Our Solar System

Saturn in infrared. Image credit: NASA / JPL / University of Arizona

Our solar system is our home, not only host to the Earth, Sun and Moon, but a plethora of other bodies. Some are vast and highly dynamic, whilst others are small and frozen solid. All have one thing in common: each is a mystery with the potential to teach us all kinds of things about the Universe and our place within it.

The story begins, as it must, before our solar system existed. Nearly five billion years ago (5,000,000,000) or maybe even longer, one or more gargantuan stars, much bigger than our own Sun, erupted in a cataclysmic firestorm which showered the surrounding space with gas and dust. This gas and dust eventually came together to form a cloud which, in turn, began to coalesce.

In the centre of this swirling aggregation of matter sat a protostar which, over the millennia, grew hotter and denser until eventually the temperatures and pressures at its core became so high that with a brilliant burst it began nuclear fusion deep within. It had become our Sun, though it was smaller and a little dimmer than today. Around the fledgling star spun the remains of the cloud from which the colossal fireball was born, and this too began to coalesce into small pieces. Bit by bit, these pieces, or 'planetessimals' began to come together to form larger lumps, and eventually planets (N.B. This brief outline does not tell the full story of planetary genesis, which I will return to more fully and accurately at a later date.).

Those planets may or may not have formed in the order they are today, but it is almost certain that they were not easily recognisable compared to their current forms. The terrestrial planets, Earth, Venus, Mercury and Mars, were likely molten balls. They were pummelled from above by a constant barrage of debris from the cloud from which they had formed, the cratered scars lingering upon the surface of Mercury and other bodies to this day.

Mercury: Closest to the sun. Credit: NASA

Venus: Wreathed in acid Haze. Credit: ESA/MPS, Katlenburg-Lindau, Germany

Mars: the Red Planet. Credit: NASA

 The gas giants, Jupiter and Saturn, possibly Neptune and Uranus too, may have appeared as they do today, though doubtless their moons were either absent or different. Saturn may not have borne rings, and Jupiter may not have been blessed with the Great Red Spot which we all know and love. Uranus may not have rolled along on its side, but their surfaces and compositions were likely similar to their present forms.

Io, moon of Jupiter, crosses the face of the Giant Planet. Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Still though, the inner Solar System was pummelled, and at the third rock from the Sun, a great cataclysm was to take place. An enormous body, a planet in its own right of a similar size to Mars, was heading toward our young planet. After a dance that would have lasted thousands of years, the two collided in a terrible event that destroyed the smaller body, hurling billions upon billions of tons of material out into orbit in a shower of molten rock and metal. As the young Earth below began to cool and heal after the great violence of the event, the orbiting debris began to coalesce again in a manner that reflected the birth of the young Sun. Slowly, over the years, from the rubble formed our beautiful Moon which, save a battering by smaller bodies, remained unchanged and formed that familiar sight that we see circling above us aeons later.

Moon of Earth. © Guy Stimpson 2012

Similar collision and capturing of bodies was taking place elsewhere in the system; Jupiter and Saturn were hoovering up moons to create some of the most dynamic and interesting sky-scapes we've ever borne witness to. Out past Mars however, another story was emerging. Here was a place where no large planets could form, for the influence of the Lord of Planets, Jupiter the Great, would either ensnare them or hurl them out into the depths of space. Instead, the detritus left over from the formation of the Sun and the planets formed a thin ring of smaller bodies, the Asteroid Belt, which lasts until today. This ring is a menagerie of small and diverse worlds, and is even thought to be the source of the moons of Mars, Phobos and Deimos.

Phobos: Left over from the dawn of the Solar System? Credit: HiRISE, MRO, NASA,  LPL (U. Arizona)

Outside the Solar system too, these primordial rocks waited, in the Kuiper Belt and Oort cloud. Every so often, a passing star or nearby perturbation gives one of these distant gatherers a little nudge. It tilts toward the sun and now, forced into an unexpected course after billions of years of waiting, it begins to hurtle manically toward the distant sun at the heart of the system. After a journey of hundreds, even thousands of years, one of the travellers finally reaches its parent star, and graces our skies with one of the most beautiful and fantastic light shows that we ever are fortuitous enough to see; a comet.

Comet Hale-Bopp in 1997, a wanderer from outer space.

Each of these bodies represents a different class of object within our local system, and each can tell us different things about the environment out of which we're born. In coming posts I will visit each of the planets and a number of smaller bodies in more detail. Be prepared for worlds of ice, underground oceans and days as long as years!

Home, sweet home.

There is only one place to begin when discussing the Universe, and that's right here at home. Our planet is home to a vast wealth of culture, diversity, colour and vibrance, yet though we may understand much about our own world today, in the past it has also been a place of mystery and enigma.

The tale of Astronomy begins in ancient times, possibly with the Babylonians. However, many cultures have formulated their own hypotheses and folk-lores around the heavens, seeing the stars and planets wheeling unassailably above as gods, demons and portents. The movements of the heavens were both employed as mystical tools to foretell great victories or terrible happenings, or used agriculturally and navigationally to stand as a sign of when to plant crops or what direction in which to sail. At times like these there was little to tell, if anything, between Astronomy and Astrology, though they could not be further apart today.

The ancient Greeks in particular began to treat Astronomy as a particular and specific science. Indeed it is from the Greek planetes, meaning 'wanderer' that we derive the term 'planet' today. The reason for this naming is that the ancient Greeks among others noticed something remarkable about the night sky, and it was this which began to open the doors to a greater understanding of the Universe around us. Firstly, they noticed that the stars at night held fixed positions, although as a group they seemed to rotate around us. Year after year they would remain in the same places relative to each other as they spun around amid the inky blackness of the heavens.

This observation led to two suppositions:

1: that the Earth rotated about its axis, and
2: that the Earth was at the centre of the Universe (whatever that might be!).

The next observation was one that, unbeknownst at the time, would eventually help to unseat that second premise. The Greeks noticed that aside from the fixed stars that wheeled above ever and anon, there were five wandering stars, the planets, whose positions did not remain fixed as the years went on. They seemed to follow their own paths, sometimes racing across the sky in one direction, and then another time drifting back from whence they came.

This apparent aimlessness of the planets perplexed the ancients for centuries. That the Sun, Moon and stars rotated around the Earth (or at least appeared to do so) was plain to see. How could these other stars be any different?

Eventually, Aristotle began work on the idea that these wanderers were fixed to seven crystal spheres, along with the Sun and Moon, called 'epicycles', which sat one inside another with the Earth at the centre. In this fashion he tried (not particularly accurately, but better than any at the time) to predict the positions of the planets from one year to another. 

Whilst this view may seem laughable to us in the modern era, it is easy to see how such mistakes could have been made without the use of telescopes, satellites and all the technology that we are so enamoured of today. Indeed, the ideas of Aristotle lasted until the 16th Century.

At this time came Mikołaj Kopernik, otherwise known to us as Copernicus. He for the first time introduced the idea that the Earth was not at the centre of the Universe, but that the Sun held that particular honour. Indeed, whilst this view was still incorrect, it was the first time that someone had begun to realise the truth about mankind's place in the cosmos.

It was then left until the coming of Galileo Galilei that the geo-centric model of the Universe could finally be shattered, although it was not permitted at the time and Galileo was punished for his work. He showed, by looking through a new invention (the 'telescope') that the planet Jupiter had four moons rotating about it. These moons are known to this day as the 'Galilean' moons in his honour, as he showed for the first time that it was possible for a something to orbit a body other than the Earth. Well, though the Catholic Church tried their best to refute the idea, geo-centrism was done for. Johanness Kepler was next on the scene, showing that the planets move in predictable ways around the sun, in an effort that finally ousted the work of Aristotle from around 1,300 years before. For the first time, one could make a calculation that would precisely pinpoint the position of a planet many years in the future with perfect accuracy. In fact, Kepler made three postulates, which are now known as Kepler's Laws:

1. A planet travels around an elliptical orbit, with the sun at one of two foci,
2. An imaginary line drawn between a planet and the sun sweeps out equal areas in equal times, and 
3. The orbital time (year) of a planet depends on it's distance from the sun (or the square of the orbital period of a planet is proportional to the cube of its semi-major axis in full).

With these three ideas, Kepler paved the way for Modern Astronomy to begin in earnest. His last law is used by astronomers today, particularly by the space-borne observatory that bears his name, the Kepler Space Observatory. This marvellous device hunts for distant planets among the stars by observing the tiny dip in light as a planet moves in front of its parent star. Using Kepler's third law, scientists can determine the distance of a planet from the star, and even its mass.

Since Kepler, Astronomy has been less a study of the mystic and supernatural, and more a study of real, testable phenomena. Since his time we have learned much about stars, planets, galaxies and all manner of astrophysical objects, yet none of it would have been possible without these early pioneers and thinkers, who often risked their reputations, even their lives, to think past conventional wisdom and seek the ultimate truths of our existence.