Star Gazing

Ken Tapping

A good analogy for the Solar System is several marbles rolling in concentric circles around a plate, with the Sun in the centre.

A better analogy is several spinning tops moving in concentric circles around the plate, with their individual spinning axes a little bit off vertical.

For example, the Earth is spinning with its axis about 23 degrees off vertical. All this fits our theories as to how the Solar System and the planets formed, with one very conspicuous exception, Uranus. That planet is leaning at an angle of 98 degrees, almost lying on its side, so that it is “rolling around on the plate.”

How did that happen?

Uranus is the seventh planet out from the Sun, after Mercury, Venus, Earth, Mars, Jupiter and Saturn. Along with Jupiter, Saturn and Neptune, Uranus planet is a “gas giant,” meaning it consists of a small rocky or icy core surrounded by an extremely thick atmosphere.

Its mean density is about is around 1.27 grams per cubic centimetre, compared with our Earth's 5.5 grams per cubic centimetre. Uranus has to be mostly gas. Being further from the Sun, Uranus and Neptune are significantly colder than Jupiter and Saturn, and because of the relatively high concentrations of methane in their atmospheres, the planets look blue. This sets them apart from Jupiter and Saturn, which appear tan-coloured, and they are often referred to as ice giants, as opposed to gas giants.

Uranus was discovered by William Herschel in 1781, although it turns out others had seen it earlier, but had not taken particular note of it. The planet orbits the Sun at 19.2 times the distance between the Earth and Sun, and takes 84 years to complete a single trip around the Sun. Uranus has a diameter of about 50,000 km, which is about four times the diameter of our world.

A “day” on Uranus is about 17 hours. Since the world is larger than ours, a point on its equator would be moving with the planet’s rotation at a speed of around 9,000 km/h,

compared with about 1600 km/h for a point on our planet’s surface.

Uranus formed around 4.5 billion years ago, along with all the other bodies in the Solar System, so how could it have come to be so different. In addition, what ever did happen to it left it in a nice tidy orbit between Saturn and Neptune, moving in the same plane as the other planets. A widely shared theory is that during its youth, when all the planets were forming, it was involved in a serious collision.

The formation of the planets involved the collisions and sticking together of lots of bits of grit, dust and ice in a collapsing cosmic cloud. However, according to all the computer simulations and observational evidence, the cloud formed into a disc, and then big lumps formed, moving in almost circular paths in the disc, and sweeping up any material that got too close.

However, the big guys generally kept to their lanes and did not get too close to one another. However, it looks as though on at least two occasions in the early youth of the Solar System, big lumps of material did collide. One of the collisions involved an object the size of Mars hitting what would become the Earth.

A great cloud of debris was blasted off, and eventually settled down to form the Moon. This left an unusual situation in the Solar System, where a planet has a Moon a good fraction of the size of the planet it orbits. The other collision was something about the size of Earth hitting Neptune.

In the case of the Earth, we wound up lucky, with our planet still spinning almost perpendicular to its orbit plane, which left us with sensible seasons. Uranus got knocked right over onto its side. The result is really crazy seasons where one end of the planet does not see the Sun at all for decades. It seems that so far our planet has been lucky. In fact, having a nice large moon has been a benefit.

Saturn and Jupiter have sunk into the sunset glow. Venus has disappeared in the dawn glow, leaving us with Mars, which shines brightly in the south after dark. The Moon will be New on the 12th


Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton.