Without evaluating how probable it may be, I was saying that if a planet is influenced by two or more stars, then its orbit (which in general would be exponentially sensitive to little changes) would behave even less predictably than the planets we see in our own solar system. It might get entirely pulled away by the second star, or it might be pulled back and forth around one star so that it never clears out an orbit to claim as its own. I don’t think the irregularity of that kind of orbit would make a planet any less a planet. I think it would just make it a more interesting planet.

To compare the main asteroid belt and the Kuiper Belt for being crowded, keep in mind I was only talking about the number of bodies possibly large enough to be dwarf planets. I was using Mike Brown’s estimate of 300 km as a lower size limit for icy bodies. For comparison, there are 5 bodies in the main asteroid belt with diameters larger than 300 km: Hygeia, Interamnia, Pallas, Ceres, and Vesta. (They are not icy bodies and are probably not dwarf planets other than Ceres, but it also depends on the timescale for relaxing to a hydrostatic state, which hasn’t been defined by the IAU.) At that distance from the sun, these largest main belt bodies are a bit less “crowded” per area of the ecliptic plane than are the five large bodies closer to the sun (Mars, Earth, the Moon, Venus and Mercury), and therefore the Kuiper Belt is far, far less crowded than the main belt in terms of bodies larger than 300 km.

However, I think you are wondering about the overall “crowdedness” including the smaller bodies in both of these belts as well as elsewhere in the inner solar system. I don’t have the numbers at hand, and keep in mind that we can’t see the Kuiper Belt as well as we can see the main asteroid belt so the following estimate is based on a model rather than pure observation. It is estimated that Pluto gets hit by roughly the same rate of small bodies (meteorites, asteroids, icy bodies) as the Earth does. To the same order of magnitude then, their orbits are about equally “cleared” or equally “crowded” in that sense. To get a sense of the densities of the Main Belt versus the Near Earth asteroids, there are less than 1000 Near Earth asteroids that are larger than a kilometer in diameter, but about one thousand times more than this in the main belt. We don’t have similar statistics for the Kuiper Belt due to the difficulty in seeing bodies that small.

This picture will give a sense for how many more there are in the main belt compared to the rest of the inner solar system: http://www.space.com/16105-asteroid-belt.html

Be advised that the pixels used to represent asteroids in this picture are not to-scale; the pixels are vastly larger than the asteroids should actually be in this image. Thus, the asteroid belt looks much more crowded than it really is. You would likely not see a thing if you flew through it, as the distances between objects are so vast. The same is true of the Kuiper Belt. The Kuiper Belt maps that show lots of pixels for the many bodies give the false impression that it is a crowded belt. In reality it is a vast empty space. When comparing these spaces, we are comparing degrees of vast emptiness.

I hope that helps!

Are there similarities between the resonance giving a “kick” to propel other objects in space and the sonar resonance in music?

Your experiment is similar to the enormous brass pendulum swinging in the lobby of the Griffith Observatory. Are the objects in your experiment and the brass pendulum swung due to differing forces?

Many of us thought the Asteroid Belt comprised rocks zooming around in a circle and occasionally colliding like carnival bumper cars. Thanks for the info about the Hilda’s, Kirkwood Gaps and other enlightening info.

Us average schmoe’s had no idea that Jupiter and Neptune are using the Kuiper belt to play soccer. Indeed, most old folks don’t know that there is a Kuiper Belt.

We never think of the inner solar system planetary positions as being crowded. Yet you’ve revealed that the distances between cubewano’s are 100 times greater! How do the distances between the asteroids in the main belt compare to the distances between inner planets?

Mercury’s foreboding fate is really scary. Jupiter is truly as you say, the big bully of the inner solar system.

Detached Objects and Scattered Disc Objects are new and profoundly fascinating topics.

This article will definitely inspire a keen interest in space science to readers of all ages. By explaining that there are more planets contained within our solar system’s heliosphere, curiosity will lead to studying and on to innovation.

Thanks for upgrading the storage files in the grey maze of my marinated brain.

Your devoted fan,

Grandma

What definition of ‘chaotic’ would that be? A planet’s motion could appear to be chaotic because we don’t yet understand all the forces acting on it.

The minimum size rule shouldn’t be entirely arbitrary. It should be as meaningful and as helpful as possible. The IAU definition does include a minimum size rule already, and there is no disagreement over it. It is a good rule. The rule is that a body must be large enough to be rounded by its own gravity. If it is not large enough for this, then the internal forces of rock or metal or ice that make up the body would withstand its gravity and keep the body in a non-round shape. Those are called “small solar system bodies”. (They are also commonly called “minor planets” although that’s not the preferred term any more.) Pluto is far larger than this minimum size limit. Nobody disagrees over this. The only disagreement is over the dynamical requirements. Why can’t a planet include rogue planets that do not orbit a star? I think rogue planets are really planets, and there are probably vast numbers of them. Why can’t a planet be in chaotic dynamics among multiple stars? Why can’t they be in non-colliding but interleaved orbits such as we found in the Kuiper Belt? Why can’t they orbit another planet? All these dynamical considerations should define dynamical subclasses of planets. When we try to use them to define ‘planet’, then we end up with definitions that don’t work for different solar systems and so it ends up being confusing and unhelpful. I can’t think of any way that science is driven forward by introducing a mixture of narrow dynamical rules along with the universally meaningful geophysical rules. I think it would actually be hindering science except that the people who study exoplanets are simply ignoring the dynamical parts of the current definition. That’s a pretty big clue that the definition should be changed by eliminating the dynamical parts and keeping just the geophysical parts (the size limits).

Even large moons don’t pass that test. On the other hand some of them may have once done so before being ‘captured’ by one of today’s planets.

Any minimum size rule for a planet would be arbitrary of course, but unless every lump of matter in the solar system is to qualify as one, a line has to be drawn somewhere.