THE SOLAR SYSTEM

Our Solar System - the Sun's realm - incorporates the planets, asteroids, comets and the icy rocky bodies at the outer reaches and beyond to the very limits of the Sun's influence. The nine main planets lie in a plane called the 'ecliptic' and orbit the Sun following a clockwise motion, at distances described by a simple mathematical formula called Bodes Law. The concept of a solar system would have been familiar to the very earliest astronomers; but our current view goes back to Copernicus and the idea of a solar system with the Sun, rather than the Earth, centrally placed.

The Sun, like other stars, was formed from a vast region of gas and dust particles, material known as the diffuse interstellar medium (ISM). Although the exact mechanism is uncertain this cloud collapses when an area of weak gravitational attraction develops. The slowly collapsing cloud - called a 'cocoon nebula' - begins to spin and material above and below the plane of rotation is drawn toward it and the cloud becomes increasingly disk-shaped 'solar nebula'. At the centre, heated by the contraction, material begins to warm up and radiate in the infrared; further contraction takes place and the core, which now internally radiates energy, slows the collapse. The diffuse sphere of material, not yet heated by fusion, is called a 'protostar'. The protostar at this stage is about twice the width of our Solar System but by the time it has shrunk to 140,000 km across and the core temperature risen to millions of degrees it is hot enough for hydrogen nuclei to fuse and become helium. This nuclear reaction, unstable at first owing to convection currents within, means that the output of the early Sun varies greatly, until its structure developed and can burn more steadily, just increasing slightly in output and size. About 4.6 billion years later, the present day, the Sun is an estimated halfway through its life-cycle.

The age of the Sun is determined by the radioactive dating of meteorites and the oldest rocks found on the Earth and Moon, which would have formed at the same time. Near the young Sun it is warm, further away it is cooler; this temperature fall-off organises the material which will in time evolve into planets and explains why the rocky terrestrial planets (Mars, Earth, Venus, Mercury) evolved near to the Sun and further out the gas giants were formed. Silicates and other minerals condensed in the warmer inner region, and water ice and other volatile compounds were able to exist as ices in the region now occupied by Jupiter, Saturn, Uranus and Neptune.

The planets grew via a process called 'accretion' in which dust grains and globular chondritic materials evolve into larger objects. Slowly colliding particles stick together through gravitational attraction (faster colliding particles bounce apart), gradually the clumps grow in size, the larger ones progressively more able to sweep up and incorporate smaller grains. By the time objects are several hundred metres to a kilometre across the gravitational interaction between them are more pronounced and the collisions more violent. Bodies able to survive and incorporate more material grow further in size, and at about 500 kilometres across are more planet like, spherical, with layered and molten interiors. It is thought that about this time, the very largest impacts upon the growing planets impart them with the rotation direction and axial inclination they will retain.

The early Solar System was a dangerous place: our Moon, Mercury, and many of the moons of the outer planets are riddled with craters, possess continent sized impact basins, and volcanic plains (like the Moon's Maria). They were able to withstand asteroid impacts but conditions on their surfaces would have been extremely chaotic under frequent and heavy bombardment and the catastrophic volcanic eruptions they would have triggered.

Beyond the orbit of Mars and the terrestrial planets lies the Asteroid Belt. We have known of asteroids there for just over about 200 years. It had been suggested that the belt represented the remains of a planet destroyed in a collision. The current view is that it comprises material that was unable to accrete owing to the gravitational influence of Jupiter.

The gas giants would have evolved like the terrestrial planets, steadily incorporating more material and growing in size. The cores of some of the gas giants could be rock; it is possible that rocky planetesimals (very large spherical asteroids) ejected from the inner solar system formed the nucleus about which ices of water and volatile compounds were swept; growing in size, they warmed up and developed substantial atmospheres. Pluto lost the race, the small amount of volatile material it accumulated froze onto its rapidly cooling surface.

Astronomers have searched for planets beyond Pluto- the fabled planet 'X'. There may be cold dark planetesimals beyond but they remain elusive, and although there is plenty of material in the Kuiper Belt (out to roughly 50 AU from the Sun), the material appears to be too widespread to have formed any planets. If this is true of the Kuiper belt, it is almost certainly true of the Oort cloud (out to roughly 1 LY), which although may contain trillions of comets, they would be too thinly distributed to form anything planet-sized.

Future

The Solar System having evolved to its current form is anything but static. The early Solar System was dangerous, but cataclysmic events are not relegated to the past- as excitingly demonstrated in the collision of Shoemaker-Levy 9 fragments with Jupiter. Studies of Lunar craters show that impact cratering has occurred in distinct episodes. Could the future hold further bombardments? It is speculated that a passing star could perturb Oort cloud objects and send comets into the middle of the Solar System. Although the probability is small, the consequences of an impact with Earth are so great that astronomers are taking the asteroid hazard very seriously, with programmes such as the Near-Earth Asteroid Tracking System (NEAT) searching for Earth crossing asteroids which could pose a danger to Earth in the future.

As well as being sent on different orbits asteroids and cometary fragments may be captured by the gas giants. But over millions of years some of the planets are certain to lose moons. Moons growing ever closer to their planet are destined to be either ripped apart in orbit or spiral inward to a spectacular finale. Such will be the fate of Mars' inner companion Phobos which presently zips round Mars in just 7 hours and 39 minutes. Phobos' eventual rendezvous with Mars is something future human settlers perhaps ought to bear in mind. Outermost Deimos, on the other hand, will continue gradually to distance itself from the Red Planet. Eventually Deimos will escape Mars' gravitational pull altogether and join the asteroid belt.

Neptune's moon Triton, one of the largest in the Solar System, is thought to have been a rogue planet which was captured by the planet's gravitational pull. In about 1 billion years time, Triton will grow nearer, and dangerously close to Neptune. As it does so, tidal forces will begin to pull it apart. These tides will massage the moon, heating it up, perhaps fuelling some volcanic activity. But there will come a time the stresses will pull the moon apart. It is predicted that the particles and debris from the break-up would initially form a doughnut-shaped cloud encircling the planet, then with time organise themselves into a flat disc. A Saturn-like ring system about vivid blue Neptune would be truly spectacular. About other planets existing ring systems may slowly vanish. Saturn's rings will eventually be eroded by meteorites which break up and wear away the rings. The diminishing rocks fall inwards and are consumed by massive Saturn; in about 100 million years the rings may be gone altogether.

Some of the changes the Solar System has in store are quite well understood, if not entirely predictable; while others, such as future bombardment episodes, are more speculative. The eventual demise of the Solar System, however, is inevitable. As the Sun runs out of fuel it will begin to expand and swell up. But although it has less fuel, it actually becomes hotter - an estimated 10% hotter every billion years. Conditions on the planets will change radically as the Sun balloons. In 3 billion years it is estimated the Sun will engulf the orbit of Mercury and, 2000 times brighter than it is now, it will be able to melt the surface of the Earth. Towards the end of the Sun's life it will become a red-giant, engulfing the orbits of Mercury and Venus. The process will take several billion years but with horrible effects for our home planet long before the Sun swells to fill most of the sky.

Soaring temperatures will make survival difficult for plants and animals as the Earth becomes increasingly arid. The greenhouse effect, which at the moment keeps much of the planet at a reasonable temperature, will trap more heat. Eventually it will become so hot that the shrinking oceans are vaporised. The atmosphere will go, as volatiles (liquids and gases) escape into space and are eroded by the solar wind, leaving the surface bathed in deadly radiation. Earth from space will in about 2 billion years look very different, no oceans, no vegetation. It will probably look as if nothing had ever lived here- perhaps rather like Mars does today; but without the polar caps. In 3 or 4 billion years the Sun is expected to grow hot enough to melt the surface of the Earth. A seething mass of molten lava, Earth would be unrecognisable.

Although the prospects for Earth 1 billion years AD are bleak, other planets may benefit from the increased solar output. Scientists interested in life in the Solar System describe a hypothetical region they name the 'ecozone' which is a band about the Sun which is just the right temperature, providing water is available, for life to exist. Today Earth is 'just right', but Mercury is too hot and Pluto too cold. As the Sun grows bigger and hotter, the ecozone moves outwards and encompasses planets and moons further from the Sun.

As the Sun expands, Mars' icecaps and permafrost will melt, releasing carbon dioxide from the south polar cap and water from the north polar cap. The release of gas that accompanies the warming of the planet will raise temperatures further, melting ice trapped below Mars' surface. With a thicker atmosphere, water is able to exist on the surface; simple microbial life could survive in the carbon dioxide rich atmosphere and gradually produce oxygen.

When the Sun is even bigger, the icy satellites of the outer planets: Jupiter's moons Europa and Ganymede, and Saturn's moon Titan will be transformed. Their icy exteriors will melt to form global oceans and in the timescales we're considering could evolve life of their own. Of the outer bodies, Ganymede seems the most likely candidate for future life. The Solar System's largest moon has a magnetic field perhaps strong enough to protect a developing atmosphere from erosion by the solar wind. Even Pluto and Charon could one day have a liquid surface while the icy bodies which make up the Kuiper belt will begin to jet gases and fragment as they warm up, leaving behind only rocky debris.

Astronomers predict the Solar System will go out with a bang. The Sun bloated to an enormous 50 to 100 times its current size is expected to retain a tiny core and surrounding it a layer of trapped hydrogen. As the last energy at the core is used, the surrounding hydrogen layer fuels an outburst from the Sun of catastrophic proportions - a supernovae. As this layer is blown from the Sun it rips through the Solar System, vaporising the Earth and leaving the planets from Mars and beyond tiny dead hulks. All that remains of the gas giants and their moons are their rocky cores. The Solar System thereafter will be cold and bleak, the husks of the planets orbiting a dim white dwarf- a tiny star with enormous mass.

Other solar systems

But somewhere else, it will be happening all over again. Although until recent times our Sun and Solar System were regarded as unique, the existence of other solar systems is not a new idea, having been suggested by the Greek philosopher Epicurus. Until now detecting planets about distant suns was impossible, the planets circulating a star are lost in the glare of an object billion times brighter, but now they are proven to exist.

Our first real clue to extra-solar planets came in 1983 when Hubble Space Telescope images recorded that the star Beta Pictoris, surrounded by a disk of gas and dust, has in fact a very thin disk - suggesting that solar system formation is well under way. Astronomers analysing the light received from the system think that it contains cometary material and the leftovers of planetary formation, and that planets could already have formed around Beta Pictoris. The Hubble Space Telescope has also imaged disks of condensing gas or 'proplyds' (proto-planetary disks) in the Orion Nebula.

New techniques have allowed the identification of the planets themselves. The first tentative discovery of a planet came in 1995 when astronomers Geoff Marcy and Paul Butler announced they had observed, using a high-resolution spectrograph, changes in the wavelength of light from the star caused by its varying velocity. This wobble in the spectra is almost certainly the gravitational effect of a nearby planet.

51 Pegasi (easily visible with binoculars in our night sky) is a G2-3 V main-sequence star and located 42 light-years from Earth. We don't know very much about its companion, but its mass is estimated to be half that of Jupiter and that its just 7 million kilometres from 51 Pegasi (Mercury orbits 58 million kilometres from our Sun), such proximity to the star would give it surface temperature of about 1000 degrees Celsius.

Since 1995 and as of May 2003 more than 107 extra-solar planets have been discovered in 93 planetary systems; with 12 of the systems possessing more than one planet. Several systems have three known planets orbiting their star: 55 Cancri and Upsilon Andromedae. The planets are large and vary from between half a Jupiter mass to nearly four times the mass of Jupiter. But even larger bodies have been found, in the region of 80 Jupiter masses. These 'brown dwarfs', as the they are known, were predicted theoretically and are thought to be failed stars- objects which could have become stars but which did not grow sufficiently and heat up enough to begin fusion. Astronomers are eager to compare other solar systems with ours. A planet about 70 Virginis orbits the star in an very eccentric orbit every 116 days and has a mass about nine times that of Jupiter. What would happen to terrestrial, Earth-like bodies, if accompanied by planets like these?

Many of the planets around stars are very close to like Pegasi 51s, but some are further away, about 200 to 250 million kilometres (more like Mars to Sun distances) where water could exist as a liquid. Such a large warm planet orbits 47 Ursa Majoris, discovered recently after analysis of eight years of observations made at the Lick Observatory. Its period is a little over three years (1100 days), its mass about three times that of Jupiter.

These early results are very encouraging. Spectrograph techniques are being refined and new techniques are being developed. Transit photometry, in which the change in the light caused by a planet passing in front of a star is detected is now possible with very sensitive telescopes making continuous observations. It is hoped that with this technique many thousands of stars could be monitored simultaneously for signs that they have planets around them. Advanced spectroscopes are expected to be capable of finding Neptune-size planets, but scientists and engineers working on plans for the next generation of telescopes, in particular NASA's Terrestrial Planet Finder and ESA's DARWIN mission. These projects will enable astronomers to study them directly, and analyse the light spectroscopically to determine their composition. Detecting water or oxygen might be a good clue to whether they may support life; but abundances of gases such as carbon dioxide, methane, and ozone could represent the signature of life itself if found in similar proportions to Earth's atmosphere.

The Terrestrial Planet Finder (TPF) will comprise either an infrared interferometer, operating on a series of formation flying spacecraft - their separate signals processed to mimic the signal from a single gigantic instrument; or a visible light chronograph, designed to reduce the starlight and enable the detection of planets and 10 times more powerful than the Hubble Space Telescope. The choice of design for the TPF will be made in 2006.

ESA's Darwin project is uses a formation of six 1.5 meter space telescopes, located at Earth's L2 Lagrange point in order that uninterrupted observations to be made. Like the proposed TPF interferometer the signals from Darwin's telescopes would be processed to emulate that of a single large telescope. ESA plans with SMART-2 in 2006, to fly two spacecraft in formation testing the technology and techniques to be used on DARWIN, which would be launched around 2014. It is possible that the TPF and DARWIN projects will be combined and operated as a collaboration between space agencies - given their cost, and technical difficulty.

Astronomers have only just begun the search for planets, and it already appears that solar systems are quite varied. Examining this variety, and observing solar systems at different stages in their development will teach us much about our own Solar System and how it evolved. With better resolving power, astronomers are certain to find more worlds to explore, smaller gas giants, and eventually terrestrial planets at distances from the star where life could exist. There are so many candidate systems, stars like ours, that some astronomers suggest the galaxy could contain more than a billion 'earths'. The first discovery of signs of life on one of them, will represent an enormous advance in human understanding- with deep philosophical, and very exciting scientific implications.