Some Interesting Facts About The Universe

••• UNIVERSE•••

All the heavenly bodies i.e. stars, planets, satellites, asteroids, meteors comets, dust and gases are called Celestial bodies, which together form the universe. The science of studying these heavenly bodies is known as astronomy and the scientists who practice astronomy are called astronomers.

Theories about Universe

Universe has been fascinating since ancient times. Various scholars have presented their views on universe. These theories can be put into two categories:

(1) Early Theories

Geocentric Theory The theory is proposed by Ptolemy in AD 140, which states that the Earth is the centre of the universe.

Heliocentric Theory The theory is proposed by Copernicus in AD 1543, according to which the Sun is the centre of the universe In 1805, British astronomer Hershel studied universe with the help of telescope and stated that the solar system is merely a small part of a galaxy.

An American astronomer, Edwin P Hubble in 1925 said that the universe has a diameter of 2.5 billion light years and is composed of several galaxies.

(1) Modern Theories

There are four main modern theories put forward to explain the origin and evolution of the universe

THE BIG BANG THEORY

It is the most acclaimed theory proposed by Georges Lemaitre in 1930s and later strongly evidenced by George Gamow which explained the modern Big Bang theory.

Theory states that, at the beginning, all matter forming the universe existed in one place in the form of a tiny ball (singular or primeval atom) with an unimaginabily small volume, infinite temperature and indefinite density. Around 15 billion years ago, a vast explosion occurred and the primeval atom disintegrated giving rise to space and time and the expansion of the universe that still continues till today and will continue further The expansion of matters further formed galaxies and these galaxies are continuously expanding and moving away from each other in an accelerated velocity and in the process cooling the universe.

The Big Bang or the expanding universe is now proven by the discovery of the Cosmic Microwave Background Radiation (CMBR) and Wilkinson Microwave Anisotropy Probe (WMAP). Another important astronomic observation called red shift also proved the expansion of universe.

The recent experiments at the Large Hadron Collider (LHC), built by the European Organisation for Nuclear Research beneath the Franco-Swiss border near Geneva, Switzerland, aims to recreate conditions just after big bang to have a better understanding of the phenomenon.

RED SHIFT THEORY

Light is formed of a band of colours called spectrum. Violet colour has the shortest wavelength and lies at one end of the spectrum and red light has longest wavelength and lies at another end The movement of a star or galaxy affects its light as seen by the observer. When the star is approaching the observer the light shifts to the blue end of the spectrum and the light shifts to the red end when the star recedes away. This is known as the doppler effect or doppler shift. The doppier shift of the galaxies show that they are receding and that the universe is in a state of rapid and accelerated rate of expansion. This astronomical observation is known as red shift.

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 it 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.

AGE OF THE UNIVERSE

Astronomers estimated that the Big Bang occurred between 12 and 14 billion years ago and the age of the universe is now accepted to be 13.7 billion years on the basis of measurements made on the Cosmic Microwave Background Radiation (CMBR). Our solar system is thought to be 4.5 billion years old.

Astronomers estimate the age of the universe in two ways:

(i) By observations made on globular clusters which are a dense collection of roughly a million stars having many old stars.

(ii) By measuring the rate of expansion of the universe called Hubble Constant (Ho) and extrapolating back to the Big Bang

The fate of the universe is determined by a struggle between the momentum of expansion and the pull of gravity. The rate of expansion is expressed by the Hubble Constant (Ho) while the strength of gravity depends on the density and pressure of the matter in the universe If the pressure of the matter is low then the fate of the universe is governed by the density. If the density of the universe is less than the 'critical density', which is proportional to the square of the Hubble Constant, then the universe will expand forever.

If the density of the universe is greater than the critical density, then gravity will eventually win and the universe will collapse back on itself, which is called big crunch This strange form of matter is now referred as dark energy. If the dark energy infact plays a significant role in the evolution of the universe, then in all likelihood the universe will continue to expand forever.

COMPONENTS OF THE UNIVERSE

Our universe has a foamy structure. Considering only the largest structures the visible universe is made up of filaments, superclusters and galaxy groups and clusters. The galaxy groups and clusters in combined form are called superclusters, which are on turn form part of walls, which are also parts of filaments.

The visible part of universe is concentrated in certain parts in a complex scaffold that surrounds a network of enormous cosmic voids or vast empty spaces, In addition to the 'normal' matter that makes up the visible parts of the universe, scientists have discovered that there are vast amounts of unseen matters called dark matters. In proportion

DARK ENERGY 68%

DARK MATTER 27%

VISIBLE MATTER 5%

The visible matter of the universe are composed of:

Hydrogen(72-75%)

Hellum (23-26%)

Other elements<2%

• The process of nuclear fusion has very slightly increased the quantities of helium, but the proportional composition is practically unchanged since, the earliest years of our universe.

Dark matters cannot be seen by any of theelectromagnetic spectrum such as radiowaves, x-ray etc. but certain phenomena such as gravitational lensing. temperature distributions, orbital velocities and rotational speeds of galaxies etc, evidence of a missing mass and justify the probable existence of dark matter, Another Invisible entity known as dark energy, is believed to be the reason why galaxies are speeding away at an accelerated rate.

Galaxy 

A large group of stars is called galaxy. There are billions of galaxies having different sizes, regular and irregular shape. Galaxies can be divided into two categories:

(I) NORMAL GALAXIES

These galaxies emit comparatively small amount of radio radiations as compared to the total radiations emitted. These galaxies are bright from the centre and gradually dim towards the edges. Each normal galaxy contains billions of stars in the form of a band, travelling together in the universe. Depending upon their shapes, the normal galaxies may be divided into three groups namely:

- Elliptical galaxies 

- Spiral galaxies

- Irregular galaxies

About 18% of the galaxies are elliptical, 80% of the galaxies are spiral and only 2% of the galaxies are irregular galaxies. The Irregular galaxies are youngest, spiral galaxies are middle aged and elliptical galaxies are quite old.

(II) RADIO GALAXIES

• These galaxies emit million times more radio radiations than normal galaxies. The radio radiations do not come from the galaxy itself, but are believed to be coming from two large radio sources.

There are 170 billion galaxies in the observable universe. The largest galaxies have nearly 400 billion stars and our galaxy 'Milky Way' has about 100 billion stars.

MILKY WAY

It is the name of the galaxy to which our Earth belongs. If one looks at the sky on a clear night, a hazy band of white light stretching across a great circle is seen, which is called the Milky Way or Akash Ganga. It is a spiral galaxy.

Recently a super massive black hole known as sagittarius "A" found in the core of the Milky Way.

STARS

• A star is a fiery luminous heavenly body that has own light and heat energy. Sun is the nearest star to our planet Earth and it takes 8.3 minute (500 second) for light to reach Earth from Sun. Proxima centauri is the nearest star beyond our solar system that is at a distance of 4.3 light years from the Earth.

• Stars may exist as single star, but are very few in the universe (only 25%). They may also occur in pairs called binary stars (about 33%) and the rest are multiple stars. Alpha centauri consists of three stars.

• Variable stars are stars that show varying degrees of luminosity. Luminosity fluctuates between periods. Delta cephei is an example. Stars of fluctuating luminosity are called cepheid variables.

• Pulsars are variable stars which emit regular pulses of electro-magnetic waves of very short duration whereas quasars are powerful quasi-stellar sources of radio radiations.

• Stars form when enough dust and gas clump together because of gravitational forces. Nuclear reactions release energy to keep the star hot. Planets form when smaller amounts of dust and gas clump together because of gravitational forces.

• Stars like the Sun change their form into other forms of stars, such as red giants, white dwarfs, neutron stars and black holes during their lifetime. The fate of a star depends upon how much matter it contains.

• High mass stars are much brighter than low mass stars, thus, they rapidly burn through their supply of hydrogen fuel. A star has enough fuel in its core to burn which makes it bright e.g. the Sun has enough fuel to keep it bright for approximately 9 billion years.

• A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A 10 solar mass star, a star that is 10 times more massive than the Sun, burns nearly a thousand times brighter and has only a 20 million year fuel supply. Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion years.

• Life Cycle of a Star Stars are born in nebulae. A nebula is a cloud of gas (hydrogen) and dust in space. Huge clouds of dust and gas collapse under gravitational forces, forming protostars. These young stars undergo further collapse, forming main sequence stars.

• Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contracts and the outer layers expand, cool and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually, collapse and explode, then becomes either a black dwarf, neutron star or black hole.

• (Transformation of stars) → Red giant White dwarf → Black dwarf.

• (Transformation of stars) → Red supergiant →

Supernova → Neutron star or Black hole.

• White Dwarf This is very small, hot star, the last stage in the life cycle of a star like the Sun. White dwarfs have a mass similar to that of the Sun, but only 1% of the Sun's diameter; approximately the diameter of the Earth.

• Supernova This is the explosive death of a star and often results in the star obtaining the brightness of 100 million Suns for a short time.

There are two general types of supernova:

(i) Types I These occur in binary star systems in which gas from one star falls on to a white dwarf, causing it to explode.

(ii) Types II These occur in stars ten times or more as massive as the Sun, which suffer runaway internal nuclear reactions at the ends of their lives, leading to an explosion. They leave behind neutron stars and black holes. Supernova are thought to be main source of elements heavier than hydrogen and helium.

Neutron Stars These stars are composed mainly of neutrons and are produced when a supernova explodes, forcing the protons and electrons to combine to produce a neutron star.

• Typical stars having a mass of three times the Sun, but a diameter of only 20 km.

• If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole. 

• Pulsars are believed to be neutron stars that are spinning very rapidly.

• Black Holes These are believed to form from massive stars at the end of their life times. The density of matter in a black hole cannot be measured.

• The gravitational pull in a black hole is so great that nothing can escape from it, not even light. It can often engulf neighbouring matters including stars and planets etc.

BLACK HOLE AND GRAVITATIONAL WAVES

When two black holes orbit each other and merge, it could cause ripples in space. These ripples would spread out like the ripples in a pond when a stone is tossed in. Scientists call these ripples of space, gravitational waves. Gravitational waves are invisible. They travel at the speed of light (186,000 miles per second).

The first direct observation of gravitational waves was made September 14, 2015 at LIGO.

THE CHANDRASEKHAR LIMIT

It is an upper bound on the mass of bodies made from electron-degenerate matter such as white dwarf. The calculation of the maximum mass of 1.44 solar masses for a white dwarf was done by Subrahmanyan Chandrasekhar. This implied that for masses above 1.44 solar masses there could be no balance between electron degeneracy and the crushing gravitational force and that the star would continue to collapse.

The collapse will continue until it is stopped by neutron degeneracy with the formation of a neutron star. But even that is not the ultimate limit, since neutron degeneracy can also be overcome by masses greater than three solar masses and the ultimate collapse is toward a black hole.

SOME FACTS OF UNIVERSE

• Universe was very hot when young. Within minutes of expansion the temperature was billions Kelvin whereas the average temperature of the universe is now only 2.725 kelvin.

• As time grows the universe is cooling and marching towards a big freeze also known as Heat Death, when the universe will be devoid of any usable heat energy. Such prediction is supported by the measurements made by the Wilkinson Microwave Anisotropy Probe (WMAP) on the current geometry and density of the universe.

• The universe spans a diameter of over 150 billion light years and is flat not spherical. The universe is ever expanding and having no centre as each galaxy is away from one another. • Some scientists predict that such expansion may fate with a radically catastrophic ending in a Big Rip, that everything would be forced to be ripped apart.

OUR CHANGING VIEW ABOUT THE UNIVERSE

• 2000 years ago, the Greek astronomers thought that the Earth was at the centre of the universe and the Moon, the planets, the Sun and stars were orbiting around it. In the 6th century, it was Aryabhatta who stated that the rotation of the heavenly bodies as observed by us is due to the rotation of the Earth around its axis.

• Aryabhatta discovered that the day and night occur due to the rotation of the Earth. He also established that the lunar and solar eclipses were caused due to the shadows of the Earth and the Moon respectively. In the 15th century Nicholas Copernicus, the polish scientist showed that the Sun was at the centre of the Solar System and planets revolve around it. So, the Sun became the centre of the universe.

• In the 16th century, Johannes Kepler discovered the laws of planetary orbits, but the Sun still remained at the centre of the universe.It was only in the beginning of the 20th century, that a picture of our own galaxy became clear. The Sun was found to be located in a corner of the galaxy.

DISCOVERY OF GRAVITATIONAL WAVES

In February 2016, scientists from Laser Interferometer Gravitational-wave Observatory (LIGO) revealed that they had detected gravitational waves. Previously gravitational waves had only been understood indirectly, via their effect on the timing of pulsars in binary star systems.

First proposed in 1916 by Albert Einstein, gravitational waves are ripples in space and time that are produced when whole black holes collide and stars explode. Through these waves scientists hope to gain valuable insight into the universe because these waves experience no barriers, unlike electromagnetic waves such as radio waves, visible light, infrared light, X-rays and gamma rays.

Laser Interferometer Gravitational wave    Observator (LIGO)-India

• LIGO-India is planned as a collaborative project between a consortium of Indian research institution and LIGO laboratory in the USA, along with its international partner Australia, Germany and UK.

The LIGO project operates three gravitational wave detectors. Two are at Hanford and one at Livingston in USA. The proposed project aims to move one advanced LIGO detector from Hanford (USA) to Hingoli in Maharashtra, India.

Gravitational waves are distortions or 'ripples' in the fabric of space-time caused by some of the most violent energetic processes in the universe.

THE SOLAR SYSTEM

• The Solar System is dominated by the Sun, which accounts for almost 99.9% of the matter of the whole system. It is the source of light and heat to us. Scientists believe that the Sun has been formed from a moving cloud of gases, which is called nebula. The Sun and the planets were born out of this cloud. The force of gravity has created them.

For over millions of years, these balls of dust and gas are moving around the Sun. The Sun by virtue of its mass and weight, controls the movement of the planets. This force is called the force of gravity. Sun is at the centre of the Solar System with eight planets (i.e.Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) and their satellites, dwarf planets, asteroids, meteors and comets that move around the Sun. Till 2006, there were nine planets in the Solar System. Pluto was the farthest planet from the Sun. In 2006, the International Astronomical Union (IAU) adopted a new definition of a planet. Pluto and other celestial bodies like Ceres, 2003 UB 313 does not fit this definition. It is no longer considered as a planet of the solar system.

THE SUN

It is believed that the Sun was born about 5 billion years ago. Since that time, it is continuously emitting huge amount of heat and light and it is expected to glow in a similar manner for the next 5 billion years or so on. The Sun is the nearest star from US.

• The Sun is the source of almost all energy on the Earth. In fact, the Sun is the main source of heat and light for all the planets. The radius of the Sun is almost 100 times of the radius of the Earth and its mass is about a million times the mass of the Earth.

• The Sun is essentially a sphere of hot gases. The disc is also called the photosphere. The layers of gas above the PHOTOSPHERE are extremely hot, but their densities are extremely low. These layers are very faint and are not visible in the presence of strong light from the disc of the Sun.

•At the time of the total solar eclipse, when the light of the Sun's disc is completely cut off, the outermost layer becomes visible. It appears like a crown round the Sun. This layer is called Corona. Solar flares are charged particles overcome from Sun's gravity and comes into the outer space. When these solar flares reach the Earth atmosphere they collide with the gaseous particles and produces a colourful effect. In South pole region, these effects are known as Aurora Australis and in North pole Aurora Borealis.

• The temperature of the bright disc is about 6000 Κ. At the centre of the Sun, the temperature is about 15 million K. The pressure is also extremely high. All these conditions are right for fusion reactions. In the Sun, four protons (nuclei of hydrogen atoms) combine to form a helium nucleus.

• The mass of the helium nucleus is slightly smaller than the combined mass of four protons. So, there is a loss of mass in the process. This loss of mass is the source of Sun's energy.

• The Sun is about 150 million km away from the Earth. Light travels at a speed of 300000 km/s. So, the light of the Sun takes about 8.3 light minutes to reach us. The light of the nearest star (Proxima or Alpha centuari) to the Sun reach Earth in about 4.3 light years.

• In August, 2018 NASA has launched first planned robotic spacecraft to study outer corona of Sun. The goal of the probe is to determine structure and dynamics of magnetic fields at sources of solar wind. It will also trace flow of energy that heats corona and accelerates solar wind.

THE PLANETS

• Planets are actually revolving around the Sun. They have their own elliptical path of movement known as the orbits. Movement around its own axis is called rotation and around the Sun is revolution. They have their own speed also.

• Planets, unlike stars, have no light or heat of their own. They are lit by the light of the stars. Stars twinkle whereas planets do not. The word planet comes from the Greek word Planetai, which means wanderers. Planets keep changing their positions with respect to the stars.

INNER AND OUTER PLANETS

• The first four planets namely Mercury, Venus, Earth and Mars are much nearer to the Sun than the other four planets. They are called the inner planets. The inner planets have very few Moons. The planets outside the orbit of Mars namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have more number of Moons.

TERRESTRIAL AND JOVIAN PLANETS

• Mercury, Venus, Earth and Mars are also called the terrestrial planets because their structure is similar to the Earth. The common features of these planets are as follows:

- a thin rocky crust

- a mantle rich in iron and magnesium

- a core of molten metals have thin atmospheres.

- the planets outside the orbit of the Mars are called Jovian

-planets because their structure is similar to that of Jupiter. 

The common features of these planets are as follows:

-these are all gaseous bodies. 

-they have rings, which are belts of small debris around them.

- have large number of satellites.

MERCURY (BUDDHA)

• It is nearest to the Sun. It is the smallest planet of the Solar System, nearly of the same size and mass as the Moon, Mercury takes 88 days for one orbit around the Sun and 59 days for one spin on its axis. It has no satellite.

• There is no atmosphere on mercury. The surface of mercury is rocky and mountainous. One side of the surface facing the Sun receives maximum heat and light. The surface of this planet does not receive sunlight or heat on its other side. One part of Mercury, therefore, is very hot while the other part is very cold.

• As Mercury is very close to the Sun, it is difficult to observe it, as most of the time it is hidden in the glare of the Sun. However, it can be observed in September and October just before sunrise in the Eastern sky as a morning star. It is also seen in the Western region of the sky immediately after sunset in March and April as an evening star.

VENUS (SHUKRA)

• Venus has no Moon or satellite of its own. Its rotation on its axis is somewhat unusual i.e. from East to West. Its size is nearly the same as that of the Earth. The mass of Venus is nearly 4/5 times that of the Earth. It takes 224.7 days for one orbit around the Sun and 243 days for one spin on its axis.

•Venus has an atmosphere that consists of mainly carbon dioxide. Minute quantities of water vapour and oxygen have also been found on Venus. The cloudy atmosphere of Venus sends back almost 3/4th of the sunlight that falls on it. Because of its thick, toxic atmosphere which is made up of Sulfuric acid clouds, it is an example of extreme greenhouse effect. It is hottest planet in Solar System. 

• Sometimes, Venus appears in the Eastern sky before sunrise and sometimes it appears in the Western sky just after sunset. Therefore, it is often called a morning or an evening star.

GOLDILOCK ZONE

The Goldilocks zones an area of space in which a planet is just the right distance from its home star so that its surface is neither too hot nor too cold. The Earth occupies what scientists sometimes call the Goldilocks zone. Its distance from our star means it is neither too hot, nor too cold to support liquid water-thought to be a key ingredient for life.

Just because a planet or moon is in the Goldilocks zone of a star. it doesn't mean it's going to have life or even liquid water. Venus and Mars are also in this habitable zone, but aren't habitable because of lack of atmosphere and solidified core.

THE EARTH (PRITHVI)

• Our Earth rotates from West to East. Our Earth is more or less like a sphere, which is slightly flattened in the North and South. The Earth is best to be described as geoid which means Earth like shape.

• Earth is the third nearest planet to the Sun. In size it ranks fifth. In size and make up the Earth is almost identical to Venus. Due to presence of water and landmasses, the Earth appears blue-green in colour from the space. Therefore, it is called blue planet.

• The Earth is the only planet where some special environment conditions are responsible for the existence and continuation of life.

OUR ONLY NATURAL SATELLITE-THE MOON

A small body revolving around a bigger body is called its satellite. The Moon is a satellite of the Earth. Planets are the satellites of the Sun. These are natural satellites. Like planets, satellites also have no light or heat of their own.

As the Moon revolve around the Earth and also around the Sun its position changes daily in relation with the Sun. As a result, the relative positions of the Moon keep on changing everyday. i.e. Moon appears different each night. The various shapes of the bright part of the Moon as seen during a month are called phases of the Moon.

The Moon takes almost the same time, 27 days and 7 hours to complete both the movements i.e. rotation and revolution. Thus, it always presents the same side towards our Earth. The Moon has no atmosphere.

The diameter of the Moon is only 1/4th of that of the Earth. The Moon is about 384400 km away from the Earth. The light reflected by the Moon reaches us in just one and a quarter seconds. Neil Armstrong, Edwin Aldrin and Collins set foot on the Moon for the first time on 21st July, 1969. They found that the Moon's surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. These cast shadows on the Moon's surface.

MARS (MANGAL)

• It is almost half the size of the Earth. The diameter of Mars is slightly more than half of that of the Earth, but its mass is only 1/10th of that of the Earth. It takes 687 days for one orbit around the Sun and 1 day for one spin on its axis. The atmosphere of Mars is much thinner than that of the Earth.

• It perhaps consists mainly nitrogen and argon. Traces of oxygen have also been found. It appears slightly reddish and therefore, it is also called the red planet. Mars has two small natural satellites named Phobos and Deimos.

• In July, 2018, scientist revealed that they had found evidence of a liquid lake beneath the surface of its's Southern pole's ice cap.

• The Mars is visible from the Earth for most part of the year. However, it is best situated for observation, when it is opposite the Sun's position in the sky with respect to the Earth. On these days, it is closer to the Earth as well.

JUPITER (BRIHASPATI)

• It is the largest planet of the Solar System. It takes 11 years and 11 months for one orbit around the Sun and 9 hours, 56 minutes for one spin on its axis. It has 67 satellites. It also has faint rings around it. Its most distinguishing feature is the great red spot. It is believed that it is a complex storm in the atmosphere of the planet ranging for a few hundred years.

• Its mass is more than the combined mass of all other planets. Because of its large mass, Jupiter exerts a strong gravitational pull on other objects which pass by it. A comet passing close to it is likely to experience this pull and stray from its original path. In 1994, the comet, shoemaker-levy, actually broke up into several pieces when it came too near this planet. The pieces then collided with the planet.

•Jupiter's bright appearance in the sky after Venus and occasionally Mars is due to its thick atmosphere that reflects most of the sunlight falling on it. It is believed that Jupiter mainly consists of hydrogen and helium in gaseous form. Its cloud like outer regions consists of methane in gaseous form while ammonia is present in crystalline form.

SATURN (SHANI)

Beyond Jupiter is Saturn which appears yellowish in colour. What make it unique in the Solar System are its three beautiful rings. It takes 29 years, 5 months for one orbit around the Sun and 10 hours, 40 minutes for one spin on its axis. It has 82 satellites. Saturn is the least dense among all the planets. Its density is less than that of water. It is similar in mass and composition to Jupiter. It is however, cooler than the Jupiter.

URANUS (ARUN)

• It was the first planet to be discovered with the help of a telescope. William Herschel discovered the planet in 1781. Hydrogen and methane have been detected in the atmosphere of Uranus. Except Venus and Uranus, all other planets rotate in the same direction in which they revolve.

• Like Venus, Uranus also rotates from East to West. The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side. It takes 84 years for one orbit around the Sun and 17 hours, 14 minutes for one spin on its axis. It has 27 satellites.

NEPTUNE (VARUN)

• It was discovered by Sir William Herschel on the basis of the law of gravitation given by Sir Issac Newton which he gave almost 180 years before it. It takes 164 years for one orbit around the Sun and 16 hours 7 minutes for one spin on its axis. Uranus and Neptune are quite faint and cannot be seen with the naked eye. That is why only 6 planets were known in ancient times. These two planets have been discovered only after telescopes came into use in astronomy. It has 8 satellites.

PLUTO

Discovered in 1930 by Clyde Tombaugh, Pluto was long considered our solar system's ninth planet. But after the discovery of similar fascinating planets deeper in the distant Kuiper Belt, Pluto was reclassified as a dwarf planet in 2006.

It takes 248 years in revolution and 155 hours in rotation. Pluto has five known moons. Charon (the largest), Styx, Nix, Kerberos and Hydra. Pluto and Charon are sometimes considered as a binary star system.

New Horizons is a NASA's spacecraft that was the first to visit dwarf planet Pluto in July 2015. Its flyby images of the dwarf planet's icy surface, as well as observations of Pluto's moon Charon, are revolutionizing our understanding of solar system objects far from the sun.

The mission is now enroute to an object deeper in the Kuiper Belt, called 2014 MU69. It reached this object on 1st Jan, 2019.

BOUNDARY OF THE SOLAR SYSTEM

The orbit of Pluto does not mark the boundary of the Solar System. It extends much beyond it. Its size has been estimated to be about 105 AU (Astronomical Unit which is equal to the mean distance between the Earth and the Sun ie. 150 million km). At the edge of the Solar System, there exist billions of very small objects.

These objects were formed very early in the gas cloud from which the Solar System evolved. Occasionally, paths of these bodies are disturbed by accidents, such as a star passing nearby, causing some of them to move towards the Sun, which appears as comets. Comets are of great interest to scientists. This is because the material of comet presents a sample of the original material from, which the Solar System was formed.

ASTEROIDS(Starlike heavenly bodies)

There are numerous tiny heavenly bodies found between the orbits of Mars and Jupiter, which revolve around the Sun. These bodies are called asteroids. Each asteroid has its own orbit and the orbits of all of them are spread over a large distance forming a band. A few asteroids have elongated orbits going beyond the orbit of Jupiter.

The size of asteroids varies from barely a pebble to a few 100 km. The largest asteroid is Ceres, has a diameter of about 1000 km. Scientists believe that asteroids are the pieces of matter that somehow could not get assembled in the form of a planet. The asteroids are small rocky bodies.

While making their rounds the asteroids often collide and break into smaller pieces. These smaller pieces or meteorites sometime fall on to the Earth. They crash to the ground and create craters. Meteor crater in Arizona, USA and Lonar lake in the State of Maharashtra, India are good examples of craters.

COMETS

It revolves around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. They become visible from the Earth only when they come close to the Sun. A comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the Sun. The tail disappears again, when the comet move away from the Sun. The tail of a comet is always directed away from the Sun.

Many comets are known to appear periodically. One such comet is Halley's comet, which appears after nearly every 76 years. It was last seen in 1986. The study of comet tails has shown the existence of molecules of carbon, nitrogen and hydrogen.

Since, these molecules can help to form complex molecules necessary for the origin of life, some scientists have suggested that the seeds of life on the Earth were brought by comets from the outer space.

METEORS, METEORITES AND METEOR SHOWERS

• Meteors are very small stone-like objects that are revolving around the Sun. Their existence becomes known only, when some of them occasionally enter the Earth's atmosphere. At that time, it has a very high speed. Due to the friction in the atmosphere it heats up. It glows and evaporates quickly.

• The path of the meteor, therefore, appears as bright streaks of light in the sky. The bright steak lasts for a very short-time. These are commonly known as shooting stars, although they are not stars.

• Some meteors are large and, so they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. They can be examined in the laboratory. They give us an idea of the composition of the material from which the Solar System was formed. When the Earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor showers. Some meteor showers occur at regular intervals each year.

•New Discovery of Exoplanets Exoplanets are planets located outside the Solar System. NASA's Spitzer Space Telescope has revealed the first known system of seven Earth-size planets around a single star. Three of these planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water. This exoplanet system is called TRAPPIST-1, named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile.

•On 7th June, 2018 scientist working for Physical Research Laboratory (PRL) of ISRO, using the PRL Advance Radial-Velocity Abu Sky Search (PARAS) Spectograph integrated with telescope at the Mount Abu, announced the discovery of host star EPIC 211945201 and expolanet EPIC 211945201 b located at a distance of 600 light years from the Earth.

•In September, 2020 scientists discovered a planet of the size of Jupiter named has 1586b by NASA's Transiting Exoplanet Survey Seettelite (TESS).