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
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
- What is the physical difference between a star and a planet?
- Are Kuiper Belt Objects asteroids? Are large Kuiper Belt Objects planets?
- Is Pluto a planet?
- Is there really a 10th planet?
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist
This page has been accessed 79135 times since September 11, 2002.
Last modified: August 28, 2006 11:46:52 AM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)