1 Introduction to Atmosphere, Space and Universe

Sunayan Saha

TABLE OF CONTENTS

1. Learning outcomes
2. Introduction
3. Origin of the universe, its space and atmosphere
4. Characteristics of deep space environment
5. Entities of the Universe, their atmosphere and weather

    5.1. Galaxy

5.2. Star

5.3. Black hole & Quasar

5.4. The solar family

6. Significance of the Earth’s atmosphere
7. Summary

1. Learning outcomes

• After studying this module, you shall be able to:
• Define the space and the atmosphere
• Tell how the space and atmosphere came into being in the universe Know about the various constituents of the universe
• Know about the properties and events in space Know the features of atmosphere in Earth and in other celestial entities

2. Introduction

       The  envelope  of gas and  or  dust particles that  surrounds the  various  celestial  bodies  of the universe, due to the force of gravity is called atmosphere. On the other hand, space is the void region that exists beyond celestial bodies. When we look at the scheme of things across scales we come across a very striking similarity. On the lower side, if we consider an atom as the most basic unit of our Universe, we will see that only a tiny fraction of it is occupied by matter i.e. the sub atomic particles like proton, neutrons and electrons and the rest are void space. Similar is true on the other side of the scale. It is the vast emptiness of the universe which is interspersed with various celestial entities like stars, planets, satellites and asteroids. These aforesaid entities are arranged in numerous local groups such as galaxies (kind of a mini universe), clusters, super clusters and filaments of galaxies. None of the above celestial units or groups are static, they are in continuous move and undergoing changes, whether small or big, by every passing moment of time. Further, evidences suggest that the universe is expanding, that is the volume of space inside it is increasing.

Scientific community agrees well to the fact that the same laws of nature govern the entire cosmos; however, we witness a variety of phenomena across space. Why it is so? Take for example, the atmosphere of our planet Earth which also encourages to flourish various life forms is significantly different than its known neighbours like Mercury, Venus, Sun or Moon. The properties of the atmosphere such as thickness, density, chemical or gaseous composition on different bodies of the universe and the weather phenomena unfolding there vary depending on many factors. These factors can be like the size and mass of the primordial matter from which an existing body has formed; mechanism of evolution; evolutionary phase (i.e. stage of formation or age); physical or chemical reactions that are taking place; location inside the universe/galaxy and distance from the neighbouring celestial bodies. An examination of facts about the phenomena happening in space and in the atmosphere of celestial bodies will help intimate ourselves with our own Universe and to dispel various myths.

3. Origin of the Universe, its space and atmosphere

Scientists have theorized that at the beginning, the entire universe (and for that matter its entire mass or energy and space) was confined within a very very tiny dot (call it, the ancient primordial matter) as happens when a dying star becomes a black hole. Enormous compression within such a tiny dimension due to the effects of gravity might have caused a condition that Einstein’s equation terms as “singularity” (a point where the laws of physics would simply break down). It is quite possible that all the matter was in the form of pure energy as compression of the massive mass of the universe in such small volume would result in unimaginable amount of heat. Then a destabilizing moment came with an explosion of the space, also termed as big bang (albeit without noise probably as propagation of sound waves requires a medium like air which was non-existent at that time) followed by formation of various types of entities (e.g. nebula, galaxy, protostar, star, binary stars, proplanetary disc or proplyd, planet, natural satellite or moon, asteroids) of the universe which is a continuous process. The astounding explosion that brought out our universe, although billions of years ago, has left its imprint all across the universe in the form of remnant energy, known as cosmic microwave background radiation (CMBR). Occasionally, gamma rays (the most energetic form of light in the universe) are produced in very intense bursts of jets so powerful that more energy is released in few seconds than the sun would produce during its entire lifespan of 10 billion years.

A star is born as a gigantic ball of very hot dense gas, not a solid. In course of time, most stars develop their own planetary systems (exception being the binary stars) where a central star is surrounded and orbited by a number of planets and some of the planets may again be orbited by one or many number of moons (natural satellites). Such an arrangement happens to conserve angular momentum (a measure of the consistency of angular motion) which is one of the many laws of nature. Another law of physics whose effect is omnipresent all across the universe is the law of gravity. Due to the immense mass of celestial bodies like many planets and moons, these attract the gases and space dust in their neighbourhood and keep them attach while rotating themselves. This envelope is what we know as atmosphere.

The primordial atomic elements of the Universe were hydrogen (H2) and helium (He). Inside the core of first generation stars where temperature condition is like that of a nuclear furnace, these lighter elements fuse together and give rise to heavier elements like carbon (C), oxygen (O), nitrogen(N). The later elements are essential requirement for formation of various life forms as can be seen on Earth. The Universe does not waste anything. Take the case of a star. When a star dies it scatters most of its masses and elements into space, leading to formation of an entity, known as nebula. A nebula can be the assemblage of stardust i.e. gas and dust of a single dying star as well as subsequent colossal collection of them (Fig. 1). In course of time, again one or many stars are born from this stardust. Infact, nebulae are the most active star producing region within a galaxy.

Fig. 1: Different nebulae (from left to right: Crab, The Glowing Eye, The Eskimo and NGC 3132) in the Universe. A white dwarf star can be located at the centres of the last three nebulae (source: Bhowmik, 2016)

Our universe, as we observe today is continuously expanding, a fact which has been inferred from the Hubble space telescope data on colour spectra where most known galaxies are found to be red shifted (exception is like Andromeda galaxy which is blue shifted) i.e. galaxies are distancing themselves from each other. This also means that the intergalactic void spaces or the space inside the universe is also increasing.

How old and big is the Universe?

There is a quote by famous Astrophysicist Sir Arthur Eddington which states that “The Universe is not only stranger than we imagine, it is stranger than we can imagine”. In the past decades nations of the world have launched some amazing instruments and satellites into our nearby space (we can call them “eyes in the skies”); these have helped unravelling a lot of mysteries about the universe, space and atmosphere. These are for example Hubble Space Telescope, ISS, WMAP (Wilkinson Microwave Anistropy Probe) and COBE (Cosmic Background Explorer) satellites. As per the present knowledge, the Universe could be as much as 13.7 billion yrs old. And how big is it? Atleast as big as the distance that light can travel in 13.7 billion yrs.

4. Characteristics of deep space environment

• Vacuum: Outer space is the closest natural approximation of a perfect vacuum. As said, it is not completely empty, but contains a low density of particles, predominantly hydrogen plasma and helium, as well as electromagnetic radiation, magnetic fields, neutrinos and dust. Even in the deep vacuum of intergalactic space there are still a few hydrogen atoms per cubic meter. Practically, deep space environment offers no effective friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories.

• Dark matter and dark energy: Theoretically our Universe is made up of only 4 % ordinary matter (the substance we can see, touch and measure), the rest 96 % is dark matter and dark energy (vaccum energy of space) that we can’t see or touch. The concept of dark matter or dark energy has not been understood well by the science community.
• Microgravity: Gravity is one of the four fundamental forces of the universe. As one move away from any planetary body, the force of gravity gradually weakens and deep inside space environment it becomes negligible, although a state of zero gravity i.e. perfect weightlessness probably never happens. Scientists use the term microgravity (one millionth of 1g, where g is acceleration due to gravity) to describe such condition.
• Hazardous Radiation: Presence of high level of UV and cosmic radiation (very high energy radiation originating outside the Solar system), which has a high potential to cause severe injury in living creatures is a distinguishing feature of the space environment. Because of the protecting shield of Earth’s magnetic field and ozone layer, such hazards are less or absent in the Earth’s atmosphere. Because of thinner atmosphere and weak magnetic field, many planetary bodies such as Mars and Moon have much higher level of cosmic radiation in their atmosphere.

5. Entities of the universe, their atmosphere and weather

5.1. Galaxy

• A galaxy is basically an assembly of vast number of stars, gas and dust along with some other exotic matter, all held together by their mutual attraction of gravity. Within it making of one or the other star is always on, hence, stars of various ages can be found inside a galaxy. Generally, if not always, the central stars are older and appear more yellow whereas those at the periphery or arms are younger and blue in colour. All the stars in any galaxy orbit the galactic centre.
• With the latest tools and techniques available with us, about 125 billion galaxies have been identified and this is not the end, the actual number can be far greater, even surpassing the count of stars in any single galaxy. About 80 % of the galaxies, observed so far, are elliptical shaped; the rests are either spiral or irregular in shape. Elliptical galaxies are older, smaller, less bright and less active(as far as star making is concerned) and houses more aged stars in comparison to spiral galaxies.
• Spiral galaxies appear to be the busiest and brightest galaxies in the universe and each of which may contain about 100-400 billion stars. Similar spiral shapes that happen in the Earth’s atmosphere  like  cyclones/hurricanes  have  the  similarity  that  both  exert  forces  on  objects around them, thus pooling everything towards centre. However, unlike in a cyclone where the centre is almost empty, in a galaxy there is a bulge at the centre where the stars are densely packed (Fig. 2).

Fig. 2: Comparison of structures of a cyclone (Hurricane Isabel, 2003) in Earth’s atmosphere (left) and a galaxy (Milky Way) in space (right). Contrary to the central bulge of mass in case of a galaxy, the centre or eye of a cyclone looks void and surrounded by white mass of clouds (sources: Johnson Space Center, USA & Bhowmik, 2016, respectively)

5.2. Star

1. Formation process: At the beginning of star formation, there are patches of whirlwind cloud in space consisting of gas and dust particles. Gravity pulls in these raw materials on itself, thus forming larger balls at the centre of each cloud. These are termed as protostars (Fig. 3). With passage of time, a protostar eventually becomes a star and the process depends upon the mass of the former. Due to higher gravitational pool associated with larger mass, heavier stars shrink faster and born quicker. Our Sun took about 30 million years to become a star.

            The dense cover of gas surrounding a protostar is blown away by the heat generated by the contracting protostar. Also, temperature at the core of a protostar increases over time due to ever increasing collision of dust and gases there. When the temperature at core becomes 10 million oC, nuclear fusion of H2 into He begins which is also accompanied by production of huge magnitude of light energy. Depending on the amount of gas and dust present at the formative stage, size of the star varies and it varies quite widely. The largest (these are also the brightest) superstars can be about 150 times more massive than our Sun whereas the smallest ones (also known as runt star or red dwarf) can be just one-tenth the size (radius) of the Sun. Red dwarf type of stars are more commonly found in the universe; those are quite dim but lives much longer.

• Stability and death: A star remains stable (such a star is termed as main sequence star) so long as the force of contraction (owing to gravity) and that of expansion (owing to nuclear fusion energy) are in balance which in turn is dependent on the reserve of nuclear fuel i.e. hydrogen in it. When most of the hydrogen gets converted to helium, the force of gravity dominates and the star starts shrinking that is it’s death phase begins. Before its death, carbon or even heavier elements are formed in stages, depending on mass of the star (the higher the mass, more heavier would be the elements formed), through nuclear fusion. During this dying phase, the outer layer expands while the core contracts, with the result that the star appears much bigger (may be about hundred times bigger than it ever was) and brighter to us; also its colour shifts from yellow towards red owing to expansion cooling of the outer layer. Hence, these are also called as red giant.
• White and black dwarf and supernova: The outer shells of red giants are ultimately detached from them and form planetary nebulae which are the precursor to new stars and solar systems. The core becomes what we call white dwarf star. The white dwarf further loses its light and heat over billion of years and becomes a cold black mass, also called as black dwarf. Big stars die faster. They first form red supergiants (a helium core surrounded by expanding shell of gas), followed by formation of heavier elements through nuclear fusion from helium at the core and finally iron with temperature exceeding even 5 billion degrees. At the end of chain of events a core is formed made of iron. In the subsequent stage, the core crushes to a size as low as 10-20 km in diameter due to immense gravitational pool. The core finally vanquishes in fraction of a second with explosion of such a mind boggling magnitude that it becomes visible from a distance of tens of thousands of light years away. This is called as supernova. It is possible that about 100 supernovae are happening somewhere in vast space of the universe each passing second (Fig. 4).
• Neutron star: A star that is 8-25 times bigger than the Sun dies as a supernova with its iron core so compacted by gravity that all its atoms are crushed into one super dense mass (one teaspoonful on Earth would weigh millions of tons) of neutrons, hence at that point it is known as neutron star (Fig. 5). These stars rotate at very high speed, producing radio waves that sweep through space like a lighthouse beacon. The neutron star whose rotating beam can be observed from Earth as regular, pulsed radio signal are called pulsars.

• Temperature and appearance: Hottest stars (surface temperatures can be higher than 40,000 oC) appear bluish whereas the coolest ones with surface temperature as low as 2500 oC look reddish; yellow stars like our Sun lies in between the above two extremes. Sun’s surface has temperature around 6000 oC.

5.3.   Black hole & Quasar

• Black hole: A black hole is created when a star at least 25 times bigger than Sun dies. Because of the initial mass of the star the gravity is so strong that the entire mass of the core instantly shrinks into a space that is infact smaller than the nucleus of a single atom and even light cannot pass through it. Light is the fastest moving entity in the universe; since light cannot escape, no other things will and hence this is called as black hole. It is said that even time itself stops inside a black hole. However, it has been postulated that Hawkings radiation (a very week form of radiation) may possibly escape the black holes.
• Quasar: Stars, planets or any other matter that happen to be in the neighbourhood of a black hole are dragged into the vortex, starts spinning around and form accretion disc which becomes extremely hot and radiate light before being swallowed up. A quasar is a compact accretion region of a massive black hole. These are among the oldest and most distant objects in the universe. The light produced by a quasar can be trillion times brighter than that of our Sun and hence visible even from a distance 10 billion light years away i.e. almost the far end of the universe (Fig. 6).

5.4 The solar family

As stated earlier, small or big, at whatever scale we look inside the Universe, there is an apparent similarity everywhere. The galaxies rotate around the universe; within galaxies, stars, many of which have families consisting of planets, moons and asteroids, orbit the galactic centre. It is quite evident that the scene inside any one galaxy or star system repeats itself in more or less

           Fig. 7 :The Solar system (source: Bhowmik, 2016)

• Sun: The Sun’s photosphere i.e. its surface is as hot as about 6000 oC (average) and guess the temperature at its core. The core is a nuclear furnace where continuous fusion reaction is taking place producing enormous energy and light, and temperature reaches there as much as 15 million oC. The sun burns about 600 million tons of H2 per second; although, it currently is as old as about 4.6 billion years, there is still enough stock to keep its fire going for 5 billion years more. Explosions of huge magnitude are always happening on the Sun’s surface, throwing its fires thousands of kilometers into the sky which also falls back again onto its surface, a phenomena similar to the restive oceans.
• Mercury: It has a very feeble and highly variable atmosphere where pressure becomes as lowas 10−14 bar. It contains hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor. Because the planet is very near to the sun, radiation and wind coming from the Sun exert strong influences on it. Solar light pushes neutral atoms away from Mercury, creating a comet-like tail behind it whose main constituent is sodium. Daytime temperature here reaches 450 oC whereas at night it becomes as cool as minus 170 oC.
• Venus: The atmosphere in Venus is known to be the hottest (temperature reaches as high as 482 oC) among all planets of the solar family, although it is not the nearest from the Sun. The reason being, very high presence of CO2.The atmosphere there is a choking, smoggy mixture of CO2 and thick clouds suffused with deadly sulphuric acid (H2SO4). Hurricane velocity winds exceeding 300 km per hour, drives the acid cloud around the planet. The Venusian sky is filled with thunder and lightning almost all the time, although rains never happen. The atmospheric pressure on its surface is about ninety times greater than that on Earth.
• Earth: So far as we know, Earth is only entity in the solar system, that has been able to support life and its atmosphere has played a great role in the evolution of life and maintaining it till present. Various natural factors cause changes in the Earth atmosphere system and in past brought warm and cold phases in cycles. In recent times, due to increased anthropogenic  interferences there is a build-up of greenhouse gases like CH4, CO2 and oxides of N2 in its atmosphere and this is threatening the very existence of life forms in our future Earth by way of global warming and climate change.

   The estimated temperature on Earth in its early days was even higher than that of Venus today. Due to volcanic activities, the gases that came out along with lava from its core produced the atmosphere. The primeval atmosphere on Earth comprised of N, CO2, CO and water vapour (H2O), some CH4and NH3. With the presence of water vapour, the hydrological cycle began i.e. cooling and condensing of water vapour as clouds at heights of the atmosphere followed by rain. As the process occurred repeatedly, the initial massive heat of Earth gradually went away and then formed the ocean and other water bodies. First life forms on Earth was conceived in these water bodies only.

The present atmosphere of Earth is composed mainly of N2 and O2 (total about 99 %) and the rest are some inert gases, CO2 and highly variable content of water vapour. The atmosphere here shows distinct vertical temperature profile.

• Mars: The temperature on Mars does not exceed the freezing point even in summer, and the land remains permanently frozen the rest of the time. It has a thin atmosphere. Storms of red dust (mainly iron oxide) whipped up by strong Martian winds rage across its surface frequently. Evidences indicate a possibility of life forms in Mars. Frozen water can be seen in its poles, and also there are features on its now dry surface that appear to have been formed by once free-flowing water. Much of the water probably has now vaporized and escaped through its thin atmosphere.
• Asteroids and Meteoroids: Lakhs of chunk of rocks (sizes as variable as a dust particle to moon)  spread  over  a  vast  swath  can  be  found  oribiting  the  Sun,  staying  between  the interplanetary orbital space of Mars and Jupiter.  This is known as asteroid belt.  Nowadays asteroids are termed as small solar system bodies. During the formative stage of the solar  system, inner planets, viz., Mercury, Venus, Earth and Mars used to be frequently hit by asteroids and comets, the signs of which can still be seen on the surface of these planets.

     Small asteroids are often knocked out of their orbits and wind up on their trajectory towards Earth. Then these are termed as meteoroids. When these objects enter our atmosphere we call them as meteors. Most meteors are small enough to burn up due to the friction offered by the atmosphere, hence they never reach the surface of the planet. We generally know them as the shooting stars of night sky. If any such falling object actually hits the surface of the Earth then we term them as meteorites.

Occasionally, a large asteroid travels quite close to Earth, however, the possibility of actually hitting the Earth is quite low such as once in every 65-100 million yrs. Most recently (in the year 2006), one asteroid (about the size of one kilometre in diameter) came close to the Earth (about the distance between Earth and moon). Had it collided with the Earth, the impact would have been equivalent to the simultaneous explosion of twenty thousands 1 megaton size hydrogen bombs!

• Jupiter: The atmosphere in Jupiter is quite stormy. Galileo spacecraft measured wind speeds of 530 km h-1 and gusts as high as 1600 km h-1on it. There is a distinctly identifiable storm zone on Jupiter, known as Great Red Spot which is a persistent turbulence system about 8km high and width wise three times that of Earth. Thunderclaps and jagged bolts of lightning shake and sear Jupiter’s atmosphere. Unlike in Earth, the violent storms of Jupiter are possibly driven by its internal heat which is supported by the fact that this planet radiates more energy than it receives from the sun. Jupiter is surrounded by atleast three rings that formed when dust from its nearest moons was tossed up into space.
• Saturn: Saturn is bit less turbulent than Jupiter, but from time to time it experiences great storms which form the coloured bands in its atmosphere. Billions of ice particles mixed with rocks and silicates (sizes varying between small sand grain to as big as one meter in diameter) encircle and orbit the planet and appear as some prominent rings of varying width.
• Uranus: It is gas giant with a rocky core. The fastest wind speeds on Uranus is as high as 700 kmh-1. The atmosphere is rich in methane. Sunlight scattered from the clouds surrounding Uranus is reflected back through these methane layers, giving this planet its blue green appearance. The temperature at the top of Uranus’s icy methane clouds is about minus 215 oC.
• Neptune: Its atmosphere is composed of H2, He and CH4.No place in the solar system is as stormy as Neptune; here storm speeds can be as high as nine times the speed of any on Earth. The planet appears aqua blue.
• Pluto: As per the new definition of planets, now Pluto is designated as a dwarf planet and it could be one of the many such in solar family. Because of its distance from Sun only very little sunlight reaches Pluto. Indirect evidences suggest that Pluto has an atmosphere and that the planet may be like a frozen snowball of gas and dust.
• Earth’s Moon: Atoms and molecules of some gases are present surrounding the lunar surface, but in very very small quantity. In fact, the density of the atmosphere at the Moon’s surface is comparable to that at the outermost fringes of Earth’s atmosphere, where the International Space Station orbits. For practical purposes, such an ambience can be equated with vacuum. Due to such small mass of gas (total weight may be lower than even 10 metric tonnes), the pressure in the “lunar atmosphere” is only around 3×10−15 atm and it also varies throughout the day. Because of the absence of a real atmosphere, moon cannot absorb appreciable quantities of radiation and does not appear layered or self-circulating. Due to low gravity condition, gases are lost to space at a high rate and it requires constant replenishment. Presence of gases like argon, helium, oxygen, methane, nitrogen, carbon monoxide and carbon dioxide has been detected in Moon’s atmosphere. Two more gases, viz., sodium and potassium are also found on moon that is unusual in the atmospheres of Earth, Mars or Venus.
• Titan: This is a moon of Saturn. As per the Voyager 2 spacecraft data its atmosphere consists of N2, Ar, CH4 and other gases; this is similar to what our Earth had 4 billion yrs ago. Hence, Titan is of special interest to us to get clue of the primordial Earth. Titan is more similar to Earth than any other members of solar family. Its atmosphere is as thick as that of Earth, although the gravitational attraction is only 14 % of that on Earth i.e. on can fly like a bird with the help of some wings, on Titan.
• Triton: It is one of the moon of Neptune; So far it is known as being the coldest place in the solar system with temperature at minus 235 oC.
• Kuiper Belt: It is situated at the outermost coast of the solar system, over a billion kilometer beyond the orbit of our last planet. Kuiper Belt Objects (KBOs) are the leftovers from the creation of our solar system. These are like dirty snowballs-clumps of packed ice and rocks of various sizes, some of them nearly as big as a thousand kilometres across fly in this belt or to say, orbit the Sun. Kuiper belt is known as one of the two places in the solar system from which comets begin their journey. Whenever, any of the giant snowballs in this belt is nudged out of orbit due to the force of gravity, it heads straight for the sun and become known as a comet. Any such comets may one day wander within Earth’s view. As the above snowball like KBO object moves towards the sun, its ice is vaporized and forms the long tail of a comet. The comets which usually make orbit in 200 yrs or less time originate in the Kuiper belt.
• Oort cloud: Well beyond the Kuiper belt, at the very edges of the solar system (about 7 trillion Km from Sun) lie another dark region known as Oort cloud. This is like Kuiper belt, however, the objects there have much longer trajectories i.e. orbit the Sun in thousands and thousands of years. This region gives rise to so-called long period comets.

Typical features of the atmosphere in some different solar system bodies and are presented through images in Fig. 8.

6. Significance of the Earth’s atmosphere

Of the all known planetary bodies, the Earth’s atmosphere is very significant and unique  one.  Because  of  the  thick  blanket  of  atmosphere  surrounding  the  Earth  and  the consequent thermal regime owing to the natural greenhouse effect, life has become possible in the Earth. Had there been no atmosphere around, average temperature of the Earth would have been as low as -38 oC. Apart from this, atmosphere supports life forms or make like congenial on Earth by serving some vital functions as mentioned below:

• Absorbs the energetic and harmful ultraviolet radiation while allow the passing of important visible radiations.
• Prevents excessive heating of surface of the Earth at day and excessive cooling at night thereby reducing the temperature variations
• Contains nitrogen, oxygen and carbon dioxide gases that are essential for plant growth and for respiration
• Protects the surface of Earth and all life existing on Earth from the direct impact of meteorites as most small meteorites are burnt up in the sky due to the friction of atmosphere.
• Serves an integral part in the bio-geo chemical cycles of C, N, O, P and S
• Helps in flow of energy and water vapour through dynamic processes of air flow
• Helps in radio communication
• Helps in movement of air crafts
• Aids in dissipation, dispersion and decomposition of pollutants

7. Summary

• Our universe is about 13.7 billion yrs old. It is as big as the distance that light can travel in 13.7 billion yrs.
• Like in an atom, only a tiny fraction of the universe is made up of matter (mass or elements that we are familiar with), the rest is void. Outer or deep space region is like a perfect vacuum.
•  The primordial atomic elements of the Universe were H2 and He.
• Dark matter and dark energy, hazardous cosmic radiation and microgravity are some important features of deep space.
• Space inside the Universe is ever expanding, a fact proved by Hubble space telescope observations.
• A star is born as a mammoth ball of very hot dense gas. With time, it develops a solid core where the lighter elements, viz., H2 and He gets converted to heavier elements like C, O, N and Fe through nuclear fusion.
• Atmosphere is the envelope of gas and or dust particles surrounding various celestial bodies due to the force of gravity.
• So far as we know, in the entire solar system, only Earth has life supporting atmosphere. However, there is strong possibility of many such Earth like planets and also alien life forms in the entire universe.
• Temperature in Sun’s photosphere i.e. its surface is about 6000 oC whereas at the core it is as high as 15 million oC. Maximum temperature at the surface of Mercury and Venus often exceeds 450oC.
• The planets Mercury, Venus, Earth and Mars have solid rocky surfaces with relatively thin envelope of gases whereas the other four i.e. Jupiter, Saturn, Uranus and Neptune have much thicker cover of gases around their solid cores.
• Compared to the Earth, the other planets of the solar system are more stormy and is topped by Neptune.
• Presence of sodium and potassium gas is typical of lunar atmosphere.
• Shooting stars of night sky i.e. the meteors are actually small sized asteroids that when pass through the Earth’s atmosphere, get burnt up due to friction. Comets are originated in the Kuiper Belt and Oort cloud, situated at the very far end of the solar system.
• Scientists infer from evidences collected so far that there are huge possibility of existence of many Earth like planets with atmosphere congenial to support life forms.