø As a result, he found that space and time were interwoven into a single continuum known as space-time.
Ø Events that occur at the same time for one observer could occur at different times for another. This was the theory of special relativity.
Ø In 1915, Einstein published his theory of general relativity. In it, he determined that massive objects distort space-time, which is felt as gravity.
Ø Gravitational lensing and gravitational waves are strong evidence for Einstein’s theory of general relativity.
Gravitational lensing
Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lie behind it.
Importance of gravitational waves
Ø The gravitational waves can work as sirens to measure the expansion rate of the universe and to understand the origin and the future of the universe.
Ø Hubble’s Law: the farther away galaxies are, the faster they are moving away from Earth ― accelerating expansion of the universe).
Ø Hubble constant: a unit of measurement that describes the rate at which the universe is expanding.
Ø Two parameters that are essential to estimating the Hubble constant are the distance of the stars from Earth and how fast they are moving away from us (their velocity).
Ø But to date, the most precise efforts have landed on very different values of the Hubble constant.
Ø Scientists have proposed a more accurate and independent way to measure the Hubble constant, using gravitational waves.
Ø A flash of light would give an estimate of the system’s velocity (system: neutron stars or black holes orbiting each other), or how fast it is moving away from the Earth.
Ø The emitted gravitational waves, if detected on Earth, should provide a precise measurement of the system’s distance.
Ø By knowing the system’s velocity and distance, a precise calculation of Hubble constant is possible.
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