Earth in the Solar System

Look up at the night sky and you see countless points of light. Almost all of them are celestial bodies, the natural objects that exist in space. Our own home, the Earth, is one such body. Understanding where it sits among the others is the starting point for all of geography. The Earth's position decides how much heat and light it receives. That in turn shapes its days, its seasons, and its very ability to support life.

Celestial bodies fall into a few clear types, and the difference between them is worth fixing firmly in mind.

A star is a huge ball of hot, glowing gas that makes its own heat and light. The Sun is a star. It looks far larger than the others only because it is so much closer to us. Stars appear to form patterns in the sky, and a recognisable group of stars is called a constellation. Ursa Major, also known as the Saptarishi, is a well-known example.

A planet does not have any light of its own. It shines only because it reflects the light of a star. Planets move around a star in fixed paths called orbits. The word planet comes from a term meaning "wanderer", because to early sky-watchers the planets seemed to wander against the fixed background of stars.

A satellite is a body that moves around a planet. The Moon is the Earth's natural satellite. Objects that humans build and send into orbit, such as communication and weather satellites, are called artificial satellites.

The Sun, the eight planets that move around it, their satellites, and a large number of smaller bodies such as asteroids and meteoroids together make up the solar system. It is an enormous family held together by the Sun's gravity. Gravity is the pulling force that keeps each planet locked in its orbit.

How did this family form? Scholars have offered different answers. O. Schmidt suggested that the Earth and the other planets formed from a cloud of gases and dust particles that circled the Sun. James Jeans, by contrast, proposed the tidal or filament hypothesis. In his view a passing star pulled a long filament of hot material out of the Sun, and this filament broke up to form the planets. Examiners often ask which scholar linked the Earth's origin to gases and dust: the answer is Schmidt.

The Sun lies at the centre and is by far the largest member. It contains about 99.8 per cent of the entire mass of the solar system, and its diameter is about 109 times that of the Earth. It is the ultimate source of heat and light for the whole system. That energy drives almost every natural process on the Earth, from the winds to the water cycle. The Sun is about 150 million kilometres from the Earth. This average Earth-Sun distance is itself a unit of measurement: one Astronomical Unit (AU) is defined as the average distance between the Earth and the Sun.

Among all the planets, the Earth stands out in one comparative measure. It is the densest planet in the solar system, with an average density of about 5.5 grams per cubic centimetre.

The Sun is not always calm. A violent eruption on its surface, called a solar flare or solar storm, hurls charged particles and radiation towards the Earth. A major solar storm can:

• Disrupt navigation: GPS and other satellite navigation systems can fail.
• Damage power grids: induced currents can overload transmission networks.
• Intensify auroras: the polar light displays grow brighter and spread wider.
• Disturb satellites: the orbits and electronics of satellites can be affected.
• Cut radio links: shortwave radio communication of aircraft flying over polar regions can be interrupted.

Note carefully what a solar storm does not do. It does not cause tsunamis or forest fires, since those arise from earthly causes, not from charged particles.

Moving outward from the Sun, the eight planets in order are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A simple sentence can help you keep the order: "My Very Educated Mother Just Served Us Noodles."

Mercury is the nearest to the Sun and completes its orbit fastest. Venus is the brightest planet in the sky. It is often called the Earth's "twin" because it is so close in size. It is also the hottest planet because of its thick atmosphere. That atmosphere is made overwhelmingly of carbon dioxide. The gas traps the Sun's heat in a runaway greenhouse effect, pushing the surface temperature to around 465°C. Human life on Venus is therefore highly improbable, despite its Earth-like size. The four inner planets (Mercury, Venus, Earth and Mars) are small and rocky. The four outer planets (Jupiter, Saturn, Uranus and Neptune) are giants made largely of gas. Jupiter is the largest planet of all. It also has the largest number of natural satellites, or moons, of any planet. Saturn is famous for its bright rings.

Note that Pluto, once counted as the ninth planet, is now classed as a dwarf planet, which is why the count today is eight.

The Earth is the third planet from the Sun and the fifth largest. As far as we know, it is the only planet anywhere that supports life. Three conditions make this possible, and they are favourite points in examinations.

First, the Earth lies at just the right distance from the Sun. It is neither so close that it bakes nor so far that it freezes, so the temperature stays in the narrow range that living things need. Astronomers call this band of right distances around a star the Goldilocks Zone, the habitable zone where it is neither too hot nor too cold for liquid water. The term guides the search for Earth-like planets in outer space.

Second, the Earth has the right kind of atmosphere, the blanket of air around it. This air contains the oxygen that animals breathe and the carbon dioxide that plants use. It also shields the surface from the Sun's harmful rays.

Third, the Earth has water in liquid form on a vast scale. From space the oceans make the planet look blue, which is why the Earth is often called the "blue planet".

The Earth is not a perfect sphere. It is slightly flattened at the two poles and bulges a little at the Equator. This shape is called a geoid, which simply means "earth-like". The planet is also always in motion. As it travels around the Sun, the Earth moves at a mean orbital velocity of about 30 kilometres per second (more precisely, 29.8 km/s).

The Earth's life-friendly air was not always there. When the planet formed, over 4,000 million years ago, its atmosphere had almost no free oxygen. Carbon dioxide and methane dominated instead. The change came from life itself. Early living organisms released oxygen as they grew, and over vast spans of time they remade the atmosphere into the breathable blanket we know today.

The Earth behaves like a giant magnet, and its magnetic field is another shield for life. A few precise facts about it are tested again and again:

• Inclination: the magnetic axis is inclined at about 11 degrees to the geographic axis, not 23 degrees.
• Northern magnetic pole: it lies on the Boothia Peninsula in northern Canada, away from the geographic North Pole.
• Magnetic equator: it passes through Thumba in South India, which is why Thumba was chosen for rocket studies of the upper atmosphere.
• Reversals: the field has flipped its direction every few hundred thousand years in the geological past.
• Protection: the field deflects electrically charged particles arriving from space towards the poles. The particles never reach the surface; instead they strike the upper atmosphere near the poles and light it up as auroras.

The Earth's surface divides unevenly between land and water. The total land area is about 1,475 lakh square kilometres, roughly 149 million sq km. Land and water stand in a ratio of about 1:2.4, so water clearly dominates; the ratio is not 1:4. Of all the Earth's water, the Pacific Ocean holds the largest share. Among the continents, Asia is the largest, and after it the order of land share runs downward as follows:

• Africa: about 20 per cent of the world's land.
• North America: about 16.5 per cent.
• South America: about 12 per cent.
• Europe: about 6.7 per cent.

The Moon is the Earth's only natural satellite. Its diameter is about one-quarter that of the Earth, and it is roughly 384,400 kilometres away. The Moon moves around the Earth and also spins on its own axis. It takes the same time, about 27 days, to do both. Because of this, we always see only one side of the Moon from the Earth.

The Moon has no air and no water of its own. So it cannot support life. Its surface is covered with craters, mountains and plains. In 1969 the astronaut Neil Armstrong became the first human to set foot on it.

The Moon's orbital motion also shifts its rising time. The Moon moves eastward around the Earth by about 12 degrees each day, so the Earth must rotate that much extra before the Moon appears again. Moonrise is therefore delayed by roughly 48 to 50 minutes every day. A quick calculation shows the pattern: if the Moon rises at 6:48 p.m. today, then three days later it rises about 144 minutes later, at around 9:12 p.m.

Between the orbits of Mars and Jupiter lie thousands of small rocky bodies called asteroids. They are sometimes described as the broken pieces of a planet that never fully formed. Together they make up the asteroid belt.

Much smaller pieces of rock that move around the Sun are called meteoroids. When one of them enters the Earth's atmosphere it rubs against the air, heats up, and burns. It appears as a brief streak of light we call a shooting star or meteor. Most burn up completely. The rare piece that survives the fall and lands on the Earth is called a meteorite.

A comet is a different kind of small body, and examiners like to contrast it with an asteroid. Three differences matter:

• Composition: a comet is formed of frozen gases held together by rocky and metallic material. An asteroid is rocky through and through.
• Origin: comets come from the cold outer reaches of the solar system, the Kuiper Belt and the Oort Cloud. They do not originate between Venus and Mercury.
• Tail: as a comet nears the Sun, its frozen gases boil off and form a glowing tail. The Sun's radiation pressure and the solar wind push this gas and dust outward, so the tail always points away from the Sun, whichever way the comet is travelling.

One comet has earned a permanent place in question papers. In July 1994 the fragments of comet Shoemaker-Levy 9 crashed into Jupiter. It was the first directly observed collision between two bodies of the solar system.

An artificial satellite stays up for a simple reason. The Earth's gravity does pull on it constantly. That pull acts as the centripetal acceleration that keeps bending the satellite's path into a curve around the planet. The satellite is moving sideways so fast that it keeps "falling around" the Earth instead of falling onto it. Satellites are launched towards the east because the Earth rotates from west to east. The rocket gains the Earth's rotational speed as a free bonus, which saves fuel. Note that this bonus is extra velocity, not escape velocity itself.

The most examined orbit is the geostationary orbit, used by telecommunication satellites. A satellite is geostationary when three conditions are met together: the orbit is geosynchronous (it matches the Earth's rotation period), it is circular, and it lies in the plane of the Equator. Such a satellite sits at a height of about 35,786 km and appears fixed over one spot on the Earth. Any claim that geostationary satellites sit at about 10,000 km is therefore wrong. Remote sensing satellites instead use low polar or sun-synchronous orbits, so they sweep the whole globe and do not stay fixed in the sky.

A related broadcasting fact often appears alongside the orbit question. FM transmission of music is of very good quality. The reason is that atmospheric and man-made noises are mostly disturbances of amplitude, and they do little harm to a frequency-modulated signal.

India runs two classic satellite families with a clean division of labour:

• INSAT series: communication, broadcasting and meteorology satellites in geostationary orbit. They were launched abroad, mostly by Arianespace, the European launch company, from Kourou in French Guiana. INSAT-3E went up in 2003 and INSAT-4A in December 2005, the satellite behind Tata Sky's direct-to-home television.
• IRS series: Indian Remote Sensing satellites for Earth observation. They assess crop productivity, locate groundwater and aid mineral exploration. They do not handle telecommunications, which is INSAT's job.
• Oceansat-2: an ocean-watching satellite that estimates atmospheric water vapour, helps predict the onset of the monsoon and monitors coastal water pollution.
• Bhuvan: ISRO's web geoportal that serves 3D satellite imagery of India, similar in spirit to Google Earth.

Remote sensing data has a wide reach. Satellite images can estimate the chlorophyll content of vegetation, greenhouse gas emissions from rice paddies and land surface temperatures of a location.

Satellite navigation is another favourite area. GPS, the American system, supports mobile phone operation, time-stamping in banking and the synchronisation of power grids. Other systems include GLONASS (Russia), Galileo (a multi-satellite navigation project of the European Union), BeiDou (China) and QZSS (Japan's own regional navigation system). India's regional system is IRNSS, now called NavIC. It uses three satellites in geostationary orbit and four in geosynchronous orbits. Its coverage is regional, extending about 1,500 km beyond India's borders, not global. A darker use of orbits is the Fractional Orbital Bombardment System, a concept in which a weapon is placed in a stable low orbit and then deorbited over a target, evading missile defences.

A rocket differs from a jet engine in one crucial way. A jet engine draws oxygen from the surrounding air, so it cannot work in the vacuum of space. A rocket carries both its fuel and its own supply of oxidiser, so it works anywhere. The most powerful upper stages use cryogenic engines, which burn liquid hydrogen with liquid oxygen stored at extremely low temperatures. Cryogenic engines belong to rocket technology, not refrigeration. India tested its indigenous cryogenic stage in 2006, at the test facility in Mahendragiri in Tamil Nadu. India was not alone in this club: the USA, Russia, China, Japan and France (through the European Space Agency) all hold cryogenic capability. The first GSLV flights, in fact, flew on a Russian cryogenic stage.

India's two workhorse launchers divide their tasks. The PSLV (Polar Satellite Launch Vehicle) lifts remote sensing satellites into polar, sun-synchronous orbits, so its satellites do not appear fixed in the sky. The GSLV (Geosynchronous Satellite Launch Vehicle) is built mainly for heavier communication satellites bound for geostationary orbit. In 1994 India demonstrated its capability to launch geostationary-class satellites. In September 2002 the PSLV-C4 carried METSAT, India's first full-fledged meteorological satellite (later renamed Kalpana-1), a payload of over 1,000 kg, into a geosynchronous transfer orbit. GSAT, launched in 2001, carried payloads to demonstrate digital broadcasts and internet services. K. Kasturirangan chaired ISRO from 1994 to 2003, the period that included the INSAT-3B launch.

India's flagship missions repay careful memorising:

• Chandrayaan-1 (2008): landed India's Moon Impact Probe on the Moon. China had impacted the Moon earlier through its Chang'e programme.
• Mangalyaan (Mars Orbiter Mission): made India the first country to put a spacecraft in Mars orbit on its very first attempt. India was the fourth space agency overall to orbit Mars, not the second.
• Astrosat (2015): India's first multi-wavelength space observatory, about 1,513 kg in a roughly 650 km orbit. Other agencies besides the USA and Russia also operate such observatories.
• Gaganyaan, SpaDeX and Axiom-4: India's human spaceflight programme, its space docking experiment, and a commercial mission to the International Space Station. All three encourage micro-gravity research.

A set of foreign missions recurs in questions:

• Cassini-Huygens: orbited Saturn, not Venus.
• Messenger: mapped and investigated Mercury.
• Voyager 1 and 2: explored the outer solar system.
• Deep Impact (2005): struck the comet Tempel 1 to photograph and study its nucleus.
• 2001 Mars Odyssey: NASA's 2001 orbiter sent to Mars.
• THEMIS: a NASA mission studying the geomagnetic substorms behind the colourful auroral displays of high-latitude skies.
• COBE: the Cosmic Background Explorer, a satellite programme that mapped the early universe's radiation.

A few supporting facts complete the picture. Space shuttles such as Discovery and Atlantis launched from Cape Canaveral on the coast of Florida. Atlantis was a shuttle, not a space station. Japan landed a spacecraft on an asteroid, while the first human embryo cloning came from South Korea, not Germany. Unmanned Aerial Vehicles (UAVs) vary widely: not all of them can land vertically, hover automatically or run on battery alone. NASA's GL-10 "Greased Lightning" is an electric, remotely piloted aircraft that takes off vertically like a helicopter and then flies like a plane.

Distances between stars are so vast that kilometres become useless. Astronomers use the light-year, the distance light travels in one year. It works as a unit only because the speed of light is always the same, about 300,000 km per second in a vacuum.

Stars live and die by their mass. A giant star has a far greater rate of nuclear reactions than a dwarf star, so it burns through its fuel quickly and dies young. Dwarf stars live much longer. The Indian-American astrophysicist Subrahmanyan Chandrasekhar proved that a star with less than 1.44 times the Sun's mass ends its life as a white dwarf. This figure is the Chandrasekhar limit, and it won him the Physics Nobel Prize in 1983. NASA's Chandra X-ray telescope is named in his honour. Three other objects are easy to confuse:

• Cepheids: variable stars that brighten and dim periodically, used to measure cosmic distances.
• Nebulae: giant clouds of dust and gas in space, the nurseries of stars.
• Pulsars: spinning neutron stars formed when massive stars run out of fuel and collapse.

A black hole is a collapsed body so dense that its escape velocity exceeds the speed of light, so no radiation can leave it. Its boundary is the event horizon, and the crushed point at its centre is the singularity. Terms like event horizon, singularity, string theory and the Standard Model all belong to the observation and understanding of the universe.

Albert Einstein's General Theory of Relativity predicts three phenomena that examiners love: light is bent by gravity, the universe is constantly expanding, and matter warps the space-time around it. Two observations serve as evidence for the continued expansion of the universe: the detection of cosmic microwave background radiation and the redshift of light from distant galaxies. The theory also predicts gravitational waves, ripples in space-time. When scientists observed the merger of giant black holes billions of light-years away, the significance was that gravitational waves were detected, by the LIGO observatories. The planned eLISA (evolved Laser Interferometer Space Antenna) mission will hunt these waves from space. It will fly three spacecraft in an equilateral triangle with sides one million kilometres long, with lasers shining between the craft.