Clues from Meteorites

The age of the solar system, derived from the study of meteorites (thought to be the oldest accessible material around) is near 5 billion years; that of the Earth is taken as 4.6 billion years. The oldest rocks on Earth are dated as 3.8 billion years. Some of these ancient rocks already have signs of advanced life forms, so-called "chemical fossils", mineral matter that has odd properties thought to result from life processes.

Surely one of the more surprising observations in the natural world is that stones can fall from the sky. Most of them are very small and burn up in the atmosphere. At night, their trails can be seen as "falling stars" or "shooting stars", faulty folk interpretations preserved in language. If large enough, these particles can make it to the ground (or into the ocean) as small molten droplets of rock. These are quite well-known from deep-sea deposits. If larger, several cm in diameter, they can survive the fall as a pebble of original rock, with a glassy crust. Occasionally, meteorites are quite large. One of these made the Meteor Crater in Arizona (see photo above). Every year, it is estimated, about 10,000 tons of stone and metal rains down on Earth, almost all objects smaller than 1 mm in size.

What are these objects and where do meteorites come from? 

Meteorites can be made of stone or iron. In fact, iron meteorites were prized objects in the earliest days of civilization, as they delivered a workable metal much harder and tougher than copper or bronze. (This is due to the high nickel content; plain iron is much softer.) By far the greater portion of meteorites is of the stony variety. A good place to find meteorites is where people have not looked before and where stones are not normally expected to occur - namely on the ice covering Antarctica. Hundreds of meteorites have been recovered from that region since Japanese geologists first discovered the place as an ideal collecting station in 1969. Some of the fragments are thought to come from the Moon and even from Mars. But the bulk is thought to be leftovers from the time of the origin of the solar system, perhaps fragments from one or more planets, formed early during the history of the solar system and soon again destroyed by collision. Such debris is abundant in the "asteroid belt", located between the orbits of Mars and Jupiter. Others of the objects may be debris from disintegrated comets, as suggested by the periodicity in meteorite showers following the demise of certain comets.
As mentioned, many meteorites studied turned out to be very old, more than 4 billion years old in fact. They contain a memory, then, of the early days of the solar system. From the very fact that there are both stony and iron meteorites it can be deduced that they have a planet as a source and that one or more planets therefore had to form very early in the history of the system.

The reason is that a planet is needed to provide the gravitational force to separate the heavy metals (iron and nickel) from the accreted dust into a metallic core. The material must have been molten, at least in part, so any parent planet was hot. The energy of heating was provided by collision and contraction, and presumably also by internal radioactive decay. It has been suggested that there were still newly made radioactive elements around after a nearby supernova explosion, which could have delivered the necessary heat for melting rock. If this is so, planet formation must have started very early after the supernova debris gathered into a growing central body and its rotating disk in the first stage of solar system formation (the "solar nebula" stage).

Within this rotating disk there were preferred orbits, where rings of gas and dust could travel around the emerging star at the center, without having to leave because of gravitational disturbance from adjacent growing planets. Each ring eventually produced a planet, starting with Mercury. The young Sun had not yet found a long-term equilibrium; it burned hot and variably and with a strong solar wind. The gas in the inner rings was blown out to the outer ones, feeding the growing large gas planets there. The inner rings concentrated solids into large bodies, making the rocky planets we know. Some of these (Venus and Earth) were big enough to replenish gaseous envelopes from their rocky bodies, and hold on to their atmospheres despite the Sun's radiation.
Whatever planet (or planets) formed next to Jupiter and inside its orbit was doomed to failure, perhaps because of gravitational disturbances from this largest of all planets, leading to collision and break-up. The material remaining in this ring makes up the asteroid belt, with a mass about 2 percent of that of the moon. The largest object is the asteroid Ceres, which is a little less than 1000 km in diameter. The rocky objects in this belt have the familiar meteorite composition, as far as can be ascertained.