2 February 2006

## No… We have 8 planets, not 9!

Posted by Dan Satterfield

I did a blog awhile back on the 8 planets in the solar system. That’s right 8 planets. Not the 9 that your grammar school and high school text books say. Well today my view of the solar system got another boost.

Seems that astronomers have detected another big object orbiting the sun. It’s official name is UB313. The discoverer wants to name it the planet Xena. Before you think this is just a large asteroid or something, let me explain that it is bigger than Pluto. UB33 is almost as big as Earth’s moon. About 3,000km across.

Many scientist believe that this is not a planet though. Anymore than Pluto is. (see my blog on Pluto)

When Pluto was discovered, our knowledge of the solar system was very poor compared to now. We now have a much better idea of how the planets formed some 5 Billion years ago. In addition we have the ability to see things far away that we could not back in the early part of the 20th century.

Suffice it to say, there will be a bunch more objects like UB313 discovered. There are literally thousands of them in what is called the Kuiper belt (Pronounced KIE-PER). They are really just dirty snowballs of left over material from when the solar system was formed.

This alone makes them of huge interest to many different sciences. NASA recently launched a probe to Pluto and the Kuiper belt that will get there in about 10 years. (I told you it was far out)

So how far away is UB313? A lot farther than Pluto. Astronomers measure distances in different ways but Miles or Kilometers are not usually one of them! A good measurement for space is the Astronomical Unit. 1AU is the distance from the Earth to the Sun.

1 AU = 1 Astronomical Unit = 149,597,871 kilometers

Now Pluto is about 35 AU from the Sun. (by definition the Earth is 1AU)

UB313 is 96 AU away!
(That is over 12 hours at light speed Captain Kirk! I canna handle eet- err I digress- the Geek in me is showing)

The Solar system has 4 rocks close into the Sun.
Mercury, Venus, Earth and Mars.
It has 4 gas giants farther out.
Jupiter Saturn Uranus and Neptune

It has a bunch of dirty little snowball objects left over that are orbiting in the Kuiper belt. Some of which get pushed out of their orbits and fly past the sun as comets every now and then.

That makes 8 planets.

Disagree?

Well, you are in good company! There are a lot of scientists who argue that they are planets. The problem is this: What IS the definition of a planet?

If you are a science teacher, I would suggest you put this to your class for discussion. Tell them to quit reading for awhile and do some real science! Anyone can do science- you don’t have to be grown up and you don’t have to have a college degree!

Later,
Dan