What is a Planet?
According to the International Astronomical Union in 2006, a planet must orbit a star, have gravity to force it into a spherical shape, and have cleared away other similar-sized objects near its orbit around the Sun. Therefore, a planet is a celestial body that accomplishes those three things.
Three Categories in the Solar System
- A planet is a celestial body that orbits a star, self-gravitates to form a spherical shape, and has cleared surrounding objects around its orbit.
- Whereas a dwarf planet orbits a star, self-gravitates to form a spherical shape, it doesn’t clear the surrounding objects around its orbit and isn’t a satellite.
- Other objects that orbit the Sun are known as “Small Solar System Bodies.” This excludes satellites.
Primary Planets
- Mercury – smallest planet and closest to the sun
- Earth – the only planet with liquid water on the surface
- Venus – spins slowly in the opposite direction from most planets
- Mars – dusty and cold with a thin atmosphere
- Jupiter – more than twice as massive as the other planets
- Saturn – has a complex system of icy rings
- Uranus – seventh planet from the Sun and almost rotates at a 90-degree angle from the plane of its orbit
- Neptune – the most distant planet that is cold, dark, and has supersonic winds.
Dwarfs
- Pluto – used to be the ninth planet and has ice mountains and frozen plains
- Ceres – heavily cratered with large amounts of underground ice
- Makemake – takes 310 Earth years to complete one orbit around the Sun
- Haumea – one of the fastest rotating large objects in the solar system
- Eris – one orbit takes about 557 years and is close to the size of Pluto
Hypothetical
Planet X is a hypothetical planet discovered by Caltech researchers. They found mathematical evidence supporting the existence of a Neptune-sized planet orbiting the Sun far beyond Pluto.
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