Sunday Space | The long-time mystery of why ice-giant Uranus is on its side

Uranus also has a lot of moons - 27 in fact. It is also the butt of all space jokes.

But more than eliciting giggles at its name (usually mispronounced, by the way), Uranus is strange. On Earth and all the other planets (as well as Pluto) in our solar system, the axis of rotation of the planet - the imaginary line our and other objects spin around in a day, is mostly perpendicular to its orbit around the sun.

Our solar system is a giant disc, and we call the imaginary line of that disc the ecliptic. All the planets lie on it and go around the sun. The tilt is the angle between how a planet spins (its rotational axis) with respect to how we go around the sun (its orbital axis). If we spin essentially straight up and down with respect to how we go around the sun, we'd have a tilt of zero degrees.

In our solar system, the planets all have different tilts. Mercury has no tilt and Jupiter has a tiny one of only three degrees. The Earth is tilted 23.44 degrees. However, the Earth's tilt does wobble a bit so it can range from 22.1 to 24.5 degrees. Mars' tilt is about 25 degrees, making it very similar to that of Earth. If an object spins in the same direction it goes around the Sun, the tilt is around 90 degrees, like it is on its side.

And this is what makes Uranus strange.

Uranus has a tilt of almost 98 degrees, meaning it is like a ball on its side rolling around. Why it does this has been a big mystery.

Some theories and models suggested maybe it got knocked - something large crashed into it and knocked it over. Or, instead of one large collision, it was a bunch of smaller objects crashing into Uranus.

However, if this is the case, it doesn't explain why in most ways, it is very similar to Neptune. Except its tilt, Uranus and Neptune are almost twins. Their atmospheres are nearly the same - which is why we call them ice giants. Their sizes are also very similar.

If things crashed into Uranus and not Neptune, they should have more differences - but that's not the case.

Now, a new model has suggested that its moons may have migrated away from the planet. In doing so, they caused Uranus to tilt, like pulling or pushing it over.

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This is exciting as we know our Moon is moving away from Earth - at about 4cm per year. In doing so, it causes changes to Earth like the tides and even the spin of our day - it is slowly getting longer.

Looking at Jupiter (it has a slight tilt) and Saturn (a tilt similar to Earth's), it appears they also are potentially affected by something similar. Maybe moons migrating away is common.

The clue to the strangeness of Uranus might just be because its moons needed some space and to get away from Uranus.