Earth and Universe


Earth is the third planet from the Sun and the only object in the Universe known to harbor life.

According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago.

Earth’s gravity interacts with other objects in space, especially the Sun and the Moon, Earth’s only natural satellite.

Earth revolves around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.



  • The solid crust is the outermost and thinnest layer of our planet. The crust averages 25 miles (40 kilometers) in thickness and is divided in to fifteen major tectonic plates that are rigid in the center and have geologic activity at the boundaries, such as earthquakes and volcanism.
  • The most abundant elements in the Earth’s crust include (listed here by weight percent) oxygen, silicon, aluminum, iron, and calcium. These elements combine to form the most abundant minerals in the Earth’s crust, members of the silicate family – plagioclase and alkali feldspars, quartz, pyroxenes, amphiboles, micas, and clay minerals.


  • Mantle material is hot (932 to 1,652 degrees Fahrenheit, 500 to 900 degrees Celsius) and dense and moves as semi-solid rock. The mantle is 1,802 miles (2,900 km) thick and is

composed of silicate minerals that are similar to ones found in the crust, except with more magnesium and iron and less silicon and aluminum.

  • The base of the mantle, at the boundary with the outer core, is termed the Gutenberg discontinuity. It is at this depth (1,802 miles, 2,900 km) where secondary earthquake waves, or S waves, disappear, as S waves cannot travel through liquid.
  • Scientists are utilizing seismic tomography to construct 3-dimensional images of the mantle, but there are still limitations with the technology to fully map the Earth’s interior.

Outer Core

The outer core is composed mostly of iron and nickel, with these metals found in liquid form.

The outer core reaches between 7,200 and 9,000 degrees Fahrenheit (4,000 and 5,000 degrees Celsius) and is estimated to be 1,430 miles (2,300 km) thick.

It is the movement of the liquid within the outer core that generates Earth’s magnetic field.

Inner Core

  • The inner core is the hottest part of our planet, at temperatures between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius). This solid layer is smaller than our Moon at 750 miles (1,200 km) thick and is composed mostly of iron. The iron is under so much pressure from the overlying planet that it cannot melt and stays in a solid state.
  • The inner core is the hottest part of our planet, at temperatures between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius). This solid layer is smaller than our Moon at 750 miles (1,200 km) thick and is composed mostly of iron. The iron is under so much pressure from the overlying planet that it cannot melt and stays in a solid state.


The lithosphere is the outermost layer of the Earth 100 km thick and is defined by its mechanical properties.

This rigid layer includes the brittle upper portion of the mantle and the crust. The lithosphere is divided into 15 major tectonic plates, and it is at the boundary of these plates where major tectonic occurs, such as earthquakes and volcanoes.

The lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time.

The lithosphere is the coolest layer of the Earth in terms of temperature, with the heat from the lower layers generating the plate movements.

The term “lithosphere” should not be confused with the use of “geosphere,” which is used to indicate all of Earth’s systems, including the atmosphere, hydrosphere, and biosphere.


The mesosphere refers to the mantle in the region under the lithosphere and the asthenosphere, but above the outer core. The upper boundary is defined as the sharp increase in seismic wave

velocities and density at a depth of 660 kilometers (410 mi). This layer should not be confused with the atmospheric mesosphere.


The word universe derives from the Old French word univers, which in turn derives from the Latin word universum.

The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.

  • The Universe                   is                  all                  of space and time and                                         their contents, including planets, stars, galaxies, and all other forms of matter and energy. While the spatial size of the entire Universe is still unknown, it is possible to measure the observable universe.
  • Further observational improvements led to the realization that our Sun is one of hundreds of billions of stars in a galaxy we call the Milky Way, which is one of hundreds of billions (perhaps trillions) of galaxies in the Universe. Many of these stars have planets.

At the largest scale galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center.

  • From studying the movement of galaxies, we know that the universe contain much more matter than we can detect in usual ways. This unseen matter is known as dark matter (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly.


  • The Milky Way is the galaxy that contains our Solar System. The descriptive “milky” is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe.
  • The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.
  • The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed “dark matter”.


  • We can see the Sun, Moon, and the five planets mentioned before with Naked Eyes.
  • The Sun has Eight Planets, All Planets revolves around the sun.
  • The eight planets have been classified into Solid Planet and Gaseous Planets. Mercury, Venue, Earth and Mars are called as Solid Planets. While Jupiter, Saturn Uranus and Neptune are gaseous Planets.


Pluto, Charon, Ceres, Eris were newly grouped as “Dwarf Planets” in the year 2008. They also revolves around the sun.

They also revolve around the sun. They are very small in size. Their Size is smaller than our Moon. So they are called DWARF PLANETS.


Thousands of Asteroids are found between Mars and Jupiter. Asteroids are Clusters of Celestial Bodies which Includes tiny Stones and big rocks which can measure upto 300 to 400 km in Diametre.


The Sudden Streaks of light seen on a starry night is called as METEOROIDS.

This can be seen when the remains of the rocky parts of the comets strike the Earth’s atmosphere and streaks of light are generated. They are not stars that fall down.


  • It is exciting to see a comet in a Night Sky. A Comet is not a star. It is a Rock made of Dust and Ice. The long tails is seen because, as the comet comes near the sun, the ice melts and reflects the light of the sun.
  • The tail of the comet is seen in the opposite direction of the sun. The tail of the comet appears because minute particles from the sun strike the gaseous part emitted from the comet.


  • Black holes are not physically large regions of space. But when you include their mass, they are among the top competitors for the largest things in the universe. And quasar OJ287 is the largest black hole we’ve spotted.
  • It’s estimated to be 18 billion times the mass of our sun and is a supermassive black hole located in the center of a galaxy. To put that in perspective, it’s an object larger than our entire solar system. Just how big can a black hole get? According to scientists, there is no theoretical upper limit.



  • The Moon is called by different names in Tamil. The moon is not a Planet.
  • Moon does not revolve around the sun directly. It revolves around the Earth. Hence it is called as a Satellite.
  • The Moon is the Satellite of the earth.


  • Dark matter is a hypothetical kind of matter that is invisible to the entire electromagnetic spectrum, but which accounts for most of the matter in the Universe.
  • The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the Universe.
  • Other than neutrinos, a form of hot dark matter, dark matter has not been detected directly, making it one of the greatest mysteries in modern astrophysics. Dark matter

neither emits nor absorbs light or any other electromagnetic radiation at any significant level. Dark matter is estimated to constitute 26.8% of the total mass–energy and 84.5% of the total matter in the Universe.


  • Two proposed forms for dark energy are the cosmological constant, a constant energy density          filling            space                         homogeneously, and scalar  fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space.
  • Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields having only a slight amount of spatial in homogeneity would be difficult to distinguish from a cosmological constant.

Solar System

The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly.

Of the objects that orbit the Sun directly, the largest eight are the planets, with the remainder being smaller objects, such as dwarf planets and small Solar System bodies.

Of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury.

The four Smaller Inner Planets are,

  • Mercury
  • Venus
  • Earth
  • Mars

The Four Outer Or Giant Planets are,

  • Jupiter
  • Saturn
  • Uranus
  • Neptune

The Solar System also contains smaller objects,

Asteroid Belt which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal.

Dwarf Planets,

  • Pluto
  • Eris

The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble- like region in the interstellar medium known as the heliosphere.

Distance and Scales

The distance from Earth to the Sun is 1 astronomical unit (150,000,000 km), or AU.

For comparison, the radius of the Sun is 0.0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of Earth’s orbit, whereas Earth’s volume is  roughly one millionth (10−6) that of the Sun. Jupiter, the largest  planet,  is 5.2

astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km) from the Sun.

Formation and Evolution

  • The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud. This initial cloud was likely several light-years across and probably birthed several stars.
  • As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula, collapsed, conservation of angular momentum caused it to rotate faster.
  • The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the centre. The planets formed by accretion from this disc, in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies.
  • Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies.

Sun (Popular Star)

The Sun is the Solar System’s star and by far its most massive component. Its large mass (332,900 Earth masses), which comprises 99.86% of all the mass in the Solar System, produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium, making it a main-sequence star.

This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.

Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn.

This region is also within the frost line, which is a little less than 5 AU (about 700 million km) from the Sun.

Inner Planets

  • The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems.
  • They are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals, such as iron and nickel, which form their cores.
  • Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.
  • The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).


  • Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses).
  • Mercury has no natural satellites; besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history.
  • Mercury’s very tenuous atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained.
  • Hypotheses include that its outer layers were stripped off by a giant impact; or, that it was prevented from fully accreting by the young Sun’s energy.


  • Venus (0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity.
  • It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere.
  • No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.


  • Earth (1 AU from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist.
  • Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed.
  • Earth’s atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.


  • Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (roughly 0.6% of that of Earth).
  • Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.
  • Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids, or ejected debris from a massive impact early in Mars’s history.

Asteroid Belt

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of refractory rocky and metallic minerals, with some ice.

They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions.

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons.

The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid.

Outer Planet

The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun.

Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of hydrogen and helium; Uranus and Neptune are far less massive (<20 Earth masses each) and are composed primarily of ices.

For these reasons, some astronomers suggest they belong in their own category, “ice giants”. All four giant planets have rings, although only Saturn’s ring system is easily observed from Earth.

The term superior planet designates planets outside Earth’s orbit and thus includes both the outer planets and Mars.


  • Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium.
  • Jupiter’s strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 69 known satellites.
  • The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, is larger than Mercury.


  • Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere.
  • Although Saturn has 60% of Jupiter’s volume, it is less than a third as massive, at 95 Earth masses. Saturn is the only planet of the Solar System that is less dense than water.
  • The rings of Saturn are made up of small ice and rock particles. Saturn has 62 confirmed satellites composed largely of ice.
  • Two of these, Titan and Enceladus, show signs of geological activity. Titan, the second- largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.


  • Uranus (19.2 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic.
  • It has a much colder core than the other giant planets and radiates very little heat into space. Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda.


  • Neptune (30.1 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and hence more dense.
  • It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.
  • Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that are in 1:1 resonance with it.


  • The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter’s (5.5 AU) and less than Neptune’s (30 AU).
  • The largest known centaur, 10199 Chariklo, has a diameter of about 250 km. The first centaur discovered, 2060 Chiron, has also been classified as comet (95P) because it develops a coma just as comets do when they approach the Sun.


  • Comets are small Solar System bodies, typically only a few kilometres across, composed largely of volatile ices.
  • They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto.
  • When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.

Trans-Neptunian Region

Beyond the orbit of Neptune lies the area of the “trans-Neptunian region”, with the doughnut- shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt.

The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice.

This region is sometimes described as the “third zone of the Solar System”, enclosing the inner and the outer Solar System.

Kupier Planet

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.

It extends between 30 and 50 AU from the Sun. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies. Many of the larger Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may prove to be dwarf planets with further data.

There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.

Pluto and Charon

The dwarf planet Pluto (39 AU average) is the largest known object in the Kuiper belt.

When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet.

Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Pluto has a 3:2 resonance with Neptune, meaning that

Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.

Charon, the largest of Pluto’s moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e. they appear to “orbit each other”). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit within the system.


Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto and about the same diameter.

It is the most massive of the known dwarf planets. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto’s distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane.