Essays

From the dawn of history until the beginning of the 17th century the knownuniverse consisted of only 8 planets:

1. Earth, 2. Sun, 3. Moon, 4. Mercury, 5. Venus, 6. Mars, 7. Jupiter,8. Saturn and the "fixed" stars. These are the ones that can be seen easily withoutany optical instruments. In Europe, the prevailing view was the Ptolemaic system withthe Earth at the centre and the other bodies revolving around it.

In 1610, Galileo first turned a telescope on the heavens and the universe wasexplored. By the end of the 17th century, 9 new planets had been discovered andCopemmican’s heliocentric theory was widely accepted. The total number of knownplanets had more than doubled to 17. In 18th Century, only 5 new planets (not counting

comets) were discovered, all by William Herschel , bringing the total to 22. Thenumber of planets in the solar system increased dramatically in the 19th century withI the discovery of the asteroids (464 of which were known at by 1899) but only 9 more"major" planets were discovered. The number of major planets rose to 31 (almostdouble the 17th century). In the first three quarters of the 20th century, 13 more majorplanets (and thousands of comets and asteroids) were discovered bringing the total upto 43. In the Space Age 27 more small moons were discovered by the two Voyagerspacecraft.In CCD Age dozens more small moons have been discovered in recentyears with large ground based telescopes and CCD cameras. There are well over 100now known.

Overview:

The solar system consists of the Sun; the nine planets, more than 130 satellitesofthe planets, a large number of small bodies like the comets and asteroids, and theinterplanetary medium. There are probably also many more planetary satellites that, have notyet been discovered.

The inner solar system contains the Sun, Mercury, Venus, Earth and Mars.Themain asteroid belt lies between the orbits of Mars and Jupiter. The planets ofthe outersolar system are Jupiter, Saturn, Uranus, Neptune and Pluto.The first thing to noticeis that the solar system is mostly empty space. The planets are very small comparedto the space between them.

The orbits ofthe planets are eclipses with the Sun at one focus, though all exceptMercury and Pluto are very nearly circular. The orbits ofthe planets are all more orless in the same plane .called the ecliptic and defined by the plane of the Earth's orbit,Theecliptic is inclined only 7 degrees fromtheplaneof the Sun'sequator.Pluto'sorbit

deviates the most from theplane ofthe ecliptic with an inclination ofl7 degrees. Theyall orbit in the same direction, counter-clockwise looking down from above the Sun’s north pole, including Venus, Uranus and Pluto also rotate in that same sense.

One way to help visualize the relative sizes in the solar system is to imaginemodel in which everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter. The Moon would be about 30 cm from the  Earth. The Sun would be 1.5 meters in diameter and 150 meters from the Earth, Jupiter would be 15 cm in diameter and 5 blocks away from the Sun. Saturn would beblocks away; Uranus and Neptune 20 and 30 blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away.

There are numerous smaller bodies that also inhabit the solar system the  satellites of the planets, the large number of asteroids orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic; and the many small icy bodies beyond Neptune in the Kuiper Belt. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids.

Traditionally, the solar system has been divided into planets, their satellites asteroids and comets . Unfortunately, the solar system has been found to more complicated than this would look.There are several moons larger than Pluto and too larger than Mercury.There are many small moons that are probably started out as asteroids and were only later captured by a planets, comets sometimes fizzle out and become indistinguishable from asteroids,the Kuiper Belt objects and others like Chiron don't fit this scheme well and some even want to consider Pluto as part of

this class,The Earth/Moon and Pluto/Charon systems are sometimes considered” double planets".                             

The nine bodies officially categorized as planets are often further classified in several ways : By compositions planets are composed primarily of hydrogen helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites, By size: small planets: Mercury, Venus, Earth, Mars and Pluto small planets have diameters less than 13000 km. giant planets; Jupiter, Saturn, Uranus and Neptune.The giant planets have diameters greater than 48000 km.

Mercury and Pluto are sometimes referred to as lesser planets The giant planets are sometimes also referred to as gas giants, By position relative to the Sun: inner planets: Mercury, Venus, Earth and Mars,outer planets: Jupiter, Saturn, Uranus Neptune and Pluto.The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system. By position relative Earth:inferior planets: Mercury and Venus, closer to the Sun than Earth.

Some Questions required solutions :

What is the origin of the solar system?

How common are planetary systems around other stars?

What conditions allow the formation of terrestrial planets?

Is there life elsewhere in the solar system? Ifnot, why is Earth special?

Is there life beyond the solar system?

Is life a rare and unusual or even unique event in the evolution of the universe

or is it adaptable, widespread and common?

Answers to these questions, even partial ones would be of enormous value.Scientists and Historians have been trying to solve the above questions, no evidentialprogress has made so far, yet a day shall come when the puzzles be solved.

Vocabulary

1. prevailing—he larger in number, quantity, or importance, holding, triumphing.2. interplanetary'—between or among planets. 3.probably—with considerable certainty;without much doubt, credibly. 4. asteroid—any of numerous small celestial bodiescomposed of rock and metal that move around the sun, minor planet, star-shaped.

5. inclined—have a tendency or disposition to do or be something; be inclined, prepared,bowed. 6. visualize—imagine; see in one's mind, envision, picture. 7.diameter—thelength of a straight line passing through the centre of a circle and connecting two pointson the circumference, diam.8. exceptions—a deliberate act of omission, exclusions.9. complicated—make more complex, intricate, or richer, elaborated, rarified.10. categorized—coordinated, organic /ed.11. widespread—widely circulated or diffused,far-flung. 12. enormous—extraordinarily l\large in size or extent or amount or poweror degree, tremendous.13. evidential—serving as or based on evidence, evidentiary.