There are 8 planets in our solar system, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. With the exception of Neptune and Uranus the other 6 planets can be seen unaided and all 8 are visible with a small telescope or binoculars.
What is a Planet?
Planets are among the many worlds and smaller objects that orbit the Sun. The formal definition of planet, as voted on by the International Astronomical Union in 2006, is as follows:
A planet is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
Under this definition, Pluto is NOT a planet, but has been deemed a dwarf planet because it has not yet cleared its orbit. This definition is under discussion, particularly by members of the planetary science community, and it may yet be further refined.
Order of the Planets
Terrestrial Planets (Inner Planets)
The Terrestrial, or “rocky” planets in our solar system orbit relatively close to the Sun. There are four terrestrial planets: Mercury, Venus, Earth and Mars. They are made mostly of silicate rocks and metals, have solid surfaces, with atmospheres that range from very thick (on Venus) to very thin (on Mercury).
Of the four terrestrial planets, Earth and Mars are considered the most hospitable to life. Earth, of course, has life. Mars may have had life in the past, and it may exist there today. Conditions on Venus and Mercury are too extreme to have habitats that could nurture life.
Gas Giants (Outer Planets)
Gas giants are large planets that contain more than 10 times the mass of Earth. Their compositions are mostly gases, such as hydrogen, and small amounts of rocky material (mostly at their cores). The four gas giants in our solar system are Jupiter, Saturn, Uranus, and Neptune.
An ice giant planet is one that is at least ten times the mass of Earth, and contains a higher percentage of what planetary scientists refer to as “ices”. These are volatile elements heavier than hydrogen and helium, and were in ice form (mainly water) when the planets first formed. In our solar system, Uranus and Neptune are often referred to as “ice giants” due to the higher amounts of such volatile they contain. Astronomers have also determined that some exoplanets could be ice giants, as well.
A ring system around a planet or asteroid is a disk made up of dust, chunks of material (ice, in the outer solar system), and small moons. This material forms a ring (or rings) around its parent body. The largest ring system in the solar system is the one around Saturn. Jupiter, Uranus and Neptune also have rings, and at least one asteroid is known to have a small ring as well.
Facts about the Planets
Click on any planet below to find out more about it.