By Jove

 by Richard Hall

If Jupiter were our Moon.

The lower of the two pictures in the image above is a photograph of the Full (Super) Moon rising above Stonehenge Aotearoa.  The upper image shows the planet Jupiter as it would appear at the same position and distance as the Moon. 

In our night sky Jupiter, due to its great distance from us, looks nothing more than a very bright star.  In reality it is a colossal world. It has an equatorial diameter of 142,984 kilometers. That is 11.2 times the diameter of the Earth or, 41.2 times that of the Moon. Jupiter has a mass equivalent to 317.8 Earths. That is 2.5 times the mass of all the other planets put together.  

Centuries ago, when astronomers were describing the nature of celestial objects, they lumped all of the large objects that orbited the Sun into a single group – the planets. However, the planets consist of a variety of different types of celestial objects. For example, the Earth is a ‘terrestrial world’-a planet made of rock and iron with a thin atmosphere. On the other hand, the composition and structure of Jupiter is very different, it is usually referred to as a ‘gas giant’.  

Jupiter is more similar to the Sun, which is a star, than it is the Earth.  Like the Sun it is composed mostly of hydrogen and helium and, it emits more energy than it receives from the Sun.  

Let’s take a journey into Jupiter.

Its surface is not solid.  In a telescope we see white bands of ammonia clouds and, between them and at greater depth, brown clouds of hydrogen sulphide. The clouds have a temperature of about -150oC and the winds have a speed of around 400 km per hour.

Image Credit: Enhanced image by Kevin M. Gill (CC-BY) based on images provided courtesy of NASA/JPL-Caltech/SwRI/MSSS.

Below the brown clouds we plunge into total darkness and, at a depth of about 1,000 km, the pressure is so great that the hydrogen gas is compressed into a liquid. We have encountered a world-wide ocean of liquid hydrogen.  This ocean has a depth of about 20,000 km. At the bottom of this ocean the pressure reaches 3 million atmospheres, and the liquid is compressed into a super-dense state of metallic hydrogen. Convection currents within this liquid metal generate enormous electrical currents and gigantic magnetic fields.

Note that we should really be calling Jupiter a ‘liquid giant’.

Like the Sun, Jupiter has its own planetary system. It has 79 moons. Four of which are the size of small terrestrial planets.  If we lived on Io, the large moon closest to Jupiter, our view of Jupiter would be similar to that shown for Stonehenge.  Io’s distance from Jupiter is 421,700 km.  The Moon’s distance from Earth is 384,400 km.

A few years ago the only planets we knew of were those that orbited our star – the Sun. However, with modern technology we have discovered thousands of worlds that orbit other stars. Indeed, it is highly probable that every star has its own retinue of planets.

Jupiter – Photo @space_samuel from Earth.

We have discovered Jupiter size planets orbiting other stars in the so called ‘habitable zone’. This is the zone around a star where a planet, like the Earth, could have liquid water and therefor living creatures.  It has often been assumed that a giant planet in this zone would exclude the possibility of an Earth-like world. This is not so because, a Jupiter-like planet could have a moon as large as the Earth.  Thus, our upper picture could be scene similar to that from an Earth-like moon orbiting a Jovian planet in a habitable zone around another star.

When can we see Jupiter in the Sky?

The Earth makes its closest approach to Jupiter on July 14th. Come along to Stonehenge Aotearoa on a Friday or Saturday evening and view this awesome world through one of our large telescopes.