Sedna (split off from wormwood=sedna thread)

[Hyndis]

Sedna has an extremely eliptical orbit that ranges something from 100-900 AU's distant from the sun.

This means that it isn't going to come anywhere near us, and infact, Sedna's orbit would make it seem as if it was just a comet of unusual size. :cool:

Also, its believed that it has a moon, as it rotates slower than expected, and its almost as red as Mars, and rather reflective. This means that its most likely make out of a lot of ice, which is what you'd expect for something so cold.

Nevermind that it is impossible to see this object without some really good telescopes, so that this object could not have been seen during ancient times.

[Shake]

Actually, since it's even smaller than Pluto (which, btw, some people were trying have removed from the list of planets), and is not even the first relatively large sized object found outside of Pluto's orbit, I doubt it will become known as the 10th planet.

Here is (http://www.space.com/scienceastronomy/new_object_040315.html) Space.com's article about Sedna. Furthermore, one could actually make a case (http://www.space.com/scienceastronomy/planet_denitions_030227.html) for Sedna being the 12th planet!

Sam, you seem to have forgotten about Quaoar (http://www.space.com/scienceastronomy/quaoar_discovery_021007.html).

Here's something about trans-Neptunian objects in the Kuiper Belt:
http://www.space.com/images/h_pluto_kbos_010523_02.jpg
... from an article about Varuna.

[Jesse]

split from this (http://www.iidb.org/vbb/showthread.php?s=&threadid=80034) thread.

[Shadowy Man]

I, personally, do not consider Sedna a planet. Then again, I don't consider Pluto a planet either.

They are both members (though probably the largest) of a distribution of objects.

I suggest the following definition for planet:

An object of significant mass (i.e. self-gravity large enough to cause it to be spherical or close to it) that occupies a unique orbit (i.e. no other major body's orbit crosses it).

A planet is not part of a distribution of objects that share similar orbits. That is why none of the objects in the asteroid belt would be considered planets. And also why Mercury, despite its small size, is still a planet.

What do you think?

[Hyndis]

Originally posted by Shadowy Man
I, personally, do not consider Sedna a planet. Then again, I don't consider Pluto a planet either.

They are both members (though probably the largest) of a distribution of objects.

I suggest the following definition for planet:

An object of significant mass (i.e. self-gravity large enough to cause it to be spherical or close to it) that occupies a unique orbit (i.e. no other major body's orbit crosses it).

A planet is not part of a distribution of objects that share similar orbits. That is why none of the objects in the asteroid belt would be considered planets. And also why Mercury, despite its small size, is still a planet.

What do you think?


What about Ceres?

http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html

Its the first item listed on the table, and its got a very circular orbit, and the asteroid itself is nice and sphereical. Of course many other objects share a very similar orbit...


And then there are some other near Earth objects with a very low orbital eccentricity, which means their orbits are roughly circular as well. I point you to here:

http://nssdc.gsfc.nasa.gov/planetary/factsheet/neofact.html

Scanning over this, I notice several asteroids with an eccentricity very close to zero, such as 0.065, and several entries of 0.074. That means that these asteroids have almost the exact same orbit as the Earth, and are very close to 1 AU.

How about an asteroid with an orbital eccentricity of 0.074 and a distance from the sun of 1.06? (asteroid 2001 GP2)

[lpetrich]

I propose this name for the next big trans-Neptunian object discovered:

Hörbiger (or Hoerbiger)

after Hanns Hörbiger, author of the crackpot Cosmic Ice Theory. Although it is riddled with errors, it is quite correct about how the outer Solar System is full of "ice blocks".

But I'm not sure if anyone wishes to honor a stopped clock just because it is right twice a day.

[Shadowy Man]

Ceres is obviously part of the distribution of objects that are the asteroid belt.

And the Earth is clearly not part of the distribution of asteroids with near-circular orbits in the 1AU range, right?

[Hyndis]

Originally posted by Shadowy Man
Ceres is obviously part of the distribution of objects that are the asteroid belt.

And the Earth is clearly not part of the distribution of asteroids with near-circular orbits in the 1AU range, right?

But there are many asteroids with near circular orbits with a orbital distance of about 1 AU, some of them probably big enough to be round due to gravity.

And there are also many round objects in a near circular orbit, except that there are also smaller and more irregularly shaped objects.

Jupiter has a pack of asteroids both preceeding and trailing the planet's orbit. The asteroids are pushed and pulled along due to gravity from Jupiter, although they don't orbit around the planet. They have the same orbit as Jupiter...does that mean Jupiter isn't a planet, as it shares its orbit with many other objects?

The problem is that there is no clear cut-off point anywhere from the scale of 1 atom to when the collection of atoms is big enough that fusion starts in the core. Its all shades of grey.

[Asha'man]

Originally posted by Shadowy Man
An object of significant mass (i.e. self-gravity large enough to cause it to be spherical or close to it) that occupies a unique orbit (i.e. no other major body's orbit crosses it).

You need to add an upper limit on the mass: small enough that the object never experienced hydrogen fusion during it's lifetime.

Otherwise you get to include small stellar companions.

The unique orbit constraint may cause a problem. What happens if a jupiter-sized object is thrown out of a solar system, and is essentially a solo object in galactic orbit? Does a planet really need to be orbiting a star?

[Shadowy Man]

Originally posted by Hyndis
But there are many asteroids with near circular orbits with a orbital distance of about 1 AU, some of them probably big enough to be round due to gravity.


Yes, and the fact that there are many show that they are part of a distribution of objects and thus not planets. Earth is not part of that distribution.

Jupiter has a pack of asteroids both preceeding and trailing the planet's orbit. The asteroids are pushed and pulled along due to gravity from Jupiter, although they don't orbit around the planet. They have the same orbit as Jupiter...does that mean Jupiter isn't a planet, as it shares its orbit with many other objects?

No, because Jupiter is not part of that distribution of objects.

The problem is that there is no clear cut-off point anywhere from the scale of 1 atom to when the collection of atoms is big enough that fusion starts in the core. Its all shades of grey.

You are correct, and because of that the name "planet" is purely a semantic distinction and not a physical one. I was just trying to provide a definition that results in less ambiguity than the current one.

[Shadowy Man]

Originally posted by Asha'man
You need to add an upper limit on the mass: small enough that the object never experienced hydrogen fusion during it's lifetime.

Otherwise you get to include small stellar companions.

You are correct.

The unique orbit constraint may cause a problem. What happens if a jupiter-sized object is thrown out of a solar system, and is essentially a solo object in galactic orbit? Does a planet really need to be orbiting a star?

Hadn't really thought about this. Certainly I was thinking about planets in stellar systems.

[Hyndis]

Originally posted by Shadowy Man
Yes, and the fact that there are many show that they are part of a distribution of objects and thus not planets. Earth is not part of that distribution.



No, because Jupiter is not part of that distribution of objects.


But all of those asteroids with a nearly circular orbit and a 1 AU distance from the sun have almost exactly the same orbit as Earth, and thus, Earth is merely one of many such objects in the same orbit. Same thing goes with Jupiter. Many other objects orbit the sun in almost the exact same orbit, in stable orbits. And some asteroids even have moons of their own!:D

Gotta love playing devil's advocate. ;)

[Shadowy Man]

Originally posted by Hyndis
Earth is merely one of many such objects in the same orbit.

True, but its mass is at least six orders of magnitude greater than any of the other objects in that orbit. It clearly is not part of that distribution of objects.

It's like calling a lone redwood in a field of grass just another blade of grass.

[Rymmie1981]

The Earth-Moon system is just that, a system. The Earth and the Moon orbit each other as do all objects in a gravitational system. Since the Earth is larger than the Moon, the Moon moves much faster with the same amount of force and falls into an orbit. The Earth also feels some of the pull, which, together with the Sun, causes tides. The point being that even though the asteroids at 1AU with near circular orbits are not significantly part of a collection of objects that affect the Earth, the Moon is one of two objects in the Earth-Moon system.

Two objects are a collection of objects. If they are not, how many objects are required to form a "collection"? What does the ratio of mass need to be in order for an object to be considered to be in a particular distribution? Does distance between objects play a role in determining the presence of a collection? Since Jupiter is several orders of magnitude more massive than its moons, are they still part of the same collection or distribution?

Just a point and some questions arising from what has been presented thus far.

It's like calling a lone redwood in a field of grass just another blade of grass.

But, they are both plants, aren't they?

[Shadowy Man]

Originally posted by Rymmie1981
The point being that even though the asteroids at 1AU with near circular orbits are not significantly part of a collection of objects that affect the Earth, the Moon is one of two objects in the Earth-Moon system.


Well, it depends on whether or not you want to call the Earth-Moon system a double planet or not. I don't know if anyone has specified a mass ratio that qualifies for a "double planet".

Two objects are a collection of objects. If they are not, how many objects are required to form a "collection"? What does the ratio of mass need to be in order for an object to be considered to be in a particular distribution? Does distance between objects play a role in determining the presence of a collection?

I am not saying that my definition is perfect. Again, we are just arguing semantics, so I was trying to set up a definition that at least helps a little bit. We don't have to call anything a planet at all; everything can just be a body orbiting the sun.

Since Jupiter is several orders of magnitude more massive than its moons, are they still part of the same collection or distribution?

The moons of Jupiter are a collection of objects that orbit the planet Jupiter.


But, they are both plants, aren't they?

Yes, and both the Earth and an asteroid are made up of rock, but that doesn't make Earth an asteroid.

[Rymmie1981]

OK, we really are arguing semantics, but precision and clarity help when defining these sorts of things. While it is true that the asteroids in the same orbit as Earth do not have a significant impact on its rotational or orbital velocity relative to the Sun, they still interact with Earth gravitationally. In fact, everything in the visible universe interacts with Earth gravitationally. One could make the arguement that the entire universe is a collection of interacting objects. The objects with a small mass may not make much difference individually, but combined they can produce significant effects.

Continuing...Though the planet and the asteroid are different in many ways that require classification into different categories of object, they are similar in the same ways that the redwood and the blade of grass are similar. The redwood has all of the elements that are extant in the blade of grass from the way the nutrients are delivered to their roots to the way they both produce food. The redwood has many things that the blade of grass does not. Bark, branches, and many leaves come to mind. The Earth has a magnetic field. The asteroid does not. Earth has an atmosphere. The asteroid does not. And so on....

Both are objects in space that orbit the same major star at the same distance. Both are made out of the same basic materials. In fact, the two objects are so similar that several of the smaller can be used to build the bigger, and the bigger can be broken down into several of the smaller. The redwood can be cut down and used as compost to fertilize a lawn, and grass clippings can be used to fertilize the redwood's ground.

See? The analogy works. The planet is not an asteroid. The redwood is not grass. But, they are as much the same as they are different.

[Shadowy Man]

Originally posted by Rymmie1981
See? The analogy works. The planet is not an asteroid. The redwood is not grass. But, they are as much the same as they are different.

I never disagreed with this.

But we still have a different classification for the "planet" Earth and the asteroids. And it is a fair distinction. The edges of the distinction may be fuzzy, but they are still there.

[Hyndis]

From dictionary.com:


1.A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
2. One of the seven celestial bodies, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn, visible to the naked eye and thought by ancient astronomers to revolve in the heavens about a fixed Earth and among fixed stars.
3. One of the seven revolving astrological celestial bodies that in conjunction with the stars are believed to influence human affairs and personalities.


They demoted Pluto in this definition. :(

But that also leads to the issue of what exactly an asteroid or comet is, and thus, in the interest of arguing semantics, I'll pull up that definition.


1. Astronomy. Any of numerous small celestial bodies that revolve around the sun, with orbits lying chiefly between Mars and Jupiter and characteristic diameters between a few and several hundred kilometers. Also called minor planet, planetoid.
2. Zoology. See starfish.


Wait a minute...Pluto has a diamter much larger than several hundred kilometers. :confused: Infact, so do many asteroids! So that means that Pluto is a planet...even though the previous definition left Pluto out.

Gotta love arguing over fuzzy definitions. :D

[Shadowy Man]

Originally posted by Hyndis
So that means that Pluto is a planet...even though the previous definition left Pluto out.

It depends on whether you really consider dictionary.com to be authoritative on the definition of scientific terms. Many times I have used words whose definitions aren't even on dictionary.com!

[Hyndis]

Originally posted by Shadowy Man
It depends on whether you really consider dictionary.com to be authoritative on the definition of scientific terms. Many times I have used words whose definitions aren't even on dictionary.com!

But the problem is that there isn't really objective way to classify an object as a planet or asteroid, depending on the various criteria on the list.

And as new and interesting things are always being discovered, it would probably be quite hard to finally pin down a definition onto what is or is not a planet.


Drop an icecube from your favorite drink into the ocean. Is it an iceberg?
Drop a piece of ice measuring about 20 cubic miles of volume into the ocean. Is it an iceberg?

An iceberg is a piece of ice floating in the ocean, but what size? When does a piece of ice from your freezer cross the line into being a full fledged iceberg, rather than just a piece of ice in the water? Or, is every bit of ice in the ocean an iceberg?

[Shadowy Man]

Originally posted by Hyndis
But the problem is that there isn't really objective way to classify an object as a planet or asteroid, depending on the various criteria on the list.

Yes, there is no perfect definition. However, I do believe that I have given a definition that does allow us to classify the objects in our solar system well and any future objects discovered in our solar system.

All of the planets have been discovered already, so it makes our life simple.

Now classifying other stellar systems will be a little trickier.