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View Full Version : What is a planet?



Haglund
2003-Oct-21, 06:04 PM
On this page (http://nssdc.gsfc.nasa.gov/planetary/planetaryfaq.html#Pluto) NASA set up three general rules for what a planet is supposed to be:
Must orbit a star
It is small enough not to have nuclear fusion reactions
Massive enough to maintain a roughly spherical shape

They also say that by those rules, Ceres might be considered a planet as well. My thoughts: At the same time, Ceres belongs to the asteroids by classification. What is interesting is that there is the Kuiper belt past Neptune, stretching between 30-100 AU from the sun, and Pluto is 39 AU from the sun. By its characteristics it could very well be a very large Kuiper object. IAU (http://www.iau.org/IAU/FAQ/PlutoPR.html) have decided that Pluto is a planet still. Since I suspect this is much because of tradition, I'm guessing it's difficult to draw any conclusions on what would make a planet based on that, because in Pluto's case it would then be about size while in Cere's case it would be other characteristics.

What do you other think about this?

Duane
2003-Oct-22, 07:24 PM
I think there is a classification for so called "rogue" planets as well--planets which are roaming free after having been flung out of a solor systems as a result of gravitational interactions with other planets.

Haglund
2003-Oct-24, 10:09 AM
A rogue planet would have the same definition as a planet, with one obvious exception :-)

Matthew
2003-Oct-25, 06:50 AM
A rogue planet would not fit the criterea of a planet. It wouldn't be orbiting a star.

Haglund
2003-Oct-25, 08:05 AM
No, but a rogue planet would be an object that once was a planet but no longer is...

Planetwatcher
2003-Oct-25, 10:07 PM
So it techniquely a rogue planet would still be a planet per say, just not a planetary body which would imply orbiting a parent star.

Matthew
2003-Oct-26, 03:51 AM
So it techniquely a rogue planet would still be a planet per say, just not a planetary body which would imply orbiting a parent star.

A planet has to orbit a star. A rogue planet is not a planet. It used to be a planet but is not. A rogue planet could be a MACHO. Like brown dwarfs or black holes. Or it could be a UOSO (Unobiting Spherical Object), or just USO for short.

Haglund
2003-Oct-26, 11:39 AM
Planets are MACHO objects, but are all of them that? M is for massive, and I guess theres a lower limit there too.

Matthew
2003-Oct-27, 10:04 AM
I don't think anyone understands what the M stands for. What is a 'massive' object in space. And how do you define it?

Haglund
2003-Oct-27, 10:47 AM
I guess there is a fuzzy line that's the lower limit.