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Updated March 14, 2011
Composition of the Solar System
Humans have been interested in the Universe and our place in it since pre-recorded history, indeed probably since before we could be considered ‘humans’ in the modern sense. The Solar System represents only the tiniest fraction of the Universe as a whole, however because it is our immediate neighbourhood and its members can be considered Earth’s immediate family, it has long been the subject of great interest. Over the centuries our understanding of the Solar System has evolved as our technologies have developed. In the 21st Century, with a battery of sophisticated telescopes, orbiting detectors at many wavelengths across the electromagnetic spectrum, and robot probes to other planets, satellites, asteroids, and comets, humankind has a deep but certainly not complete knowledge of the solar family.
The dominant figure in an intriguing cast of characters is the Sun. Consisting almost entirely of hydrogen (74%) and helium (24%), our local star contains some 99.8% of the mass of the entire system, its volume big enough to comfortably swallow a million Earths.
The Sun is located in one of the spiral arms of the Milky Way Galaxy, one house in an immense ‘city of stars’ of some 400 billion members. The Sun revolves about the centre of the Milky Way every 200 million years or so, and the galaxy itself is in motion relative to other galaxies in the ‘neighbourhood’. But wherever the Sun goes, the planets move with it like the occupants of a moving vehicle, so such motion can be discounted when considering just the Solar System itself. It’s perhaps easiest to consider the Sun to be fixed in the middle, and everything else to be in motion relative to it.
This wasn’t always thought to be so. Although Aristarchus proposed a heliocentric (Sun-centred) system in the 3rd Century BCE, the idea lay dormant for many centuries until once again suggested by Nicholas Copernicus in 1543. Although the idea solved many of the problems in observational astronomy, it was considered heretical for a time. The idea of Earth hurtling around the Sun in a billion-kilometre annual round trip, or rotating on its own axis once a day at over 1,000 km/hr, seemed nonsensical to its inhabitants. Eventually through the work of such acclaimed scientists as Johannes Kepler, Galileo Galilei, and Isaac Newton, the evidence supporting the heliocentric system became overwhelming.
In our ‘family’ analogy, the Sun can be thought of as the (single) parent. The immense pressure and, especially, temperature at its core causes hydrogen nuclei to be fused into helium nuclei, a process known as thermonuclear fusion. A byproduct of this process is the energy that we experience as heat and light that is fundamental to life as we know it. The Sun also produces a constant stream of energized particles known as the solar wind. Other, more harmful forms of radiation are also produced, but we are largely protected from these by Earth’s atmosphere and magnetic field. The Sun spins on its axis roughly once per month, but its spin rate is faster nearer the equator than near the poles. This ‘differential rotation’ causes extremely powerful and complex magnetic fields, which drive an assortment of solar activity including sunspots, flares, prominences, and coronal mass ejections. The Sun is a fascinating object that has been intensively studied as the closest and (to us) most important of the stars.
The entire solar system condensed out of an immense cloud of gas – the ‘solar nebula’ – some 4.6 billion years ago. All of the constituents of the solar system are considered to be of similar age. Because this nebula was spinning as it collapsed, all of its major components co-exist on a relatively flat plane perpendicular to the solar rotational axis. The plane defined by the Earth’s orbit around the Sun is known as the ecliptic; the Moon and planets can always be found in a narrow band of the sky very close to it. The constellations situated in the direction of the ecliptic are known as the signs of the zodiac.
The immediate family members are the major planets, which collectively account for some 0.1% of the Solar System’s matter. There are generally thought to be nine of these – in order, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto – although some modern planetary scientists question the inclusion of Pluto in this category. The first eight are in two distinct groups: the four inner, rocky ‘terrestrial planets’ (each some 5,000 to 13,000 km in diameter) and then the four outer, ‘gas giant planets’ (50-143,000 km). They were formed out of leftover material from the collapse of the solar nebula, including elements heavier than helium that were themselves forged in the explosive destruction (supernova) of more ancient stars.
The planets were not formed whole, but were formed as rings of material around the Sun started to clump together into planetesimals through gravitational attraction. This process, known as ‘accretion’, featured a large number of violent collisions, especially in the earliest years of the Solar System. One such collision between early Earth and a Mars-sized object is believed to have resulted in our Moon. After about a billion and a half years, or one-third the Solar System’s current age, this process was largely complete. However, by no means is it fully so, and cosmic collisions remain an occasional fact of life in this relatively crowded neighbourhood.
Most of the planets have their own satellites, or moons, in orbit around them. This is especially true of the gas giants, which have large families of dozens of rocky, icy moons, each such system in effect a mini-solar system.
In addition to the major planets, there are also two systems of minor planets, better known as asteroids. The Main Asteroid Belt is largely confined to a loose ring in the transition zone between Mars and Jupiter. In the late 18th century a group of astronomers used the Titius-Bode Law to predict that there should be a planet somewhere between the two. Instead of finding a single such large body, they began to find large numbers of small ones. Guiseppe Piazzi discovered the minor planet 1 Ceres on January 1, 1801. It remains the largest of the asteroids in this zone, at something less than 1,000 km in diameter. Within a decade, three more were found at similar distances, and as detection techniques improved after 1850 or so, discoveries became routine. Nowadays there is something in excess of 70,000 catalogued asteroids, which collectively have less mass than the Moon. It is widely thought that the gravitational interference from Jupiter prevented the accretion of the various smaller pieces. Indeed, the zone may originally have been much more heavily populated, only to be swept relatively clean by the gravitational broom of giant Jupiter.
A small subset of the Main Asteroid Belt are the so-called ‘Earth-crossing asteroids’ which as a group must be considered a potential threat to life on our planet. There is substantial evidence that major extinction events in Earth’s past were caused by collisions with such objects. Furthermore, scientists accept that there is a 100% chance that such collisions will happen at some point in the future. It is therefore of critical importance that we observe our immediate neighbourhood and take a census of potential ‘doomsday asteroids’. At present over 580 objects have been designated Potentially Hazardous Asteroids, the large majority of them discovered through remote sensing techniques within the past decade.
A second asteroid belt outside the orbit of Neptune was theorized by Gerard Kuiper back in 1951. Over 40 years later, the first member of the so-called Kuiper Belt was observed. In the dozen years since, some 800 additional objects have been discovered in this region, some of which are slightly larger than Ceres. There is a raging debate among astronomers whether Pluto and its moon Charon should simply be considered the largest members of this class of objects, or the smallest of the planets. There are valid arguments both ways and no clear-cut division in the continuum.
Comets are occasional visitors to the inner solar system due to their highly elliptical (cigar-shaped) orbits, which were likely altered by a past close encounter with a massive planet (presumably Neptune itself). As they approach the Sun, volatile materials on the comet’s surface, such as ice, are transformed from a solid directly into a gas in a process called sublimation, which can be considered melting and boiling all rolled into one. The new gases form a huge temporary atmosphere loosely held by the weak gravity of the cometary nucleus (typically 1-10 km in diameter), which despite being extremely diffuse can act as a very efficient reflector of sunlight. The cloud around the nucleus is called the comet’s ‘head’ or ‘coma’, while gases and dust also are blown away by the solar wind to create the comet’s ‘tail’. Often there are two tails, one of gas and one of dust. Occasionally comets can achieve naked-eye visibility, and roughly once per decade a so-called Great Comet garners widespread public interest.
The Kuiper Belt is now thought to be the source of the so-called short-period comets, which orbit the Sun in periods under 200 years. Although comets and asteroids are different types of objects, there is clearly a relationship between them. It seems likely that many small asteroids may be extinct comets (all volatile materials sublimated away) or potential comets (too far from the Sun for sublimation to occur unless orbit changes and the object passes closer to, and is heated by, the Sun.)
Long period comets appear to come from a second, more distant reservoir. The Oort Cloud is a spherical collection of perhaps trillions of comets, located some 1,000 times further from the Sun than the Kuiper Belt. Calculations of their orbits indicate that long-period comets return only after many thousands if not millions of years. In 1950 Jan Oort compared the orbits of many long period comets to predict the existence of this vast reservoir.
In early 2004 a nearly 2000-km asteroid, recently named Sedna, was discovered in a strongly elliptical orbit well past Pluto. Currently some three times the distance of Pluto even at its closest, Sedna’s orbit has been calculated to take it some 10 times that distance, with a total round trip time of 10,500 years. Whether it will be classified as an object from the outer Kuiper Belt, the inner Oort cloud, or some new class of solar system object intermediate to the two, remains unknown at this time. However, it is distinctly possible that Sedna is simply the first of another burst of new discoveries.
The solar system also contains immense quantities of smaller debris on the scale of dust, sand grains, or pebbles. A flat disc of interplanetary dust can be seen under ideal conditions as the zodiacal light. This material continues to be attracted by the gravitational fields of the planets; Earth accumulates some thousands of tonnes of meteoritic dust each day. The larger particles disperse a flash of light as they are annihilated by Earth’s atmosphere; these are known as sporadic meteors. Very occasionally, an object encounters Earth, which is large enough to survive passage through Earth’s atmosphere. These may be sighted as a bright fireball, and may deposit one or more meteorites on Earth’s surface. Such objects provide scientists valuable insights as to the early history and composition of our Solar System.
The debris trails left in the wake of short-period comets continue to orbit the Sun, each particle, or meteoroid, in its own orbit, which is very similar to that of the comet. Over long periods, these particles spread out around the comet’s entire orbit. In certain cases, these elliptical rings of particles intersect Earth’s orbit. Our home planet passes through these so-called meteoroid streams on the same date(s) each year, resulting in a meteor shower where significant numbers of meteors appear to radiate from one area of the sky. Each such meteor is visual proof that the bombardment of Earth is an ongoing, dynamic process.
Although we have learned a great deal about our neighbourhood the discoveries continue to mount at an ever-increasing rate. The first three months of 2004 have seen three successful missions to Mars, including two rovers and one orbiter; an extremely close flyby of Comet Wild resulting in the most detailed photographs of a comet ever taken and collection of sample particles for return to Earth in 2006; the closest recorded ‘near miss’ of an asteroid yet predicted by astronomers; the discovery of Sedna, the largest object found in our solar system since Pluto. In the coming months Project Cassini will arrive at Saturn for a detailed survey of the ringed planet, sending a probe down into the dense atmosphere of its mysterious moon Titan. And thanks to the Internet, we have unprecedented access to each new find. Nearly 50 years into the Space Age, it remains an exciting time for those interested in the Big Picture.