In the search for extraterrestrial life, we look for planets at a similar distance from their sun as Earth is from ours: the so-called Goldilocks zone. However, we are very fortunate to have the bounty of materials that exists in Earth’s crust, and our civilisation would not have developed without them. What are the odds of another planet being similarly equipped? It would need to have had a similar geological history to Earth’s. Is this likely?
• Planets form when supernova debris clumps together. Their compositions reflect the local mix of stellar residues and whatever they managed to retain in the face of heat and collisions while settling into an orbit. Stars that formed later in the galaxy’s evolution should host Earth-sized planets with Earth-like general compositions. They should retain a modest amount of volatile compounds, but be too small to drown themselves in troublesome gas-giant atmospheres.
Such bodies are hard to detect; we can barely guess at their frequency. But in our own solar system, three are crowded suggestively into near-Goldilocks orbital ranges (Venus, Earth and Mars).
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Any such planet necessarily begins as a fairly homogeneous midden of local supernova trash. Gravity, collisions and radioactivity produce enough heat to separate chemical and physical products, forming a core of molten heavy metals and a rocky mantle and crust. After that, geological chemistry, convection, volcanism and sometimes biology segregate minerals and metal ores.
We have no reason to suspect that Earth-like geology should not develop around numerous life-friendly planets – say a billion in our galactic disc. Nearly every life-friendly planet is probably mining-friendly as well, unless covered by ocean – an extra 10 kilometres depth of water would have sufficed to cover Earth.
Jon Richfield, Somerset West, South Africa
• There is another important point to consider. Many civilisations developed without exploiting heavy minerals or metals at all. The most striking example is the human migration into the Pacific.
Polynesians spread across the Pacific and built sophisticated societies using only the calcium minerals found in shell and bone. They had no significant hard woods and limited access to plants and animals.
Similarly in Australia, Aboriginal and Torres Strait Islander people survived for at least 40,000 years without exploiting metals and built a complex culture. When James Cook arrived from Europe, he was surprised to find the Aboriginal peoples uninterested in trading supplies for metal products and came to realise that their culture supplied them with all they felt necessary for a complete life. His metals, weapons and fabrics were seen as superfluous. Cultures with similar technologies also arose in the Amazon basin.
Stephen Johnson, Eugene, Oregon, US
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