From the dawn of history until the beginning of the 17th century the known universe consists of 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 could be seen, easily without any optical instruments. In Europe, the prevailing view was the Ptolemaic system with the Earth at the centre and the other bodies revolving around it.

In 1610, Galileo first turned a telescope on the heavens and the universe was explored. By the end of the 17th century, 9 new planets had been discovered and Copernicus’s heliocentric theory was widely accepted. The total number of known planets 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. The number of planets in the solar system increased dramatically in the 19th century with 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 (almost double the 17th century). In the first three quarters of the 20th century, 13 more major planets (and thousands of comets and asteroids) were discovered bringing the total up to 43. In the Space Age 27 more small Moons were discovered by the two Voyager spacecrafts. In CCD Age dozens more small moons have been discovered in recent years with large ground based telescopes and CCD cameras.

The solar system consists of the Sun, the eight official planets, at least three ‘dwarf planets’, more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered).

The inner solar system contains the Sun, Mercury, Venus, Earth and Mars.

The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, and Neptune (Pluto is now classified as a dwarf planet)

The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them.

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth’s orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun’s equator. The above diagrams show the relative sizes of the orbits of the eight planets (plus Pluto) from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun’s North Pole); all but Venus, Uranus and Pluto also rotate in that same sense.

One way to help viz. the relative sizes in the solar system is to imagine a model 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 size of a grape). The Moon would be about 30cm (about a foot) from the Earth. The Sun would be 1.5m in diameter (about the height of a man) and 150 m (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 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 numberous smaller bodies that inhabit the solar system, the satellites of the planets, the large number of asteroids (small rocky bodies) 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 are not generally true for comets and asteroids.

Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites.

There are several moons larger than Pluto and two larger than Mercury.

There are many small moons that are probably started out as asteroids and were only later captured by a planet.

Comets sometimes fizzle out and become indistinguishable from asteroids.

The Kuiper Belt objects (including Pluto) and others like Chiron don’t fit this scheme well.

The Earth/Moon and Pluto/Charon Systems are sometimes considered ‘double planets.’

The eight bodies officially categorized as planets are often further classified in several ways

By Composition

Terrestrial or Rocky Planets Mercury, Venus, Earth, and Mars

The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

Jovian or Gas Planets Jupiter, Saturn, Uranus, and Neptune

The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lot of satellites.

By Size

Small Planets Mercury, Venus, Earth, Mars.

The small planets have diameters less than 1300km. Giant planets Jupiter, Saturn, Uranus and Neptune.

The giant planets have diameters greater than 48000km.

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.

The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.

By Position Relative to Earth

Inferior Planets Mercury and Venus.

Closer to the Sun than Earth.

The Inferior planets show phases like the Moon’s when viewed from Earth.


Superior Plants  Mars to Neptune.

Father from the Sun than Earth.

The superior planets always appear full or nearly so.

By History

Classical Planets like Mercury, Venus, Mars, Jupiter and Saturn. Known since prehistorical times.

Visible to the unaided eye.

In ancient times this term also referred to the Sun and the Moon, the order was usually specified as, Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, based on the time for them to go ‘all the way round’ the sphere of the fixed’ starts).

Modern Planets Uranus, Neptune.

Discovered in modern times.

Visible only with optical aid.


The IAU has recently decided that ‘classical’ should refer to all eight planets (Mercury thru Neptune, including Earth but not Pluto). This is contrary to historical usage but makes some sense from a 21st century perspective.

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? If not, 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 evidential progress has made so far, yet a day shall come when the puzzles are solved.