By THOMAS STEVENSON.

Until now, the dwarf planet known as Pluto has been barely a speck in the night sky. It was always a place of mystery and wonder, far beyond our reach. School textbooks have often been less flattering, calling it little more than a ball of ice and rock.

Pluto was discovered in 1930 by a chap named Clyde Tombaugh, who was looking for a “Planet X” in the depths of the solar system. The existence of a planet in its vicinity was predicted by earlier astronomers, but this world immediately provided some surprises.

Like Earth and the other planets, Pluto travels around the sun in an elliptical path called an orbit. However, its orbit is tilted relative to the other planets’ paths. It actually goes inside the orbit of Neptune in some areas, but they don’t intersect, so the two objects will never collide.

Now, a NASA probe called New Horizons* has flown past Pluto and its moons, giving us clear images – and plenty more questions.

We already knew that Pluto itself is smaller than our moon, yet it has no less than five of its own. It also shares its part of space with a massive ring of asteroids called the Kuiper Belt, which New Horizons is on its way to explore. Objects in this belt sometimes fly through and clutter Pluto’s orbit, which is the main reason it was ‘demoted’ to a dwarf planet in 2006.

What we’ve just learned is that Pluto is a far more complex and interesting place than anybody expected. It’s so far from the sun that temperatures on its surface never exceed -218°C. That is cold enough to freeze even nitrogen, which is a very light gas making up 79% of Earth’s atmosphere. So people already guessed that Pluto is a bit frosty. What’s new are the features seen in images like these – whole mountains made of water ice!

Some of the mountains are higher than the Southern Alps, and are more like the Canadian Rockies in size. They would undoubtedly look beautiful from the surface, but we might have to wait a few decades for that kind of view.

What’s most interesting is what isn’t being seen. So far, the team at NASA haven’t found a single crater on Pluto, which is very strange indeed. Considering that it moves through the Kuiper Belt, where untold numbers of asteroids hang out, you’d expect something to crash into it. Even on Earth we have craters made by meteorites, but most of them are covered up by geological processes.

If Pluto isn’t showing craters now, it’s very unlikely that it never had any. So it seems that something is happening under the surface, where we used to think there was only inert rock and ice.

If you’re into the physical sciences, I hope the probe’s flyby has given you some things to think about. If you’re not at all sciencey, it’s still worth paying some attention to the event. This month we have all witnessed history being made! Mere Earthlings have finally got their eyes to all the major bodies in the solar system – and you were alive to see the last of these pioneering probes. When you’re old and retired, you can tell your grandchildren about how you felt when you first saw pictures like this. So enjoy knowing a bit more about the universe than Clyde Tombaugh did back in 1930.

In other news, folks at the Large Hadron Collider just discovered a new form of matter based on things called pentaquarks. If you happen to be a textbook author, I suggest you get that new edition started.

* It is very fitting that the New Horizons probe is powered by plutonium, the element named after the very dwarf planet it’s visited.

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