Into the unknown

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If we had tried to include Pluto in our scale model of the Solar System, we would have had a problem. Unlike the others, it has a quite eccentric orbit, so it spends some of its time actually closer to the Sun than Neptune. At other times in its 248-year orbit of the Sun, it is much farther out. Currently, it is about where this board is compared with Neptune, but over the years we'd have to keep on moving the board farther away, till in 2114 it would be on the edge of Northwood Hills.

For years before the decision to change its status in 2006, astronomers realised that if they allowed Pluto to be a planet, there would have to be many planets, as more similar-sized bodies were being discovered. It's actually quite tricky to decide what is a planet and what is a lesser body, and even now there are people who disagree with the decision. To be a full planet, a body has to orbit the Sun, and to be big enough that its gravity pulls it into a spherical shape, rather than looking like a potato or a peanut as many of the smaller bodies do. It also has to be big enough to dominate its orbit, which means that there can't be other bodies in nearby orbits.

It's the last bit that has caused so much bother. There's plenty of space debris around that crosses the orbit of even Jupiter, so you could argue that Jupiter isn't a planet. But a lot of this is mathematical juggling with figures, and Pluto is still a planet, although a dwarf one. The same definition has upgraded Ceres from being a minor planet to a dwarf planet, so fans of Ceres are happy.

The same sort of argument extends to stars. When is a star a star? There are some objects called brown dwarfs, that produce their own light,  more like a star than a planet, but without having the same level of nuclear reactions inside. In 2014, a brown dwarf was discovered quite close to the Sun, making it the fourth closest star system beyond the Sun. There could be others, though they would be quite faint. There could even be free-floating planets, not attached to any star.

That's why this board is called Into the Unknown. We don't really know what's out there, and if there are any big discoveries we'll have to change this commentary. But we don't think we'll discover any aliens lurking around the outer fringes of the Solar System and if you see any flying saucers here they will probably be frisbees!

Photo: Pluto and its large moon Charon, photographed by the New Horizons probe in 2015.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute