Solution Rocky Planets Are Rare

Here's metal more attractive.

William Shakespeare, Hamlet, Act 3, Scene 2

As far as we know, the only surviving witnesses to the birth of the Solar System are a group of metal-rich meteorites called chondrites. (Their name comes from the Greek word chondros, which means "grain" or "seed": it refers to the appearance of the many small spherical inclusions, known as chondrules, which occur within them. Chondrules are typically 1 to 2 mm in diameter, and are composed mainly of the silicate minerals olivine and pyroxene.) Using the known decay rates of various radioisotopes found in chondrites, we can calculate when these meteorites formed. The best estimates imply that chondrites formed about 4.56 billion years ago, which is the accepted age of the Solar System. Chondrites, it seemed, formed within the first few million years of the Solar System's history.188

figure 46 Chondrules are spherical inclusions of silicate in chondrites. Their origins remain a matter for debate. Chondrules are clearly visible in this cut surface of the AH 77278 chondrite. This specimen is 8 cm across.

Chondrites occasionally fall to Earth, and when they do they are studied intensively. Indeed, chondrites have been studied for over two cen turies, and much is known about their chemical and physical makeup. One thing remains mysterious, though: the precise nature of the chondrules.189

An embarrassing number of hypotheses have been presented to explain the enigma of chondrule formation. (A surfeit of hypotheses is a sure sign that we do not understand something. In the case of chondrule formation, this lack of understanding is not surprising. Not only did chondrules form a long, long time ago, but they appear in no other type of rock. Geologists have no other specimens with which to compare them.) The ideas range from the suggestion that chondrules are drops of lava ejected from extraterrestrial volcanoes to the suggestion that they form when lightning discharges through dust balls. All we know for sure is that chondrules must have been flash-heated to temperatures above 1800 K, and then cooled quickly. One interpretation is that, about 4.5 billion years ago, a brief flash of heat spread through the Solar System.

In 1999, the Irish astronomers Brian McBreen and Lorraine Hanlon proposed a new theory of chondrule formation: they suggested a gamma-ray burster (grb) might have been involved.190 Suppose a GRB occurred within 300 light years of the nascent Solar System. It would have pumped enough energy into the protoplanetary ring of dust and gas to fuse up to 6 x 1026 kg of material (100 times the Earth's mass) into iron-rich droplets, which would quickly cool to form chondrules. The chondrules would then absorb gamma-rays and X-rays from the GRB.

If McBreen and Hanlon are correct, then the Solar System could be a rarity in possessing chondrules. They estimate that, on average, only 1 star in 1000 would be close enough to the burster for chondrule formation to occur. The significance is that the high-density chondrules may have settled quickly into the plane of the protoplanetary disk and aided the formation of the planets. In other words, planetary systems like our own — complete with rocky terrestrial planets — would be scarce. And, with only a small number of Earth-like planets on which to develop, ETCs might be vanish-ingly rare.

The idea that chondrule formation was initiated by a GRB is interesting. However, other suggestions seem to offer more plausible mechanisms for making chondrules. Furthermore, these other mechanisms do not imply that there is anything particularly special about our Solar System. So, as a solution to the Fermi paradox, this is not high among the list of contenders.

A discussion of metal-rich meteorites brings to mind a related solution to the Fermi paradox that surfaces occasionally: perhaps planets with workable lodes of metallic ores are rare. The reasoning is simple: if alien intelligences cannot find and work metal, then they will be unable to develop technology — and thus will be unable to construct the radio telescopes or starships that would enable them to contact us.

This solution has been well examined by several SF authors. One group of authors has dismissed the suggestion in thought-provoking stories. Even if Earth's surface composition is unusual among planets (see page 183 for one reason why this might be the case), they believe this does not necessarily mean that technology is impossible. The technology would inevitably be different from ours, but the results might be the same. (For example, perhaps extraterrestrials produce electricity using biological means rather than generators?) A different group of authors — either less imaginative or more realistic, depending upon one's point of view — argue that technology cannot develop without the materials we take for granted.

We shall return to the issue of technological progress in a later section. However, whether or not technology is possible in the absence of metals (and this is something we may never know), it seems perverse to attempt to resolve the Fermi paradox by supposing Earth is the only planet in the Galaxy with workable lodes of metallic ores. A scarcity of such planets may yet be another factor acting against the existence of ETCs, but surely this cannot by itself explain the silence of the Universe.

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