The Sun

The Sun is the central and most prominent celestial body in the Solar System. It provides the essential energy that sustains life on Earth and shapes the characteristics of the other celestial bodies within its influence.

It is classified as a star which is defined as a massive, luminous ball of hot plasma that provides light, heat, and energy to all the planets, moons, and other objects stably orbiting around it by its gravitational pull.

Our Sun has a diameter of about 1.4 million kilometers. Its mass accounts for about 99.86% of the total mass of the Solar System.

The Sun is primarily composed of hydrogen (about 74%) and helium (about 24%) by mass, with heavier elements like oxygen, carbon, neon, and iron, in the remaining 2%.

Inside the central reigon, known as the core, the immense pressure and temperature, around 15 million degrees Celsius, create the conditions necessary for fusion reactions to occur. In this reaction, hydrogen nuclei (protons) combine to form helium nuclei, releasing a tremendous amount of energy in the form of light and heat.

Energy produced in the core travels outward through the radiative zone in the form of photons which are then absorbed and re-emitted by ions in its dense plasma. Above the radiative zone, the material becomes less dense, and energy is transported primarily through convection.

Then, there is a photosphere which is the visible surface of the Sun and emits the sunlight we see. It has a temperature of around 5,500 degrees Celsius. Above the photosphere is the chromosphere, which is visible during solar eclipses as a reddish ring around the Sun's edge.

The outermost layer is the corona, which extends millions of kilometers into space. It's extremely hot, with temperatures exceeding a million degrees Celsius and is visible during total solar eclipses as a faint, halo-like glow.

Planets

The International Astronomical Union (IAU) defines a planet as a celestial body that meets three criteria:

(a) It orbits a star (in our case, the Sun).
(b) It is spherical in shape due to its self-gravity,i.e., it has achieved hydrostatic equilibrium.
(c) It has become gravitationally dominant in its orbital region and has removed or absorbed most other objects in its path.

In our solar system, there are two types of planets :

1. Inner Planets (Terrestrial Planets) : These planets are smaller in size and are made up of rocks. We call them inner planets because these are closer to the Sun. These are Mercury, Venus, Earth and Mars.
2. Outer Planets (Gas Giants) : These planets are relatively larger in size and made up of gases, often referred to as the gas giants. These are called as the outer planets as these are farther from the Sun. These include Jupiter, Saturn, Uranus and Neptune.

Mercury

Mercury is the closest planet to the Sun in our solar system. It is the smallest planet with a diameter of around 4,880 kilometers.

It is about 57.9 million kilometers away from the Sun, and completes an orbit around it, in approximately 88 Earth days.

It takes approximately 59 Earth days and 16 hours for Mercury to complete one full rotation on its axis, which is notably slow compared to its orbital speed around the Sun.

Mercury has a large iron core that makes up about 42% of its volume, giving it a relatively high density.

During its daytime, surface temperatures can reach up to around 430 degrees Celsius, while during its nighttime, temperatures can plummet to as low as -180 degrees Celsius.

Mercury is so close to the Sun, it won't be able to hold its own moon thus it has no moon.

Venus

Venus is the second planet and about 108.2 million kilometers away from the Sun. It completes one orbit in approximately 225 Earth days.

Venus is often referred to as Earth's "sister planet" due to its similar size and composition. It is very similar in size to Earth, with a diameter of about 12,104 kilometers.

Venus has an incredibly dense and thick atmosphere composed mostly of carbon dioxide (about 96.5%) with traces of nitrogen and other gases. This thick atmosphere creates a strong greenhouse effect, resulting in extremely high surface temperatures.

Venus rotates on its axis in the opposite direction compared to most other planets,i.e., it rotates on its axis from east to west. This means that its rotation is considered "retrograde." It takes about 243 Earth days to complete one rotation.

Like Mercury, Venus also has no moon revolving around it.

Earth

Earth is the third planet from the Sun and the only planet known to support life. It's a diverse and dynamic world with a wide range of ecosystems and environments.

Its average distance from the Sun is about 149.6 million kilometers which is defined as 1 Astronomical Unit (AU). This distance from the Sun allows for temperatures that are suitable for a variety of habitats. The average temperature of the Earth (15 degree Celcius) is a fundamental part of what allows it to be habitable.

It takes 365 days to complete one revolution around the Sun which is defined as a year and completes one full rotation on its axis in 24 hours, which defines a day.

While the rotation goes on, some parts of the planet are facing the Sun whereas the opposite parts are not. This defines the day and night patterns on a planet. Also, these two times define the concepts of sunrise and sunset.

Earth's axis is tilted about 23.5 degrees relative to its orbital plane around the Sun. During the revolution, this tilt causes different parts of the planet to receive varying amounts of sunlight, leading to the distinct seasons we experience.

Earth is a rocky planet with a diameter of about 12,742 kilometers. Its outermost layer, the crust, is divided into tectonic plates that move and interact, causing earthquakes, volcanic eruptions. Beneath the crust lies the mantle which is semi-solid and drives plate tectonics and other geological activities, and then there is the core which is a solid sphere of iron and nickel under immense pressure and temperature due to its depth.

Its atmosphere is a mixture of gases that extends several hundred kilometers above the surface and is composed mainly of nitrogen (about 78%) and oxygen (about 21%), with trace amounts of carbon dioxide, water vapor, and noble gases.

Earth has got one natural satellite or moon which is simply known as "the Moon".

Mars

Mars is the fourth planet from the Sun in our solar system and is often referred to as the "Red Planet" due to its reddish appearance which comes from iron oxide (rust) on its surface.

It has a diameter of about 6,779 kilometers. It has a thin atmosphere primarily composed of carbon dioxide, with traces of nitrogen and argon. It completes one rotation on its axis in 24.6 hours.

It is 228 million kilometers away from the Sun and completes one revolution around the Sun in approximately 687 days.

Mars's significant orbital eccentricity and axial tilt cause large seasonal changes to the polar ice caps' coverage and temperature swings between -110 °C to 35 °C on the surface.

Numerous space missions have been sent to Mars to study its surface, atmosphere, and potential for past or present life. Notable missions include the Viking landers, the Mars rovers (Spirit, Opportunity, Curiosity, and Perseverance).

While no direct evidence of life has been found, scientists continue to explore the possibility that microbial life could exist beneath the planet's surface, where conditions might be more favorable.

Mars has two moons, Phobos and Deimos.

Jupiter

Jupiter is the largest planet in our solar system with a diamter of 139,820 kilometeres.

It is predominantly composed of hydrogen and helium, similar to the composition of the Sun. Its atmosphere is made up of several layers of gas, with the top layer consisting of clouds primarily composed of ammonia crystals and other compounds. The iconic bands and swirls seen on Jupiter's surface are created by powerful jet streams that separate different atmospheric regions.

One of the most famous features on Jupiter is the Great Red Spot, which is more than twice the size of Earth. It is a massive storm that has been raging for centuries.

It is 742.49 million kilometeres away from the Sun and makes a complete orbit around it, in 4,333 Earth days. One year here is as long as 12 years on Earth but it has a rotation period of about 10 hours.

Jupiter is so massive that it contains more than twice the combined mass of all the other planets in the solar system.

Due to its enormous gravitational field, Jupiter plays a protective rolefor us. By deflecting comets and asteroids that might otherwise hit Earth, it helps to create a more stable environment for life to evolve here.

Since this giant planet commands thousands of small objects in its orbit, it is estimated that it has 95 moons.

Saturn

Saturn is the sixth planet from the Sun and the second largest planet of our solar system. It has a diameter of about 116,000 kilometers.

It is known for its spectacular ring system which is composed of numerous individual rings that are thin and primarily made of ice particles or rocky debris ranging in size from tiny grains to larger chunks. Together, these rings give a very stunning look to the planet.

It is a gas giant which primarily consists of hydrogen (around 96%) and helium (around 3%) with no solid surface. Its atmosphere features distinct cloud bands and storm systems, including its famous hexagonal-shaped storm at the north pole.

Saturn is 1.4 billion kilometers away from the Sun and takes about 29.5 Earth years to complete one orbit. It rotates relatively quickly, with a day lasting approximately 10.7 hours.

One of Saturn's most unique features is its hexagonal polar storm, which is believed to be created by the planet's rapid rotation and the interaction of different wind patterns. It was first observed by the Voyager spacecraft in the 1980s and later studied extensively by Cassini.

As of June 2023, Saturn has 146 moons in its orbit.

Uranus

Uranus is the seventh planet from the Sun and third largest planet of our solar system, with a diameter of 50,000 kilometeres.

It is an ice giant whose small rocky core surrounded by a thick layer of icy and watery materials, all enveloped in a deep atmosphere of hydrogen and helium.

It has two sets of rings. The inner system of nine rings consists mostly of narrow, dark grey rings. Then there are two outer rings: the innermost one is reddish like dusty rings, and the outer ring is blue.

All the planets rotate on their axes from west to east direction, except Venus and Uranus. These two planets rotate from east to west. Uranus completes one rotation in about 17.2 Earth hours.

Uranus orbits the Sun at an average distance of about 2.9 billion kilometers and takes about 84 Earth years to complete one orbit. Its unique tilt gives a sideways orientation due to which, each pole experiences nearly 42 years of continuous daylight followed by 42 years of darkness.

Uranus has a relatively featureless and thick atmosphere that primarily consists of hydrogen, helium, and trace amounts of methane. The presence of methane in the upper atmosphere gives the planet its distinctive blue-green color.

It has 27 moons known upto yet.

Neptune

Neptune is the eighth and farthest planet from the Sun in our solar system.

It is primarily composed of hydrogen and helium, along with trace amounts of methane, which absorbs red light, reflects blue light, and gives the planet the vibrant blue color. This chacteristic is similar to Uranus.

Its atmosphere is characterized by fast-moving winds and the presence of large dark storm systems with wind speeds reaching up to 2,100 kilometers per hour.

The Great Dark Spot, similar to Jupiter's Great Red Spot, was a large dark storm in Neptune's atmosphere, but it has now disappeared. Other dark storm systems have also been observed on its surface.

Neptune orbits the Sun at an average distance of about 4.5 billion kilometers and takes about 165 Earth years to complete one orbit.

It rotates on its axis relatively quickly, with a day lasting about 16 hours and 6 minutes.

Neptune has a system of rings, although they are faint and difficult to observe. The rings are composed of ice particles and dust.

Dwarf Planets

A dwarf planet is the planet which does not obey the third criterion for the definition of a planet. It means that it does not clear or accumulate other objects from its path. The dwarf planets are farthest from the Sun, than any other planet. These include Pluto, Eris, Ceres, Makemake, and Haumea. Most dwarf planets are located in the Kuiper Belt, a donut-shaped region of icy bodies beyond the orbit of Neptune, except Ceres which lies in between Mars and Jupiter.

Pluto

Pluto was discovered in 1930 when it was named the ninth planet in the solar system. However, it was recognized as a full-fledged planet in the 1990s but in 2006, it was officially classified as a dwarf planet.

It is the biggest dwarf planet in size and the second largest in terms of mass. Its surface has plains and mountains composed of water ice and nitrogen ice.

It does not have a circular or elliptical orbit like other planets do, and its orbit crosses that of Neptune. This means that sometimes Pluto is closer to the Sun than Neptune is.

Pluto orbits the Sun about 5.8 billion kilometeres away and takes 249 Earth years to complete one revolution.

One day on Pluto lasts 153 hours, or about 6 Earth days.

Ceres

Ceres is the only dwarf planet located in the inner solar system. It lies in the asteroid belt between Mars and Jupiter.

Its radius is just 476 kilometers but it contains considerable amount of matter.

Ceres is at an average distance of 410 million kilometres from the Sun and completes one revolution in 1680 Earth days.

It completes one rotation in about 9 hours which is quite shorter than any other planet.

Scientists suspected this unique dwarf planet might even possess sporadic water vapor—possibly from ice ejected by small impacts, no evidence of the atmosphere has been found yet but scientists would like to search for possible signs of life on Ceres.

Makemake

Makemake is a small dwarf planet with a radius of 715 kilometers. It is around 46 Astronomical Units away from the Sun and takes 305 Earth years to complete one revolution.

It completes one rotation every 22 and a half hours, making its day length similar to Earth and Mars.

It appears reddish in color due to the presnce of tholins that form whenever solar ultraviolet light interacts with ethane and methane.

Makemake may develop a very thin atmosphere, most likely made of nitrogen near perihelion when it is closest to the Sun.

Eris

Eris is little smaller than Pluto, with a radius of 1163 kilometeres.

It is at 68 Astronomical Units away from the Sun and completes one orbit in 556 years.

It completes one rotation every 25.9 hours, which is similar to the Earth.

Eris most likely has a rocky surface similar to Pluto. Its surface temperatures vary from -217 degrees Celsius to -243 degrees Celsius, which is extremely cold.

It is often so far from the Sun that its atmosphere collapses and freezes, falling to the surface as snow or methane ice. As it gets closest to the Sun in its faraway orbit, the atmosphere thaws.

Haumea

Haumea is the first known Kuiper Belt object to have rings. Its size is roughly similar to that of Pluto.

Haumea is not round, but like an egg or a football in shape which is believed to be due to its extremely fast rotation.

It is one of the fastest rotating large objects in our solar system, which completes one rotation on its axis in just 4 hours.

It is 43 Astronomical Units distant from the Sun and takes about 285 Earth years to complete one revolution.

Moons

A moon, also known as a natural satellite, is a celestial body that orbits a planet in the space. It is relatively small with respect to its planet and is held in orbit due to the gravitational attraction between them. Based on the orbits, a moon is classified in two separate categories: (a) Regular moon, which has prograde orbit (in the direction of its planets' rotation), lies close to the plane of its equators and is of bigger size as compared to the other category. (b) Irregular moon, whose orbit can be pro- or retrograde (against the direction of its planet's rotation), often lies at extreme angles to its planet's equators and is generally less than 10 kilometers in diameter. Moons come in various sizes, shapes, and compositions, and they can exhibit a wide range of features, including surfaces with craters, mountains, valleys, and even atmospheres and hidden oceans beneath their surfaces. Currently, 297 moons are known to be present in our solar system. Some notable moons of our solar system are :

Earth's Moon

Earth's Moon is the only place beyond Earth where humans have set foot.

It's the fifth largest moon in the solar system but appears to be the largest and brightest object in our night sky.

It is simply called "the Moon" because people didn't know other moons existed until 1610.

The Moon orbits the Earth from an average distance of 384,400 km and completes one revolution in 27 days.

It has a solid, rocky surface cratered and pitted from impacts by asteroids, meteorites, and comets. It has a very thin and tenuous atmosphere which is not breathable.

More than 105 robotic spacecraft have been launched and 24 humans have visited to explore the Moon.

Ganymede

Ganymede was discovered by Galileo Galilei in 1610 using a homemade telescope. Alongwith it, three more moons were discovered namely, Io, Europa, and Callisto.

Ganymede is the largest moon of the solar system with a diameter of 5269 kilometeres, which is even larger than the planet Mercury.

It is the only moon known to have its own magnetic field, which causes auroras, which are ribbons of glowing, hot, electrified gas, in regions circling its north and south poles.

Although its surface is icy but, the rocky sea bottom might be in contact with salt water. Thus, the development of primitive life might be possible here.

Io

Io is the most volcanically active world in the solar system, with hundreds of volcanoes, some erupting lava fountains dozens of kilometers high.

Its volcanoes are at times so powerful that they are seen with telescopes on Earth.

It is 3464 kilometeres in diameter which is slightly larger than Earth's moon.

It takes Io about 42.5 hours to complete one orbit around Jupiter. The revolution is so fast, because Io is in a tug-of-war between Jupiter's powerful gravity and precisely timed pulls from two neighboring moons Europa and Ganymede.

Europa

Europa has the smoothest surface of any solid body in the solar system. Its icy surface may hide a buried ocean full of more water than Earth.

It is primarily made of silicate rock and has a water-ice crust and probably an iron-nickel core.

It has a very thin atmosphere, composed primarily of oxygen. It also has reddish-brown cracks slicing the surface, which give a look of a veiny eyeball.

Europa may be one of the most promising places in our solar system to find present-day environments suitable for some form of life beyond Earth.

Callisto

Callisto is Jupiter’s second largest moon and the third largest moon in our solar system.

Its surface is rocky and is the most heavily cratered of any object in our solar system. Images captured by passing spacecraft show bright white spots standing out against darker regions. Scientists think the bright areas are mostly ice and the darker patches are areas where the ice has eroded.

Research reveals that there may be an ocean located deeper beneath its icy surface. If this happens to be true, it’s possible to create a potential habitat for life.

Callisto has hydrogen and oxygen in its atmosphere and carbon dioxide on its surface.

Titan

Titan is the largest moon of Saturn and the second largest moon in our solar system, with a diameter of 5150 kilometers.

It is an icy world whose surface is completely obscured by a golden hazy and dense atmosphere consisting of nitrogen, and a small amount of methane.

It is the sole other place in the solar system known to have an earthlike cycle of liquids raining from clouds, flowing across its surface, filling lakes and seas, and evaporating back into the sky.

Although there is so far no evidence of life on Titan, its complex chemistry and unique environments are certain to make it a destination for continued exploration.

Asteroids, Comets and Meteors

Asteroids

Asteroids are rocky airless fragments which are leftovers from the beginning of the solar system. They vary in size from small pebble-like objects to larger bodies several hundred kilometers in diameter.

Most known asteroids are within the asteroid belt, located between the orbits of Mars and Jupiter. According to NASA, the currently known asteroid count exceeds 1.1 million. Yet the collective mass is still less than the mass of Earth’s Moon.

They orbit around the Sun due to its incredible gravitational force. Their orbits may be elliptical or inclined towards a planet.

Since their orbits are not entirely stable and can be disrupted by close passes of the planet Jupiter, thus they collide with Earth and impact our world in massive ways by triggering tsunamis and fires, along with enormous amounts of dust and soot.

Comets

Comets are cosmic snowballs of ice, volatile gases, rock, and dust that orbit the Sun. They are are often referred to as "dirty snowballs" due to their composition.

Comets are thought to be remnants from the early solar system.

When a comet comes close to the Sun, it heats up and spews dust and gases into a giant glowing head, known as coma and forms a tail that points away from the Sun and stretches for millions of miles. There is a solid core known as the nucleus.

There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud.

One of the most famous comets is Halley's Comet, which is visible from Earth roughly once every 76 years.

Meteors

There are three terms namely meteor, meteoroid, and meteorite for the same object, depending on where it is.

Meteoroids are objects in space that range in size from dust grains to small asteroids.

When meteoroids enter a planet’s atmosphere at high speed and burn up, the fireballs or “shooting stars” are called meteors.

When a meteoroid survives a trip through the atmosphere and hits the ground, it’s called a meteorite.

Scientists estimate that about 44,000 kilograms of meteoritic material falls on the Earth each day, but it is vaporized in the atmosphere, leaving a bright trail fondly called "shooting stars."

Several meteors can usually be seen on any given night. Sometimes the number increases dramatically—these events are termed meteor showers.

Interplanetary Space

The interplanetary space or medium refers to the vast regions of space that exist between the different bodies within a solar system. It includes interplanetary dust, cosmic rays, and hot plasma from the solar wind. It is mostly a near-vacuum environment, and its density is very low, decreasing in inverse proportion to the square of the distance from the Sun. The temperature has extreme variations due to the lack of thermal regulations. The objects in direct sunlight can become very hot, while those in shadow can become extremely cold. Additionally, interplanetary space is bathed in solar radiation, including visible light, ultraviolet radiation, and other forms of electromagnetic radiation emitted by the Sun. All the celestial objects orbit the central star in interplanetary space following elliptical, circular, or even hyperbolic trajectories, depending on their initial velocities and the gravitational forces acting on them. Spacecraft travel through this environment to reach their destinations, using gravity assists from planets to adjust their trajectories and conserve fuel. It serves as a medium for the interactions between numerous components of the solar system and helps in understanding mechanisms of the same.

Oort Cloud

Beyond the orbits of the planets, there are regions full of icy objects known as the Kuiper Belt and the Oort Cloud. Similar to the asteroid belt, the Kuiper Belt is a region of leftovers from the solar system's early history. It seems to be a thick disk or a donut. The Oort Cloud is a predicted or hypothetical collection of spherical layers trillions of of icy bodies, comets and planetesimals farther away than everything else in the solar system. It surrounds our solar system at a distance between about 2,000 and 100,000 astronomical units (AU) from the Sun, in a bubble shape. It is the most remote and unexplored region of the solar system. It was named after the Dutch astronomer Jan Oort, who proposed its existence in the mid-20th century. No missions have been sent to explore the Oort Cloud yet, but five spacecraft will eventually get there. They are Voyager 1 and 2, New Horizons, and Pioneer 10 and 11. However, it is so distant that the power sources for all five spacecraft will be dead centuries before they reach its inner edge. Thus, its exploration remains a challenge for future space missions. A rough illustration of the Oort Cloud is shown in the diagram at the right side.