Thursday, August 17, 2006

IAU planet definition Q&A sheet

Yesterday, I blogged about the proposal by the International Astronomical Union (IAU), to increase the number of planets in our solar system from nine to twelve. The IAU has posted a very informative Questions & Answers sheet on their website to help us understand better what it all means. From IAU:
Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”?

A: No. The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of the Earth.

Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called planets?

A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the “barycentre”) that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be “satellites” rather than planets.

Q: Why is Pluto-Charon a “double planet” and not a “planet with a satellite”?

A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “planet”. The reason they are called a “double planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is “planet-like” and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a “double planet.”
(Thanks phi824)

Previously: And soon, there will be twelve planets

1 comment:

Sailom said...

This proposal is the most sensible one as far as it takes some objective parameters into account: gravity, mass, barycenters between two objects etc.

However, there seems to be quite a lot of criticisms about the satellite/planet distinction.

1. There are much larger satellites than Ceres (I do agree it is a planet under the new definition).

2. This gap in size is much bigger than it was between Mercury and Ganymede.

3. We might discover one day an extrasolar planet with a satellite bigger than the Earth

4. There are stars orbiting other stars in the universe... All these objects are called stars anyway.

5. A planet has been found wandering in interstellar place on its own without orbiting any star at all.

My own conclusion is that the current proposal is indeed a good compromise at present. However, it doesn't follow the full logic of defining an object ONLY in terms of its composition and its characteristics just like we do with stars and asteroids.

In other words, satellites should be called planets as well if they match the definition applied to all the other objects orbiting the sun.

This problem is enough to spread confusion in the future...