Pluto and the Kuiper Belt


More than forty times further from the Sun than the Earth are the beginnings of the outer reaches of our Solar System. Out here is a cold and lonely place. The first things we encounter as we move past Neptune and out into this frontier are the Kuiper Belt objects. The most famous of these is the dwarf planet called Pluto. Pluto is a small rocky world, with a biggish moon called Charon and two smaller moons called Nix and Hydra. Pluto is less than a fifth the size of the Earth, and this far from the Sun is an incredibly cold -380F on its surface. The outer layers of Pluto are frozen nitrogen and underneath that probably frozen water. A thin atmosphere hovers over Pluto, made of nitrogen, methane and carbon monoxide – but perhaps only during summer on Pluto, which comes every 248 years, the length of time it takes to go around the Sun. Unlike the major planets, Pluto’s orbit around the Sun is not very circular, instead it is a great ellipse. This means that it loops in closer and further away from the Sun, instead of going around it in a perfect circle. Sometimes it actually crosses where Neptune orbits and is then closer to the Sun than Neptune.

After Pluto was discovered in 1930 it became known as the 9th planet of the Solar System. Starting in 2002 though, astronomers began to discover other objects like Pluto, far out in the Solar System. One of them, called Eris, was bigger than Pluto. This made scientists think that they shouldn’t call Pluto the 9th planet anymore, but instead that Pluto, Eris and these other new worlds should all be called minor or dwarf planets. Where these objects are – some are a hundred times further from the Sun than the Earth – is known as the Kuiper Belt, and so all of these new worlds are considered to be Kuiper Belt objects.

Think about it

Where did these Kuiper Belt objects come from? These small, cold worlds formed billions of years ago along with all the other planets in the Solar System, just much further away from the Sun. About 4 billion years ago then the ice giant planet Neptune and hundreds of thousands of these objects were much closer to each other than they are now. Neptune’s strong gravitational pull threw these smaller objects around, and this resulted in Neptune moving further from the Sun and these small worlds being rearranged to their present positions.

Illustration from a computer model of how Neptune and the other giant planets may have rearranged the young Solar System and pushed smaller objects into what is now the Kuiper Belt. On the left are many small planetary objects shown as white dots. The orbits of the big planets are the circles. Neptune is the dark blue circle and may have started out closer to the Sun that Uranus! As time went by (the middle and right hand pictures) Neptune ended up moving further from the Sun, all the little objects got thrown around by the planets, until after tens of millions of years the only ones left were in the Kuiper Belt, orbiting further away from the Sun.



No spacecraft has ever visited Pluto or any of the other Kuiper Belt objects – yet. The NASA mission New Horizons is on its way to Pluto and will arrive in 2015. It will take pictures and make measurements of Pluto and its moons as it shoots past. It will be traveling about 9 miles a second going past Pluto, much too fast to slow down or stop. It may then go on to look at more distant Kuiper Belt objects.

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