Jupiter, as any keen astronomer or long-suffering student in a science classroom will tell you, is our Solar System’s fifth planet and it’s largest. This massive gas giant has two and a half times the mass of all the other planets. Its appearance, with its distinctive marbled surface of white, grey, orange and brown, has intrigued sky-watchers since the dawn of human civilisation. The Romans named it after the senior god in their pantheon and it was an important feature in ancient Chinese and Indian astrology. This celestial mammoth of hydrogen (and a small amount of helium) has also the honour of possessing the Solar System’s largest collection of orbiting moons, or ‘satellites’.
At last count, Jupiter had the grand total of SIXTY-FOUR satellites of varying sizes. The largest is Ganymede, the largest moon in our corner of space. Composed of silicate rock and ice, she is larger than the planet Mercury.
A recent discovery by a group of scientists at the Carnegie Institute for Science in Washington DC, will mean a small hurried revision to that count. Scott Sheppard, affiliated with Carnegie’s department of terrestrial magnetism, examined data and images taken by the Magellan-Baade telescope and forwarded from an observatory in the Chilean Andes. In one of the black-and-white images, among several blots and points of white light representing stars and galaxies, Sheppard noticed something unusual. Two moons, only about a kilometre (0.62 miles) across and 2 kilometres in diameter with enough surface area for a small town. These miniscule (in astronomical terms) satellites, were indeed of such an insignificant size that they had been completely missed by previous studies of Jovian satellite imagery, and even by most detection equipment. Contrast this with Jupiter’s four largest moons, the Galileans, which can be seen by anyone with a telescope, a back garden and a clear sky.
Provisionally dubbed S/2011 J1 and S/2011 J2, the two moons are believed to be asteroids originating from the belt of material between Mars and Jupiter. It is theorised that they left the belt many millions of years ago and while trying to pass Jupiter, were ensnared by its gravity. Both of these new Jovian moons orbit so far away that they take between 580-730 Earth days to revolve around their mother planet. J1 and J2 are ‘retrograde satellites’ – they orbit backwards, in the opposite direction to Jupiter’s own axial rotation. Their orbits are also quite erratic and inclined. Fifty-two of the other known moons also are retrograde, and Sheppard had predicted that there may be up to fifty more still awaiting discovery. As far as the two Js are concerned, they will be renamed after a year of observations and their new names must be ones related to Jupiter/Zeus, the Graeco-Roman king of the gods.
At this point in time, very little is known about them, especially their appearance and what elements they are composed of.
Scientists are excited about the new discovery. Sheppard notes that J1 and J2 could provide answers to the formation of Jupiter and the planetary system as a whole. At a time when telescopes and man-made satellites have their lenses trained on the furthest reaches of space and dozens of planets & other phenomena are being found outside the Solar System every year, we have still much to learn and understand about what is lying around in our own ‘astronomical backyard’.
This blog post was based on material from the article “Two New Moons Found Orbiting Jupiter” by Jason Major (National Geographic News, February 2, 2012) and info from Wikipedia.