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Earth is one of the planets that orbit the Sun, which is actually a star like
those in the night sky. The Sun is by far the largest object in the solar system,
1,392,000km in diameter, and contains 99.8% of all the mass in the entire solar system. Even Jupiter, the largest
planet, is microscopic in comparison.
There is considerable debate amongst astronomers about which solar system objects should actually be counted as planets. In August 2006 the International Astronomical Union adopted a new definition of "planet" which excluded Pluto, reducing the total of officially recognised planets from nine to eight. As this was a controversial decision I've still included it on this page.
I am not including Sedna,
because although it was widely described as the tenth planet by the media at the time of its discovery
it was never officially classified as such. It is in fact a
Kuiper Belt or Oort cloud object.
The image down the right-hand side of the page shows the relative distance of each planet from the Sun. The size
of the planets and Sun is not to scale, because if it was most of them would be too small to see. The table below lists the
equatorial diameter and average distance from the Sun of each planet:
| Planet |
Eq. Diameter
(km) |
Av. Distance
(million km) |
No. of
Moons |
| Mercury |
4879 |
57.9 |
0 |
| Venus |
12,104 |
108.2 |
0 |
| Earth |
12,756 |
149.6 |
1 |
| Mars |
6794 |
227.9 |
2 |
| Jupiter |
142,984 |
778.6 |
63 |
| Saturn |
120,536 |
1433.5 |
56 |
| Uranus |
51,118 |
2872.5 |
27 |
| Neptune |
49,528 |
4495.1 |
13 |
| Pluto |
2390 |
5906.4 |
3 |
I've given the diameter at the equator because the planets are not perfectly spherical, especially Jupiter, Saturn,
Uranus and Neptune, which are the "gas giant" planets. They do not have a solid surface like the Earth, but are huge
balls of gas that get steadily more dense towards the centre, so there is no clear distinction between atmosphere and
surface. Consequently, as they rotate they flatten slightly, and have a significant bulge around the equator.
Also, I've given average distance from the Sun because the planets move around it in elliptical orbits rather than
circular ones. Pluto's orbit is so elliptical that at times it is closer to the Sun than Neptune.
The solar system also contains comets and asteroids, which are small irregular lumps of rock,
and the solar wind, which is a stream of electrically charged particles flowing out from the surface of the Sun. It
isn't known how far this actually extends, but it certainly goes far beyond the orbit of Pluto.
Most of the planets can be found with the naked eye or a pair of binoculars, although you will generally need a telescope to
see any detail. The page for each planet specifies what can be seen in each case. Any of the following astronomy magazines
will give details of where they are in the sky at the moment:
NOTE: Most of the images of the planets and other astronomical objects used in this section come from various NASA resources,
and are used in accordance with their guidelines. Some images are from ESA sources.
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