Planet comes from the Greek word for wandering star (the inspiration for the blog name), the name was given to objects with an unusual movement across the night sky compared to the stars, including the Sun and Moon as well as Mercury, Venus, Mars Jupiter, & Saturn. This definition has changed substantially through history as more about the solar system was discovered, new planets found and orbits understood.
The modern definition comes from the International astronomical union defines a planet as:
A celestial body that:
- Is in orbit around the Sun; this excludes the Moon and the Sun itself
- Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; any object which is not large enough for its gravity to cause it to pull it into a sphere
- Has cleared the neighbourhood around its orbit; removing similar sized bodies and many of the smaller bodies from its surroundings
So bodies which don’t orbit the sun can’t be a planet for example. Ganymede and Titan are both larger than Mercury but because one orbits around Jupiter and the other Saturn they are classed as satellites, whereas Mercury which goes round the Sun is a planet. As a side note, Exoplanets cannot be technically called a planet, as they orbit other stars which are not the Sun. Asteroids are not large enough for their own gravity to pull themselves into a ball and so aren’t planets. The third point over cleaning the neighbourhood is the main reason why Pluto is no longer considered a planet, It has not cleared its surrounding orbit in the Keuper belt.
This definition leads to a wide group bodies being included as planets, from large balls of gas to small rocky worlds closer to the sun, and so can be further split down:
- Gas Giants – (Jupiter and Saturn) dominantly made of hydrogen with some helium, but not big enough to undergo fusion like in stars
- Ice Giants – (Neptune and Uranus) Giant planets with less than 20% Hydrogen/Helium with the rest being water, ammonia, and methane
- Terrestrial planet (Mercury, Venus, Earth & Mars) the smallest types of planets which are rocky dominated by silicates and metals
All of these meet the main criteria for planets, however there is a different range of sizes, masses and compositions, the study of these planets, as well as the dwarf planets, moons and other small bodies provide a range of environments to explore, they give an understanding of our neighbourhood and how the solar system and earth came to be as they are.