What are the requirements for being a planet?
I was always under the impression that for an object to be a planet it had to have a satellite orbiting around it, a moon, that is why Pluto can be called a planet even though it is so small. My question then is, why are Mercury and Venus planets and what are the parameters required for planet status.
Not all planets have moons (you've pointed out that Venus and Mercury do not), and it's not a requirement.
The definition of planet is mostly a historical distinction. Planets must be orbiting the Sun (or another star), and must be "large," whatever that means. Beyond that, there are no parameters -- after all, there are only 9! Historically, things have been called planets, and we stick to that. Some astronomers think Pluto should not be called a planet because it doesn't qualify in their minds as "large" and it has a weird orbit. Really, though, it's just a label.
August 2006 Update by KLM: this month the International Astronomical Union (IAU) voted to update the definition of what makes a planet. According to their decision a planet must satisfy the following three criteria:
- It must be an object which independently orbits the Sun
- It must have enough mass so that gravity pulls it into a roughly speroidal shape
- It must be large enough to "dominate" its orbit (ie. its mass must be much larger than anything else which crosses its orbit
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