PV Gap

Michele Said:

Compared to all the other planets in the solar system, how is the Pluto-Charon system unique?

We Answered:

A is accurate.

Pluto actually as three satellites. Charon, Nix and Hydra. Pluto and Charon form a double, dwarf planet pair rotating about a center of gravity near a point on the surface of Pluto. They are locked in an odd dance as they orbit the Sun dragging Nix and Hydra along for the ride.

Jessie Said:

If you were to consider Pluto a planet, would there be 14 planets in the solar system?

We Answered:

Actually, it might be 17:

if Pluto were to be considered a planet, then it would be a "double-planet" as the barycentre of the system Pluto-Charon is outside Pluto's body.

In the case of the Earth-Moon system, the barycentre is well inside Earth, so that the Earth-Moon system is not considered a double-planet, even though the Moon is far enough from the Earth to be more in orbit around the Sun than around Earth (just do the math: F = G * M * m / d^2 and you will see that the gravitational force of the Sun on the Moon is greater than that of Earth).

However, if you want to bring Ceres into the "fold", then you have to consider bringing in the other three bodies who have lost their original planet status when Ceres lost it (in the 1860s):

Pallas, Juno and Vesta.

Let's remember that they lost their planet status essentially for the same reason that Pluto lost it. So, if you bring Pluto back along with the dwarf planet Ceres, then you must also bring these back.

So the complete list would be:

Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon*, Eris, Haumea, Makemake.

A total of 17. Plus the one or two left to be discovered in the Oort cloud (according to a statistical survey done by Japanese observers in the mid-1990s and an analysis of comet-orbits published in 1999).

* If Pluto is made a planet, then it will be difficult to avoid making Charon a planet as well, given that they form a bound pair with the barycentre in open space between them. The question would become: is the pair counted as one planet? If so, what do you call the "planet"? "Pluto-Charon"?

Other objects that have lost their "planet" status are:
the four satellites of Jupiter discovered by Galileo;
the Sun;
the Moon.

They all lost their planet-status around 1687, when Newton mathematically proved that the Sun was at the centre of the solar system; that is when astronomers decided to call objects going around the Sun "planets" and objects going around planets were given the new name "satellites".

Wilma Said:

planets in our solar system and pluto not being a planet anymore?

We Answered:

There have been no new planets discovered in the solar system. We have discovered a large number of objects - Dwarf Planets or as I prefer Trans-Neptunian objects, that look very much like pluto, which os one of the reason pluto isn't classed as a planet any more.

Georgia Said:

What is the new 9th planet in the solar system when Pluto was replaced?

We Answered:

Pluto wasn't replaced. It was simply classified as a PLANETOID rather than a planet. The reason being is that scientists have discovered several other similar sized objects even further out than Pluto in the Kuiper Belt. That is an area at the edge of our solar system thats littered with space debris like an asteroid belt.

Personally I believe that Pluto should remain a planet since its orbit is entirely within the Kuiper Belt.

Marcia Said:

Why is pluto not a planet in the solar system anymore?

We Answered:

To be considered a planet under current definitions the orb must clear its orbital path of other objects. Pluto resides within the Kuiper belt. This belt contains many objects besides Pluto. Therefore, the orb called Pluto is not defined as a planet.