Hi Lyndsay,

Nice try, but the answer is no, the fact that Pluto has other objects orbiting around it does not necessarily mean it is a planet. Astronomers have found asteroids that have other asteroids orbiting around them, as described in this NASA story from 1999: http://nssdc.gsfc.nasa.go...nia_pr_19991006.html

The other planets that have moons (Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) are all significantly larger than their moons. The radius of Pluto's largest moon Charon, is nearly half the size of Pluto's radius. In contrast, our Moon's radius is only about 1/4 of the Earth's radius.

All of the planets dominate their neighborhoods and have pretty much cleared the path of their orbits of debris like asteroids and comets. In spite of the fact that Pluto appears to have captured several objects in its gravitational pull, Pluto does not appear to have cleaned up all of the debris along its orbit.

Back when Pluto was first discovered and labeled as a planet, we actually knew very little about it. Our knowledge has increased since then and the astronomers and planetary scientists who decided to reclassify Pluto as a dwarf planet based their decision on the currently known properties of Pluto.

The planets in our solar system can be grouped into two categories:

(1) The rocky terrestrial planets - Mercury, Venus, Earth, and Mars

(2) The gaseous Jovian planets - Jupiter, Saturn, Uranus, and Neptune

We don't know a lot about Pluto's composition, but we know Pluto definitely does not fit into the Jovian planet category. Unfortunately, Pluto doesn't really fit into the terrestrial planet category either. Pluto is much smaller than the other terrestrial planets and it appears that Pluto's composition may have more in common with Kuiper belt objects. In fact several Kuiper belt objects that are nearly as large as Pluto have been discovered. Scientists faced a dilemma when these objects were discovered - either we had to start calling all of these objects planets, or Pluto needed to get demoted.

We will know a lot more about Pluto when the New Horizons mission arrives there in a few years.

Kris

Hi Lyndsay,

Yes, that's right, Pluto has 3 moons that we know of: Charon, Nix, and Hydra. I think the problem is that there is an additional requirement for a planet, namely that it has to have 'cleared its neighborhood.' The problem is that Pluto hasn't done that; there are lots of small Kuiper Belt objects near it.

Best,

Christina

Hello Lyndsay,

The problem is that Pluto really isn't the dominating body, at least in the sense of the Earth and the Moon. The main moon of Pluto is Charon which is about half as big as Pluto and a little over a 1/10th the mass. This essentially means that both Pluto and Charon orbit about a point in space lying between them (roughly 1000 km away from pluto and 19,000 km away from Charon (for the Earth the center of mass lies inside the Earth so the Earth exhibits a slight wobble due to the moon). In other words Charon is too big relative to Pluto to be called a moon. That is why Pluto-Charon is now known as a double planet. The additional moons complicate the picture but this is one of the reasons why pluto is no longer regarded as a standard planet. Other reasons include its composition which is more like what are known as Kuiper Bekt Objects and its highly inclined orbit (i.e. it does not lie in the same plane as the rest of the solar system's planets.

To turn your question around you could ask why would you call Charon a moon when it is almost as big as Pluto?

Cheers - David

I am not an expert in this, but I think probably not. The larger-than-Pluto object that was found out near Pluto's orbit also has a moon (the two objects are called http://www.space.com/scie...d_moons_021003.html)

This "dominating body controlling movement" is debated, of course, but I think the essence of the idea is that a planet should be clearly the biggest thing in the general region of its orbit, controlling the things in that region in a way nothing else does. It is basically a way of saying that if there a whole bunch of relatively small object in a general region (like the asteroid belt or out around Pluto) they should be classed differently than planets. I guess that depends a bit on what you condider to be the orbit of an object - how big a region does it have to dominate?

I expect peolple will keep arguing about it for a while!

Terry