Visions of the Cosmos

Planetary Science

The Solar System

Credits for Diagrams NASA

A meeting of the International Astronomical Union in Prague in August 2006 was held and a vote of  424 astronomers decided on the definitions of ther Planets

A decision established three main categories of objects in our solar system.

  • Planets: The eight worlds starting with Mercury and moving out to Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

  • Dwarf planets: Pluto and any other round object that "has not cleared the neighbourhood around its orbit, and is not a satellite."

  • Small solar system bodies: All other objects orbiting the sun.

By this classification the asteroid Ceres and four Kuiper Belt Objects Pluto, Xena, Sedna and Quaoar  were classified as dwarf planets.   The definitions seemed to beg the question that our Moon, the four large moons of Jupiter, Titan and the large moon of Neptune Triton are quite large bodies - in fact Ganymede and Titan are larger than Mercury although they are less massive since their densities are much lower.

The other small objects orbiting the Sun are the asteroids (which includes the 'dwarf planet' Ceres) and the Kuiper Belt Objects and very far out the comets of the Oort Cloud. 

. It is quite useful to have a statistical Table of some of the major properties of the planets.  The table below is taken from the New Zealand Astronomical Yearbook 1997 by Stan Walker and Grant Christie.  The data is approximate but gives a good idea of the properties of the planets.  As far as moons of the outer solar system are concerned new very small ones  are always being discovered!

Statistical Table

Planet Mean Distance from Sun kilometres Distance in Astronomical Units A.U. Equatorial Diameter Kilometres Moons Orbital Period Rotational Period
Mercury 57,900,000 0.39 4,880 0 87.96 days 58.64 days
Venus 108,200,000 0.72 12,100 0 224.68days 244.3 days
Earth 149,600,000 1.00 12.750 1 365.26days 23 hr 56.07 mins
Mars 227,900,000 1.52 6,800 2 686.95days 24 hr 37.34 mins
Jupiter 778,300,000 5.20 143,000 63+ 11.862years 9 hr 50 mins
Saturn 1,427,000,000 9.54 120,500 18+ 29.456 years 10 hr14 mins
Uranus 2,871,000,000 19.19 51,100 27+ 84.07 years 12 hr
Neptune 4,497,000,000 30.06 50,500 13+ 164.81 years 15,48 mins
Pluto 5,915,000,000 39.53   3 248.5 years 6days 9hr

Sun's Equatorial diameter 1,392,000 kilometres rotational period  25.29 days

An Astronomical Unit is the mean distance of the Earth from the Sun

None of the planets pursue a completely circular orbit.  All the orbits are elliptical.  The nearest point to the Sun is called the


This line up shows the 12 planets that were proposed last week, with a wedge of the Sun at far left. Ceres, Pluto, Charon and 2003 UB313 now called Xena are barely visible. Now Charon will continue to be considered Pluto's satellite, and the three other worlds will be dubbed "dwarf planets" rather than full-fledged planets. The planets are drawn to scale, but without correct relative distances  Credit Marti Kornmesser International Astronomical Union IAU

The order in which the bodies of the Solar System will be deal with are as shown below and this page will be the main introductory page for this section of the web - (a sort of 'home page')

Click here for a special feature on European Astrofest 2007


Introductory Page

The Solar System and Other Planetary Systems

Mercury - Companion to the Inferno              


Earth An Oasis in the Cosmos

Earth's Moon 

Planet Mars

Asteroids and Meteors   Access this page through the home page not from this page



Moons of Jupiter Four large moons and the minor moons.  The Galileo Mission   

Moons of Saturn.

Titan The Cassini-Huygens Mission 

Uranus and Neptune

The Moons of Uranus

Moons of Neptune mainly Triton

The Kuiper Belt Objects including Pluto and the other 'dwarf planets'

The Late Heavy Bombardment

Extra-Solar Planets


This -The Solar System page acts as a sort of 'home base' for the section of the web-site on Planets


To return to the main 'Home Page' of the web-site click on the blue hyperlink  Home

To proceed to the section on STARS click on the blue hyperlink  STARS