Planets with peculiar pitches

The oddballs of the Solar System are the planets Venus and Uranus. Venus is the second planet from the Sun and one of the four terrestrial planets – that is, it is composed mainly of rock.

With a surface temperature of 460 degrees Celsius it is the hottest planet and has an extremely dense atmosphere that presses on to its surface, generating enough pressure to crush a submarine.

Venus is similar to the Earth in size, mass and surface gravity, but it has an odd feature unique in the Solar System – its year is longer than its day.

A year on Venus is 225 days – that’s how long it takes it to orbit the Sun once – but its day, the time it takes to spin once on its axis, is 243 days long.

Venus has the slowest rotation on its axis of any planet, of 6.5 kilometres an hour at its equator, compared with the Earth’s 1670kmh.

The reason is not entirely clear. It could be due to gravitational effects from other planets tugging at Venus, along with its proximity to the Sun that introduces tidal locking effects.

An object is tidally locked if the time it takes to spin on its axis is the same as the time it takes to orbit its host star or planet.

Our Moon is tidally locked with the Earth.

It takes the Moon about 27 days to turn once and about 27 days to orbit the Earth.

The result is that we only ever see one face of the Moon, despite the fact that it is rotating on its axis. A similar but not so extreme tidal tug could be restraining Venus’ spin.

The other oddball planet is Uranus.

It is very different from Venus. It has a diameter four times that of the Earth but is a gas giant.

Twenty times further from the Sun than the Earth, a year on Uranus is 84 Earth years, but it spins fast on its axis, making its day just over 17 hours long.

As a result, its equatorial rotation speed is nearly six times faster than the Earth’s. But what makes Uranus an oddball like Venus is its tilt.

Imagine an apple with a knitting needle driven vertically through its core.

Now tilt the needle towards the left. At 24.5 degrees of tilt you have the tilt of the Earth.

Continue turning until the knitting needle is lying flat and the apple is on its side (90 degrees tilt), then tilt further until at 98 degrees you have the tilt of the planet Uranus.

Continue further all the way round to 180 degrees, where the top and bottom of the apple have changed places, then bring the tilt back a few degrees to 177 degrees and you have the tilt of Venus.

The tilt of Venus is so extreme that its north pole is almost facing south and its south pole is almost facing north.

The tilt of Uranus means that it is spinning on its side. That has profound consequences.

If the tilt is greater than 90 degrees, the planet is turning on its axis in a different direction from the direction of its orbit.

All the planets orbit anti- clockwise around the Sun, so this means Venus and Uranus are the only two planets to spin in a clockwise direction.

Only on these two planets does the Sun rise in the west and sink in the east.

Pluto has retrograde rotation but is now classified as a dwarf planet.

The tilt of a planet is responsible for its seasons. The greater the tilt, the more extreme the seasons.

If the Earth’s tilt was 90 degrees, the poles would be tropical for six months and experience Arctic-like conditions for six months.

Uranus’ tilt means its poles each experience, in turn, a 42-year summer in constant daylight followed by a 42-year winter in pitch dark.

This tilt is thought to have been caused by a collision with a planetsized object during the formation of the Solar System.

So these are the Solar System’s oddballs – Venus with its north facing south and its day longer than its year, and Uranus with its extreme seasons, and both planets with their retrograde spin.