To be or not to be, is that a planet I see before me?

“So how much further is there to go?”

We’re almost there, only 4 billion miles to go.

“Another 4 billion miles?”

But the good news is that it will only take us 7 hours.

“That’s not so bad then.”

Just long enough for a good look at these planets. Where we are now is just at the edge of the solar system. This very cold and very dark place is a bit like the Oort cloud in that it was invented before it was seen. Just a year after Jan Oort named his cloud in 1951, Gerard Kuiper proposed there should be lots of small planets or bits of rubble on the edge of the solar system.

“This is a different lot of rubble to the Oort cloud?”

This is much closer to the sun than the Oort cloud. The other difference is that no-one, well no human, has ever seen the Oort cloud but just a few years ago in 1992 the first object was spotted with powerful telescopes in just the place that the Kuiper belt was meant to be.

“What was it called?”

1992QB1.

“That’s a bit of a boring name isn’t it?”

Things in space seem to get either strange names or boring names made up of letters and numbers. Astronomers think there are up 70,000 more of these things flying around out here and a few have been given proper but odd names, like Quaoar (which has a boring name - 2002 LM60) and Varuna. Sedna is the most remote object in the solar system ever seen by telescope and seems to be going around the sun between the Kuiper belt and Oort cloud, almost 4 billion miles from the Sun.

“So are these objects really planets?”

No they’re too tiny, much smaller than the earth’s moon so they aren’t proper planets. The big ones are sometimes called planetoids, the medium to small ones planetesimals and the tiny ones asteroids. The first proper planet in the solar system is Pluto, though some people starting saying that it was too small to be a planet as it is only two third’s the size of the earths moon.

“So how can you tell the difference between a moon and a planet?”

Moons are little planets but they are in orbit around a proper planet rather than around the sun.

“So even though Pluto is smaller than the Earth’s moon, Pluto is a planet and the moon is just a moon?”

Well Pluto’s been called a planet since it was first discovered in 1930 but that has just changed. Astronomers have come up with a rather complicated way of deciding if a planet really is a planet. In 2006 the International Astronomical Union decided that to be called a 'planet' you have to go around the Sun, being round (or nearly round) and have 'cleared the neighbourhood' around your orbit. They decided poor Pluto has met only two of these conditions. As it crosses the orbit of the next planet Neptune it hasn’t cleared its neighbourhood, so it is now officially a dwarf planet.

“And Neptune is a planet even though Pluto crosses its orbit? That’s not fair.”

I totally agree. It isn’t fair at all, but I don’t think we heard the end of this planet story just yet. I’m sure the International Astronomical Union might change their minds again. In the mean time Pluto has been given a number 134340.

“What if a dwarf planet had a moon going around it, what would that be called?”

Well they still can still call that a moon because no one has defined that yet but technically both would be dwarf planets. That’s just what Pluto has. Look there’s Pluto over there.

“There’s two of them or am I seeing double?”

The bigger one’s Pluto and the smaller one is Charon its moon which isn’t much smaller than Pluto. There are two other much smaller moons too, Hydra and Nix. We've got a much better view than they get from earth. Even the Hubble space telescope pictures of Pluto are fuzzy, but you can still see Charon. A space probe was launched in 2006, the New Horizons probe, but that won't reach Pluto until 2015.

“Nine years just to reach Pluto?”

It is a still long way away, on average 4 billion miles from the sun or about 5 light hours. Plutelings, if they existed, would have a slightly different view of the sun to earthlings. Firstly at minus 213oC in broad daylight sunbathing takes on a different complexion. Also the sun looks about 40 times bigger on earth than on Pluto, so rather than a nice golden disc the sun will look more like a bright point of light. The size of the sun as seen on Pluto actually changes during the Pluton year, about 248 earth years, almost doubling in size in what I suppose you would call the Pluton summer.

“How can the sun get bigger?”

The sun doesn't but Pluto has a very eccentric orbit so that it changes from being 2.8 billion miles from the sun to a very chilly 4.6 billion miles. A sun 2.8 billion miles away looks bigger than the same sun 4.6 billion miles away. This eccentricity helped it to sneak inside the orbit of Neptune in 1979, so for a few years it wasn’t the most distant planet. This was a short lived change in the solar system’s natural order; after February 1999 it was back to being the most distant planet for another 220 years or so.

“Not quite.”

Why?

“Well, you just said in 2006 Pluto became a dwarf planet so Neptune will always be the most distant planet from now on.”

You really are getting the hang of this space stuff you know.