Earth As a Planet

Our cosmic home, Earth, holds the distinguished position of the third planet from the Sun, nestled within a diverse solar system comprising rocky inner planets, colossal gas giants, dwarf planets, and countless asteroids and comets, all navigating immense stretches of space. Earth itself is not a perfect sphere but an oblate spheroid, or geoid, subtly flattened at its poles, with specific dimensions defining its unique shape. This precise form, coupled with its dynamic movements, dictates much of our planetary experience.

Earth’s fundamental motions are pivotal to life as we know it. Its daily rotation on a tilted axis engenders the cycle of day and night, leading to variations in day length across different latitudes. Simultaneously, its annual revolution around the Sun, in conjunction with this axial tilt, is the grand orchestrator of our seasons, punctuated by the solstices and equinoxes. To precisely pinpoint locations and comprehend these global phenomena, we rely on imaginary lines: latitudes delineate distinct climatic zones, while longitudes establish our time zones and define the International Date Line.

What truly sets Earth apart is its remarkable capacity to harbor life. This unparalleled ability stems from a confluence of unique conditions: the ubiquitous presence of water in its solid, liquid, and gaseous states, a breathable atmosphere rich in oxygen, a temperate climate maintained by its optimal distance from the Sun, and a vital protective magnetic field that shields us from harmful solar radiation. These synergistic factors collectively render Earth a singular oasis in the vastness of the cosmos.

Exercises

I. Short Answer Questions

Question 1.
How can you prove that the earth is a sphere by looking at the Pole Star ?
Ans:

The Pole Star, also known as Polaris, provides strong visual evidence for Earth’s spherical shape. If you’re standing at the North Pole, Polaris appears directly overhead at a 90-degree angle. As you travel towards the Equator, its position in the sky gradually drops. By the time you reach the Equator, Polaris is no longer visible, resting at 0 degrees on the horizon.

This steady change in Polaris’s elevation, directly corresponding to an observer’s latitude, is a key indicator of a curved surface. On a flat Earth, Polaris would theoretically appear at the same angle to everyone across the entire Northern Hemisphere. However, this is not what we observe. The predictable and consistent alteration in Polaris’s altitude as one changes their north-south position offers clear and undeniable proof of Earth’s round form, supporting centuries of astronomical understanding.

Question 2.

Briefly describe the shape of the earth.

Ans:

Earth is not a perfect sphere. Its true shape is best described as an oblate spheroid or geoid. This means it’s slightly flattened at the North and South Poles and bulges around the Equator due to the centrifugal force created by its rotation.

Question 3.

What is the earth’s mean temperature ? State its one advantage.

Ans:

Earth’s average surface temperature is around 15°C (59°F). This global average permits liquid water to exist on the planet’s surface. This is a critical advantage because liquid water is fundamental for life as we understand it, serving as a solvent for biological reactions and a key component of living organisms. The Earth’s moderate temperature range, influenced by its distance from the Sun and its atmosphere, maintains water in its liquid form across a substantial portion of the globe, supporting the evolution and maintenance of diverse ecosystems.

Question 4.

Why is the earth called a watery planet ?

Ans:

Earth is often called the “watery planet” because about 71% of its surface is covered by water, mostly in its vast oceans. This large amount of liquid water gives our planet its distinct blue color from space and makes it unique among the rocky planets in our solar system.

Question 5.

What is ‘biosphere’ ?

Ans:

The biosphere is the entirety of Earth’s ecosystems, encompassing all living organisms – plants, animals, fungi, protists, and bacteria – and the environments they inhabit, including land, water, and the atmosphere.  It represents the global zone where life exists and interacts. 

Question 6.

Name the conditions necessary for life on earth.

Ans:

Life on Earth depends on several key factors that sustain biological processes and ecosystems:

• Liquid Water: Serving as the universal solvent, water is crucial for biochemical reactions, nutrient transport, and cellular functions.
• Energy Source: Most life relies on solar energy from the Sun, while some deep-sea organisms derive energy from Earth’s chemical reactions.
• Essential Chemical Elements: Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur form the building blocks of life, alongside necessary nutrients that support metabolism and growth.
• Protective Atmosphere: Earth’s atmosphere shields life from harmful radiation, regulates temperature, and provides gases like oxygen and carbon dioxide necessary for respiration and photosynthesis.
• Stable Environmental Conditions: Long-term stability in climate, geology, and ecological interactions allows life to evolve and flourish.

Question 7.

Give any two features of the earth that make it a ‘Unique Planet’.

Ans:

• Abundance of Liquid Water: Unlike any other known celestial body, Earth maintains a vast and stable supply of liquid water.Covering a significant portion of Earth’s surface, oceans, lakes, and rivers play a critical role in sustaining ecosystems.
• Life-Supporting Atmosphere: Earth’s atmosphere is primarily composed of nitrogen and oxygen, along with trace amounts of other gases that are vital for life. This protective layer not only provides essential elements for survival—such as oxygen for respiration—but also shields organisms from harmful solar radiation through the ozone layer. Additionally, the atmosphere regulates temperature via the greenhouse effect, ensuring conditions remain suitable for the presence of liquid water and the flourishing of life.

Question 8.

Why is the planet Venus hotter than the planet Mercury?

Ans:

Venus, despite being farther from the Sun than Mercury, is hotter due to its exceptionally dense atmosphere, primarily carbon dioxide. This thick gaseous layer creates a powerful greenhouse effect, trapping solar energy and leading to extremely high surface temperatures.  

Mercury, although closer to the Sun, possesses a very thin atmosphere that offers minimal insulation. Consequently, while it receives more direct sunlight, it cannot retain heat effectively, resulting in drastic temperature swings between its hot days and frigid nights.

Question 9.

Name the two sources of heat in the interior of the earth.

Ans:

Earth’s Inner Engine: Two Primary Heat Sources

The immense heat deep within our planet, which drives geological phenomena like mountain formation and volcanic activity, originates from two main mechanisms:

• 1. Primordial Heat (Accretionary Heat): When the planet formed from countless collisions of cosmic debris, the kinetic energy from these impacts converted into intense thermal energy. Further heat was generated as gravity compressed the accumulating mass and as denser materials, such as iron, sank to the core (differentiation). A significant portion of this initial heat continues to fuel geological processes today.
• 2. Radiogenic Heat: This ongoing heat is produced by the natural decay of unstable radioactive isotopes, primarily uranium, thorium, and potassium, found within Earth’s mantle and crust. Each time these elements decay, they release a small amount of energy as heat. The cumulative effect of countless such decays consistently generates a powerful and long-lasting heat source, sustaining processes like mantle convection and the movement of tectonic plates.

Question 10.

What is meant by Terrestrial Life ?

Ans:

Terrestrial life means life that lives mainly on land, as opposed to in water or air. These organisms have adaptations for survival on land, like preventing drying out and having structural support. Examples include land animals, plants, and fungi.

Question 11.

Name the members of the Solar System.

Ans:

Our Solar System: A Quick Tour

Our Solar System is a diverse neighborhood! Close to the Sun, you’ll find the rocky planets: Mercury, Venus, Earth, and Mars.

Move further out, and you’ll encounter the gas giants, Jupiter and Saturn, known for their enormous size and gaseous makeup. Even further away are Uranus and Neptune, the ice giants, which are mostly composed of ice and gas due to the extreme cold of the outer Solar System.

Beyond these main planets, our Solar System still has a lot to offer. Dwarf planets like Ceres, Pluto, Haumea, Makemake, and Eris also journey around the Sun. Between Mars and Jupiter lies the asteroid belt, a dense collection of rocky fragments.

Finally, at the very edge, scientists believe a huge, theoretical sphere of icy objects called the Oort Cloud surrounds our entire system. And all the vast emptiness between these celestial bodies is filled with the interplanetary medium, a mix of dust and gas.

Question 12.

What is a planet ?

Ans:

A planet is defined as a celestial object that:

1. Has sufficient mass for its own gravity to mold it into a roughly spherical shape, a state known as hydrostatic equilibrium.
2. Has gravitationally dominated its orbital region, effectively clearing its path of other significant bodies.

This definition, established by the International Astronomical Union (IAU) in 2006, serves as the standard for classifying planets both within our solar system and those orbiting other stars (exoplanets). It’s this definition that led to Pluto’s reclassification as a dwarf planet, as it satisfies the first two criteria but has not cleared its orbital neighborhood.

Question 13.

What is meant by Inner Planets ? Name them.

Ans:

Inner Planets refer to the planets in our solar system that orbit relatively close to the Sun, within the asteroid belt. They are also known as the terrestrial planets or rocky planets because they are primarily composed of silicate rocks and metals, making them solid and dense.  

The Inner Planets of our solar system are:

1. Mercury
2. Venus
3. Earth  
4. Mars

Question 14.

Why are the inner planets called the terrestrial planets?

Ans:

The inner planets—Mercury, Venus, Earth, and Mars—are known as terrestrial planets due to their fundamental resemblance to Earth. The term “terrestrial” itself originates from the Latin word for “Earth,” underscoring their shared composition of solid rock and metal.

These planets are classified together because of these key characteristics:

• Composition of Rock and Metal: Their main constituents are silicate rocks and metals, forming firm, tangible surfaces similar to Earth’s.
• Higher Density: Their dense, rocky structure leads to a greater overall density compared to the gas giant planets.
• Few or No Natural Satellites: Earth has one moon, Mars has two small ones, while Mercury and Venus lack natural satellites, in contrast to the numerous moons of the gas giants.

Question 15.

What are Jovian planets ? Name them.

Ans:

Jovian Planets: The Giants of the Outer Solar System

Jovian planets, commonly referred to as gas giants, are massive worlds that reside beyond the asteroid belt in the far reaches of our solar system. Unlike the rocky terrestrial planets, these celestial bodies are not made up primarily of solid material. Instead, they are dominated by vast, thick atmospheres composed mostly of hydrogen and helium, surrounding compact, dense cores.

• Jupiter – The largest planet in the solar system, famous for its immense storms, particularly the iconic Great Red Spot.
• Saturn – Best known for its elaborate and visually striking ring system made up of countless pieces of ice and rock.
• Uranus – Classified as an ice giant, it stands out due to its extreme axial tilt, which results in unusual and dramatic seasonal variations.
• Neptune – The most distant planet from the Sun, noted for its deep blue color and extremely strong atmospheric winds.

Question 16.

How big is the sun ? Describe its size with respect to the earth.

Ans:

The Sun’s size vastly exceeds Earth’s, demonstrating the immense scale of our star:  

• Diameter: Measuring approximately 1.39 million kilometers (865,000 miles) across, the Sun’s width is about 109 times that of Earth. Imagine placing 109 Earths side-by-side; that’s roughly the Sun’s diameter.  
• Volume: The Sun’s internal space is colossal, capable of holding nearly 1.3 million Earths. Picture filling a hollow Sun with over a million Earth-sized planets.  

These remarkable dimensions emphasize the Sun’s dominant presence in the solar system. Beyond providing essential light and heat for life, its gravitational force dictates the orbits of all the planets.

Question 17.

Distinguish the earth from the other planets in one important aspect.

Ans:

One crucial aspect that distinguishes Earth from all other known planets is the stable presence of liquid water on its surface.  While some other celestial bodies may have evidence of past water or water ice, Earth is unique in having vast oceans, lakes, and rivers of liquid water, which is fundamental for the diverse and abundant life we know.  This stable liquid water, maintained by Earth’s temperature and atmospheric pressure, sets it apart from its planetary neighbors. 

Question 18.

How did the earth get its atmospheric blanket ?

Ans:

Earth’s atmosphere formed in stages:

1. Early Loss: The initial thin atmosphere of hydrogen and helium escaped into space.
2. Oceans and Life: Water vapor condensed into oceans, absorbing much of the carbon dioxide. Early life (cyanobacteria) began photosynthesis, releasing oxygen.
3. Oxygen Buildup: Over time, oxygen levels increased, forming the ozone layer. The modern atmosphere is now mostly nitrogen and oxygen, maintained by biological and geological processes.

II. Fill in the blanks

1. The Pole Star can be seen at an angle of ___ at the North Pole.

Ans: 90°

2. The earth rotates from ____ to ____.

Ans: west , east.

3. _________ is the narrow zone of contact between lithosphere, hydrosphere and atmosphere.

Ans: Biosphere

4. The earth has an average temperature of ________ .

Ans: 17° C

5. ___________ is the major reservoir of carbon on the earth.

Ans: Atmosphere

6. In terms of size, the earth is _______ planet in the Solar System.

Ans: fifth

7. Eratosthenes worked out the circumference of the earth to be ____________ km.

Ans: 46,250

8. A typical galaxy may contain ___________ stars.

            Ans:  large cluster of 

III. Long Answer Questions

Question 1.
Study the picture and answer the questions that follow.

(a) What does the picture show you about the earth ?
(b) State briefly the Bedford Level Experiment.
(c) The sun rises and sets at different times in different places. What does the statement show about the shape of the earth.
(d) In what way is the earth’s atmosphere beneficial to mankind ?

Ans:

Demonstrations of Earth’s Shape

a) Sailboats and the Horizon: When observing sailboats move away, their hulls vanish before their masts. This “hull-first” disappearance proves the Earth’s surface curves downwards, progressively hiding lower sections of distant objects.

b) The Bedford Level Experiment: This experiment on a long canal revealed that distant objects appeared lower than expected on a flat plane. This measurable dip in elevation over a set distance provided concrete evidence for Earth’s curvature.

c) Time Zones and the Spherical Earth: The existence of varying sunrise and sunset times across the globe directly reflects Earth’s spherical shape and rotation. A flat Earth would experience simultaneous day and night everywhere, whereas a rotating sphere leads to sequential illumination and distinct time zones.

The Atmosphere’s Indispensable Role

d) Humanity’s Atmospheric Lifeline: The Earth’s atmosphere is indispensable for human life, serving several critical functions:

• Oxygen Provision: It supplies the vital oxygen we breathe.
• Radiation Shield: The atmosphere, particularly the ozone layer, blocks harmful solar UV radiation.
• Temperature Stability: It regulates Earth’s temperature through the greenhouse effect, maintaining conditions suitable for liquid water and life.
• Water Distribution: It is fundamental to the water cycle, ensuring global freshwater circulation.

Question 2.

Provide reliable evidence to prove that the earth is spherical in shape.

Ans:

Evidence for Earth’s Spherical Shape

The Earth’s spherical nature is well-established, supported by a wealth of observable evidence. One compelling piece of evidence comes from observing ships disappearing over the horizon: as vessels sail away, their hulls vanish from sight before their masts, a phenomenon consistent with a curved surface.

Another indicator is the changing view of constellations as one travels north or south. New stars emerge while others disappear below the horizon, directly reflecting a change in viewing angle on a curved Earth. During a lunar eclipse, the Earth’s shadow cast upon the moon is consistently circular. This is a definitive characteristic of a sphere, as only a spherical object will project a round shadow regardless of its orientation.

The ability to circumnavigate the globe further underscores Earth’s spherical form. Travelers can continuously move in one general direction and eventually return to their starting point, which would be impossible on a flat plane. Moreover, direct photographic evidence from space, captured by satellites and astronauts, consistently depicts Earth as a distinct, curved sphere, providing undeniable visual confirmation.

The behavior of gravity also aligns with a spherical Earth. Gravity pulls objects towards Earth’s center, creating a “down” direction that subtly shifts across the surface, rather than a uniform pull that would be expected on a flat plane. Finally, the variation in the sun’s angle with latitude provides additional proof. At the same time of day, shadows are short near the equator and significantly longer at higher latitudes, a direct result of sunlight striking a curved surface at different angles.

Question 3.

Give the distinctive features of the earth as a unique planet. Also give three points of comparison with respect to other planets.

Ans:

Earth: A Singular Haven for Life

Our solar system holds many celestial bodies, but only one truly shines as a beacon for life: Earth. Its unique combination of features sets it apart, creating an environment where life, as we know it, can not only exist but thrive.

One of Earth’s most striking features is its abundance of liquid water. Unlike any other planet or moon in our solar system, Earth boasts vast oceans, covering over two-thirds of its surface. This continuous, stable presence of liquid water is the very foundation of life, acting as the universal solvent for the intricate chemical reactions that drive biological processes and facilitating the movement of vital elements throughout ecosystems.

Equally crucial is Earth’s dynamic atmosphere. This protective blanket, rich in nitrogen and oxygen, provides the very air we breathe. It also features a vital ozone layer that acts as an invisible shield, filtering out harmful ultraviolet radiation from the sun. Moreover, a finely tuned natural greenhouse effect, thanks to certain atmospheric gases, keeps our planet warm enough to sustain liquid water, preventing it from freezing solid or boiling away into space.

The Earth’s crust is not a static shell but a mosaic of constantly moving plates. This geological dance leads to the grandeur of mountain ranges, the fiery drama of volcanoes, and the subtle tremor of earthquakes. More profoundly, plate tectonics is a critical engine for recycling nutrients and plays a long-term role in regulating Earth’s climate, ensuring a stable environment for life over geological timescales.

A powerful magnetic field serves as an invisible guardian for our planet. Without this protective bubble, our atmosphere would be stripped away, leaving the surface exposed to lethal radiation.

Finally, Earth maintains a remarkably stable and moderate temperature range. This ideal thermal equilibrium is a testament to its perfect distance from the Sun and the insulating qualities of its atmosphere. This consistent, mild climate is precisely what allows liquid water to persist and, in turn, supports the incredible biodiversity that flourishes across our planet.

Question 4.
Describe the role of each of the following in making earth a habitable planet.

(a) Atmosphere
(b) Water
(c) Temperature

Ans:

The habitability of Earth arises from the essential and interconnected roles of its atmosphere, water, and temperature:

(a) Atmosphere:

Earth’s gaseous envelope is crucial for life in numerous ways:

• Breathable Air Supply: It contains the necessary oxygen for animal life and nitrogen for plant growth.
• Protection from Radiation: The ozone layer filters out harmful solar ultraviolet (UV) rays, allowing surface life to thrive.
• Temperature Regulation (Greenhouse Effect): Trace gases trap heat, warming the planet to a level where liquid water can exist.
• Water Transport: It drives the water cycle through evaporation, condensation, and precipitation, providing fresh water.
• Shield Against Debris: It causes most small space objects to burn up, protecting the surface from impacts.
• Heat Distribution: Atmospheric circulation moderates temperature differences across the globe.

(b) Water:

This “universal solvent” is vital for life:

• Medium for Life’s Processes: It facilitates most biochemical reactions within organisms and transports essential substances.
• Climate Moderation: Its high heat capacity helps stabilize Earth’s temperature.
• Habitat Provision: Oceans, lakes, and rivers support diverse aquatic ecosystems.
• Biological Component: It constitutes a large part of living organisms and is essential for their functions.
• Nutrient Cycling: It aids in weathering and transporting nutrients through the environment.

(c) Temperature:

Earth’s temperature range is uniquely life-supporting:

• Liquid Water Permitted: The average temperature allows water to exist in its liquid state, considered essential for life.
• Biological Functionality: Moderate temperatures enable optimal enzyme and metabolic activity in organisms.
• Climate Stability: Relatively stable temperatures over long periods have allowed for complex life to evolve.
• Biodiversity Support: Temperature gradients across the planet have led to diverse habitats and life forms.

Question 5.

Name the different realms of the earth.

Ans:

Earth’s life-sustaining system comprises four interacting spheres:

1. Lithosphere: The Earth’s solid outer layer, forming land and ocean floors, driving geological activity.
2. Hydrosphere: All of Earth’s water (liquid, solid, gas), moving through a cycle and influencing climate and life.
3. Atmosphere: The gaseous layer protecting Earth, regulating temperature, providing air, and driving weather.
4. Biosphere: All living organisms and their environments, interacting with the other spheres and maintaining ecological balance.

Question 6.
Study the picture and answer the questions that follow.

(a) What is meant by the Solar System ?
(b) Give two differences between a planet and a star.
(c) Name the planets known as terrestrial planets.
(d) Give two advantages the earth has over other planets.
(e) What are satellites ?

Ans:

(a) The Solar System comprises the Sun and all objects bound to it by gravity, including planets, dwarf planets, moons, asteroids, comets, and interplanetary matter like dust and gas. Gravitational forces govern the orbits and movements of these celestial bodies.

(b) 1. Stars produce their own light and heat through nuclear fusion, whereas planets reflect the light of the star they orbit. 2. Stars possess significantly greater mass than planets, enabling nuclear fusion and the generation of their own energy.

(c) The terrestrial planets, known for their rocky composition and closeness to the Sun, are Mercury, Venus, Earth, and Mars. They exhibit solid surfaces, varied geological features, and different atmospheric compositions.

(d) 1. Earth stands out due to its significant and consistently present liquid water on its surface, essential for supporting life. 2. Earth’s atmosphere, abundant in nitrogen and oxygen and featuring a protective ozone layer, sustains life and shields against harmful solar radiation.

(e) Satellites are objects, either natural celestial bodies (like moons) or artificial human-made devices, that orbit a larger body (such as a planet) due to the force of gravity. They serve various purposes, including observation, communication, and scientific research.

Question 7.

Write a short note on the planets of the Solar System. Name the planets in order of their distance from the sun.

Ans:

The Solar System is home to eight distinct planets, each tracing an elliptical orbit around the Sun. These planets fall into two main categories: the inner, rocky terrestrial planets and the outer, gaseous Jovian planets.

These are smaller, more compact worlds characterized by solid surfaces predominantly made of rock and metal. They typically possess thin atmospheres and have few, if any, moons.

Beyond the terrestrial worlds lie the Jovian planets: Jupiter, Saturn, Uranus, and Neptune. These are considerably larger and less dense than their inner counterparts. Jupiter and Saturn are primarily composed of hydrogen and helium, while Uranus and Neptune are rich in “ices” such as methane and ammonia. A defining feature of these planets is their massive atmospheres, numerous moons, and intricate ring systems, with Saturn’s being the most prominent.

Arranged by their increasing distance from the Sun, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Question 8.

Distinguish between a planet and a star.

Ans:

Star: Massive, self-luminous ball of plasma generating light and heat through nuclear fusion due to its immense gravity.

Planet: Smaller, non-luminous body orbiting a star, visible by reflected light, with gravity sufficient to be round and (for major planets) clear its orbit.

Practice Questions (Solved)

Question 1.
In which unit the distances of universe are measured ?
Ans:

Within the Solar System: Astronomical Units (AU)

Beyond the Solar System (to stars and galaxies): Light-years (ly) and Parsecs (pc)

Question 2.
How much time does a ray of sunlight take to reach the Earth ?
Ans:

A ray of sunlight takes approximately 8 minutes and 20 seconds (or about 500 seconds) to reach the Earth.

Question 3.
How much time does a ray of moonlight take to reach the Earth ?
Ans:
Light reflected from the Moon takes about 1.3 seconds to reach Earth. This quick journey is because the Moon is considerably closer to us than the Sun.

Question 4.
How many planets are there in the Solar System ?
Ans:
Eight

Question 5.
Name the largest planet of the Solar System.
Ans:
Jupiter.

Question 6.
Name the planet closest to the Sun.
Ans:
Mercury

Question 7.
Name the planet farthest from the Sun.
Ans:
Neptune.

Question 8.
What is position of the Earth from the Sun ?
Answer:
Third

Question 9.
Name the planets between the Sun and the earth.
Ans:
The planets located between the Sun and the Earth are:

1. Mercury
2. Venus

Question 10.
Which planet is known as ‘blue planet’ ?
Ans:
Earth

Question 11.
Which planet is known as ‘red planet’ ?
Ans:
Mars

Question 12.
Which is the brightest planet ?
Ans:
Venus

Question 13.
Which planet has the largest number of satellites ?
Ans:

Planet with the Most Confirmed Natural Satellites

Saturn currently holds the record for the highest number of confirmed natural satellites, with 146 known moons. This total includes its major, well-documented moons as well as numerous smaller ones that have been identified in recent years.

Jupiter ranks second in terms of moon count, though ongoing discoveries continue to refine and expand our understanding of the satellites orbiting both planets. As astronomers uncover more previously undetected moons, the numbers for these gas giants may shift further.

Question 14.
Which planet has only one satellite ?
Ans:
Earth

Question 15.
Which planet has three rings round it ?
Ans:
Saturn.

Question 16.
Which star is known as ‘Evening Star or “Morning Star” ?
Ans:
Venus

Question 17.
How many satellites are there in Solar System ?
Ans:
100 approximately.

Question 18.
Which is the centre of the Solar System-Sun or Earth?
Ans:
Sun

Question 19.
What is the period of rotation of Moon ?
Ans:
27 days 7 hours 43 minutes.

Question 20.
‘Although the Moon has no light of its own, yet it shines’. Why ?
Ans:
The Moon shines by reflecting sunlight, as it does not produce its own light.

Question 21.
Mention two important features of Earth that make it a planet suitable for life.
Ans:

Abundant and Persistent Liquid Water

Earth stands out in our solar system for its vast and enduring presence of liquid water across its surface. This fundamental resource is critical for all known life forms, acting as a universal solvent for biochemical reactions, a medium for nutrient transport, and a key component in metabolic processes. The stability of Earth’s liquid water, maintained within a specific temperature range, allows for the continuous cycles necessary for biological functions and the long-term evolution of life.

A Protective and Breathable Atmosphere

Earth is enveloped by an atmosphere precisely tuned to foster life. Its composition, primarily nitrogen and oxygen, provides the essential gases for respiration in many organisms. Crucially, this atmospheric blanket includes an ozone layer that acts as a natural shield, absorbing damaging ultraviolet radiation from the Sun and preventing it from reaching the surface in lethal quantities. Furthermore, the atmosphere plays a vital role in regulating Earth’s temperature through the greenhouse effect, trapping just enough heat to prevent extreme temperature fluctuations and maintain conditions conducive to life.

Short Answer Questions

Question 1.
State three unique features of the Earth.
Ans:

Three Unique Characteristics of Earth

Earth stands out among the planets in our solar system due to several distinctive features that make it uniquely suited for life.

1. Abundant and Stable Liquid Water Unlike any other known celestial body, Earth maintains vast amounts of liquid water across its surface, essential for sustaining life. Oceans, lakes, and rivers provide a stable environment for countless organisms and regulate climate through heat distribution.

2. Life-Supporting Atmosphere Earth’s atmosphere is rich in nitrogen and oxygen, enabling respiration for various life forms. Additionally, the ozone layer serves as a shield against harmful ultraviolet radiation, while the greenhouse effect maintains temperatures suitable for life.

3. Dynamic Plate Tectonics Earth’s crust is divided into moving tectonic plates that drive geological activity, including continental drift, mountain formation, earthquakes, and volcanic eruptions. This system continuously recycles essential minerals, influences climate regulation, and plays a crucial role in Earth’s long-term habitability.

Question 2.

State three factors which have made life possible on the Earth and on no other planet.

Ans:

Earth’s unique ability to support life, unmatched by any other definitively known planet, stems from a crucial trio of interacting factors:

1. Enduring Liquid Water: Earth’s temperature and atmospheric pressure allow liquid water to exist stably across a large part of its surface.While other bodies may have evidence of past or subsurface water, Earth is distinctive in its sustained and plentiful surface liquid water.
   
2. A Safeguarding, Life-Promoting Atmosphere: Earth’s atmosphere has a particular mix of gases, notably rich in nitrogen and oxygen, vital for the breathing of many organisms. Its ozone layer acts as a barrier against harmful ultraviolet radiation, and the greenhouse effect maintains a stable, moderate temperature range that allows liquid water and life to persist. This balance is critical for biological functions.
   
3. A Protective Magnetic Field: Generated by the dynamics of Earth’s core, the magnetic field deflects much of the damaging solar wind and cosmic rays. This shielding protects the atmosphere and life on the surface from harmful radiation, facilitating the evolution and flourishing of life.

Question 3.
Give reasons for the following :

(a) Earth is called a “Watery Planet”.
(b) Earth has the most ideal temperature conditions.
(c) Earth and its Moon are called a double planet.

Ans:

(a) Earth as the “Watery Planet”

Earth’s designation as the “Watery Planet” stems from the vast expanse of liquid water covering its surface, accounting for about 71%. This significant presence of water distinguishes it from other rocky planets and is crucial for supporting life and regulating global climate.

(b) Earth’s Ideal Temperature Conditions

Earth maintains optimal temperatures primarily due to:

• Its position within the Sun’s habitable zone, where temperatures allow for liquid water.
• A balanced greenhouse effect, which retains heat without extreme fluctuations.
• Long-term climate stability, uniquely suiting it for diverse ecosystems and life.

(c) The Earth-Moon System as a “Double Planet”

The Moon’s notably large size relative to Earth has prompted some scientists to term the Earth-Moon system a “double planet.” Unlike most moons, Earth’s Moon exerts a considerable gravitational influence.

Question 4.

Draw a fully labelled diagram of water cycle and explain its working and significance with reference to the diagram.

Ans:

The water is evaporated from the surfaces of open oceans, seas, rivers, lakes and from the surface of living organisms. The evaporated water gets condensed, precipitated and form the water droplets. These water droplets are returned to the earth in the form of rain and snow or it may drop directly into the oceans. If it falls on land, it again flows back into the oceans through rivers and streams. The evaporation rainfall cycle repeats several times to get the average rainfall.

Question 5.

(a) What makes our Sun as an ordinary star in the universe?
(b) What heavenly bodies are the members of the Solar System?
(c) Arrange the nine planet of the solar system in order of their sizes.
(d) Why do planets, being opaque bodies, shine in the sky?

Ans:

The Sun: Our Cosmic Anchor

At the heart of our solar system lies the Sun, a magnificent main-sequence star. This colossal, incandescent sphere, stretching about 1.39 million kilometers across, is a powerhouse of energy. Its core reaches an astounding 15 million Kelvin, where nuclear fusion continuously transforms hydrogen into helium. The Sun’s surface, in contrast, blazes at a scorching 5,778 Kelvin.

Our Solar System: A Celestial Ballet

Circling this stellar giant is our solar system, a dynamic collection of astronomical bodies. It’s home to eight recognized planets, along with numerous dwarf planets like Pluto, and a multitude of natural satellites, or moons. Closer to the Sun, the main asteroid belt is densely populated with rocky fragments. Farther out, comets carve out elongated, icy trajectories. Beyond Neptune’s orbit lie the icy reaches of the Kuiper Belt and the scattered disc, both teeming with frozen remnants from the solar system’s genesis. Throughout this grand cosmic design are subtle wisps of interplanetary dust and gas, subtly linking all its components.

The Planetary Arrangement

The planets in our solar system are arranged in a precise order, based on their increasing distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Illuminated by Sunlight

Every celestial object within our solar system, with the sole exception of the Sun itself, is visible to us only because of the sunlight it reflects.

Question 6.
Distinguish between the following pairs.

(a) Superior and Inferior planets.
(b) Planet and Planetiods
(c) Periodical and Non-periodical comets
(d) Meteors and Meteorties
(e) Planet and Satellite.
(f) Inner and Outer planets.

Ans:

Here’s a breakdown of the differences between these astronomical terms:

Superior vs. Inferior Planets

Superior planets are those whose orbits around the Sun lie outside Earth’s orbit. Think of them as the planets further out from the Sun than we are. Conversely, inferior planets have orbits that are entirely within Earth’s orbital path. They’re the planets closer to the Sun than Earth.

Planets vs. Planetoids (Asteroids)

A planet is a sizable, spherical celestial body that orbits a star and has cleared its orbital neighborhood of most other debris through its gravitational influence. Planetoids, more commonly known as asteroids, are generally smaller, often irregularly shaped rocky or metallic objects that also orbit a star but haven’t gravitationally cleared their orbital paths.

Periodic vs. Non-Periodic Comets

Periodic comets are those that follow a predictable, recurring orbital trajectory, reappearing in the inner solar system within a relatively short and consistent period, typically less than 200 years. In contrast, non-periodic comets have highly elongated orbits that can take thousands or even millions of years to complete, or they might be comets that pass through our solar system only once before exiting permanently.

Meteors vs. Meteorites

Meteors are the bright streaks of light we observe in the night sky, caused by small pieces of space rock burning up due to friction as they enter Earth’s atmosphere. Meteorites are the remnants of these space rocks that survive their fiery atmospheric journey and successfully land on Earth’s surface.

Planets vs. Satellites

Planets are the primary celestial bodies that orbit a star directly. Satellites, on the other hand, are objects that orbit planets or other smaller astronomical bodies. Earth’s Moon is a classic example of a natural satellite.

Inner vs. Outer Planets

The inner planets are the four rocky, denser worlds located closer to the Sun and situated within the asteroid belt: Mercury, Venus, Earth, and Mars. The outer planets are the much larger, less dense gas or ice giants found beyond the asteroid belt: Jupiter, Saturn, Uranus, and Neptune.

Question 7.

(a) How many satellites are there in the Solar System?
(b) Name the planet having no satellite.
(c) Which planet has the largest number of satellites.

Or
Name the largest known satellite of any planet.
Ans:

Natural Satellites in the Solar System

(a) The Solar System’s Moons The Solar System hosts over 900 known natural satellites, which orbit planets, dwarf planets, asteroids, and other celestial bodies. Among these, approximately 420 moons are associated with the eight major planets. However, as new discoveries are made, this number continues to evolve.

(b) Planets Without Moons Mercury and Venus are the only two planets in our Solar System that do not have any natural satellites. Their proximity to the Sun and weaker gravitational influence make it challenging for them to retain moons.

(c) The Planet with the Most Moons Saturn holds the record for the highest number of confirmed natural satellites, currently boasting 274 known moons. These include both its major, well-studied moons and a large collection of smaller ones identified through ongoing observations.

Question 8.
Why are the following planets not inhabitable ?

(a) Mercury
(b) Venus
(c) Jupiter
(d) Neptune
(e) Saturn

Ans:

Here’s why those planets are considered uninhabitable for life as we understand it:

(a) Mercury: Its proximity to the Sun results in extreme temperature fluctuations, from scorching days to frigid nights, coupled with a negligible atmosphere offering no protection.

(b) Venus: A runaway greenhouse effect creates incredibly high surface temperatures and a toxic, high-pressure atmosphere filled with sulfuric acid clouds.

(c) Jupiter: Being a gas giant, Jupiter lacks a solid surface. Its immense pressures, extreme temperatures within its atmosphere, and intense radiation belts make it hostile.

(d) Neptune: This gas and ice giant is extremely cold due to its distance from the Sun and experiences violent winds, lacking a solid surface and a stable, moderate environment.

(e) Saturn: Similar to Jupiter, Saturn is a gas giant without a solid surface, featuring extreme internal pressures and low upper atmospheric temperatures.

Question 9.
Name the following

(a) Two planets which are nearer to the sun than the Earth.
(b) Two planets which are farther from the sun than the Earth.
(c) The planet farthest from the Sun.
(d) The planet nearest to the Sun.

Ans:

(a) Planets closer to the Sun than Earth:

• Mercury
• Venus

(b) Planets farther from the Sun than Earth:

• Mars
• Jupiter (Saturn, Uranus, and Neptune are also correct)

(c) The planet most distant from the Sun:

• Neptune

(d) The planet closest to the Sun:

• Mercury

Question 10.

(a) Which is the unit adopted for measuring the distance in the Universe ? Give a reason for its choice.
(b) Find out the number of years to reach the following celestial bodies from the Earth : Nearest star from the Earth (Distance 150 million km)

Ans:

Here’s a rephrased and unique version of the provided text:

(a) The immense scale of the cosmos demands a tailored unit for gauging astronomical distances, a role ideally suited to the light-year. This unit is defined by the incredible distance light travels within one year, providing a far more practical and comprehensible measure than conventional units, which would otherwise yield unwieldy and astronomically large figures. Consequently, the light-year is an essential instrument for astronomical measurements.

(b) Light from the Sun reaches Earth, a journey of roughly 150 million kilometers, in approximately 8 minutes and 20 seconds. This duration, when converted to years, is approximately 1.58×10−5 years.

Question 11.

Which unit is used for measuring distances in the universe?

Ans:

Cosmic distances are typically measured using:

• Astronomical Units (AU): For distances within our solar system, based on Earth’s average distance from the Sun.  
• Light-years: For interstellar distances, representing the distance light travels in a year.  
• Parsecs (and Megaparsecs): For larger interstellar and intergalactic distances; one parsec equals 3.26 light-years.

Question 12.

What makes the Sun so hot ?

Ans:

The Sun’s intense heat is a product of nuclear fusion happening in its core. Tremendous gravitational pressure in this region creates extreme temperatures, soaring to around 15 million degrees Celsius. Under these superheated conditions, hydrogen atoms fuse to form helium, releasing vast amounts of energy as heat and light. This continuous fusion process, precisely described by Einstein’s famous equation E=mc², is the engine behind the Sun’s constant glow.

Question 13.
Name the three planets which have rings around them.
Ans:

1. Saturn
2. Uranus
3. Neptune

Question 14.
Give reasons for the following :

1. Mercury completes its orbit in less time than the Earth.
2. Venus is considered as the Earth’s twin.
3. No life is possible on Saturn.
4. Pluto is the coldest planet.
5. Comets appear very rarely.

Ans:

Solar System Facts

• Mercury’s Speedy Orbit: Mercury zips around the Sun faster than Earth because it’s much closer, experiencing a stronger gravitational pull that demands higher orbital velocity to stay in orbit.
• Venus, Earth’s Sibling: Venus is often called Earth’s twin due to their comparable size, mass, and density, even though their surface environments are dramatically different.
• Saturn’s Inhospitable Nature: Life as we know it can’t exist on Saturn because it’s a gas giant with no solid surface, extreme atmospheric conditions, and no stable liquid water or breathable air.
• Pluto’s and Uranus’s Chill: However, Uranus is even colder on average, largely because of its unusual axial tilt affecting its energy distribution.
• Comets’ Elusiveness: Comets appear infrequently because many have incredibly long orbital paths, taking centuries or millennia to return. Even short-period comets can take decades to reappear, and many are simply too dim to see without specialized equipment.

Question 15.

What do you understand by ‘Terrestrial Planet’ ?

Ans:

A terrestrial planet, also known as a rocky planet or telluric planet, is a planet that is primarily composed of silicate rocks or metals. These planets have a solid surface, making them distinctly different from the gas giants, which are primarily composed of hydrogen and helium.  

Here are some key characteristics of terrestrial planets:

• Solid Surface: This is the defining feature. They have a firm ground that one could theoretically stand on.
• Composition: Primarily made of rocky materials (silicates) and metals (like iron and nickel).  
• Density: Generally have a higher density compared to gas giants due to their rocky and metallic composition.
• Size: Typically smaller in size compared to the gas giants.  
• Few or No Moons: They tend to have few or no natural satellites.  
• Presence of Geological Features: Their surfaces can exhibit features like mountains, valleys, volcanoes, and craters.  
• Secondary Atmospheres: If they have atmospheres, these are typically formed later through volcanic outgassing or impacts, unlike the primary atmospheres of gas giants that captured gases from the early solar nebula.  
• Metallic Core: They usually possess a central core made of heavy metals.  

In our solar system, the four inner planets closest to the Sun are the terrestrial planets:  

• Mercury
• Venus  
• Earth
• Mars

Question 16.

Why does the moon appear big in the sky though it is smaller than other heavenly bodies ?

Ans:

The Moon’s seemingly large size in our sky is primarily due to its relative closeness to Earth, giving it a significant angular size in our field of view. Despite being much smaller than many other objects in space, its nearness makes it appear prominent. Furthermore, the Moon illusion, a perceptual phenomenon, makes it appear even larger when viewed near the horizon than when it’s high above. ¹ This effect is thought to involve how our brain interprets distance cues and makes size comparisons with objects on the ground.

Question 17.

The shape of the earth is not exactly that of a sphere Why?

Ans:

The Earth’s true shape is not a perfect sphere; rather, it’s a complex form influenced by three main factors.

Primarily, the planet’s rotation leads to an equatorial bulge. The centrifugal force generated by Earth’s spin causes matter to move outwards at the equator while simultaneously flattening the poles. This dynamic transforms the Earth into an oblate spheroid, a shape akin to a squashed sphere, deviating from a perfectly round form.

Secondly, the Earth’s gravitational field is not uniform. If the oceans were to extend globally without land barriers, their surface would assume an irregular, undulating shape known as the geoid. This geoid represents the Earth’s shape as defined by gravity, highlighting its departure from a smooth, spherical ideal.

Lastly, the Earth’s surface features, though comparatively small, add to its irregularity. Mountains towering above sea level and deep oceanic trenches carved into the crust contribute to localized deviations from a perfect sphere. While these topographic variations are minor in the grand scheme of the Earth’s size, they nonetheless contribute to the overall complexity and non-spherical nature of our planet.

Question 18.

Why is Mercury the hottest planet of the Solar System?

Ans:

The planet in question, while close to the Sun, isn’t the hottest. Its incredibly tenuous exosphere provides almost no thermal regulation, causing extreme temperature fluctuations. Its remarkably thick atmosphere, predominantly carbon dioxide, creates a runaway greenhouse effect, trapping heat and resulting in surface temperatures that surpass even those of the closer planet.

Question 19.

Why is Neptune the coldest planet of the Solar System?

Ans:

Here’s why Neptune isn’t the coldest:

• Distance vs. Internal Heat: Although Neptune is farther from the Sun and receives less solar radiation, it possesses a significant internal heat source. This internal heat warms Neptune’s atmosphere from within, preventing it from reaching the absolute coldest temperatures in the solar system. The exact cause of this internal heat is still researched but is likely residual heat from its formation and ongoing gravitational contraction.  
• Uranus’s Lack of Internal Heat and Axial Tilt: Uranus, despite being closer to the Sun than Neptune, has a significantly lower internal heat output. It radiates roughly the same amount of heat it receives from the Sun. This lack of a substantial internal heat source contributes to its colder average atmospheric temperature. Furthermore, Uranus has an extreme axial tilt of about 98 degrees, causing its poles to experience very long periods of sunlight followed by equally long periods of darkness. The winter pole experiences extremely low temperatures, contributing to Uranus holding the record for the coldest temperature ever measured on a planet in our solar system: -224°C (-371°F).

Question 20.

‘Venus is considered Earth’s Twin’. Why ?

Ans:

Why Venus Is Considered Earth’s Twin

Venus is often referred to as Earth’s twin due to several striking physical similarities, despite vast differences in climate and habitability.

1. Comparable Size Venus has a diameter of approximately 12,104 km, which is very close to Earth’s 12,756 km, making it the closest planetary match in size.

2. Similar Mass Venus possesses about 81.5% of Earth’s mass, making it the most comparable planet in terms of gravitational influence and internal structure.

3. Related Density Both planets share similar densities, indicating they have comparable bulk compositions—primarily silicate rock and metal.

4. Closest Planetary Neighbors Venus and Earth are adjacent in the Solar System, likely forming from similar materials within the same region during planetary formation.

5. Shared Composition Venus and Earth are classified as terrestrial planets, meaning they consist mainly of rock and metal, unlike the gas giants.

6. Geologically Young Surfaces Both planets exhibit relatively young surfaces with fewer impact craters compared to older planetary bodies. This suggests ongoing geological processes, although Venus lacks active plate tectonics like Earth.

Question 21.

(a) Describe the position of the Earth in Solar System.
(b) What is the size of the Earth ?
(c) Which is the satellite of the Earth ?

Ans:

(a) The Earth holds the third position from the Sun within our Solar System. It orbits the Sun, along with seven other planets (Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune), as well as dwarf planets, asteroids, and comets. 

(b) The Earth is a moderately sized planet. Its equatorial diameter is approximately 12,756 kilometers (7,926 miles), while its polar diameter is slightly smaller at about 12,714 kilometers (7,900 miles), making it an oblate spheroid (slightly flattened at the poles and bulging at the equator). This makes Earth the fifth-largest planet in our Solar System by both mass and diameter.

(c) The Earth has one natural satellite, which is simply called the Moon.