How to find intelligent life elsewhere in the universe

In the 19th century there was a proposal to let martians know there was intelligent life on Earth. At the time it was widely accepted Mars was inhabited and some of that life had to be intelligent. Our telescopes showed polar caps, which melted in spring, launching a wave of darkening of the desert that diffused down towards the equator. This was interpreted as vegetation growing as meltwater reached it. Then of course there were the canals, mapped by Lowell, which were suggested to be massive engineering projects by technically advanced Martians to manage the declining resources of a dying world. Those canals were later proved to be due to a combination of poor observing conditions and wishful thinking.

The plan was to dig a trench in the Sahara desert, marking out a huge right-angled triangle. The trench would then be filled with kerosene and ignited. It was hoped that the Martians would see this through their telescopes and interpret that fiery triangle as an indication of intelligent life. After all, huge right-angled triangles rarely if ever turn up naturally. The underlying idea about the triangle was probably sound, because even alien creatures, living under very different conditions, are likely to have geometry in common.

Our ship is entering the planetary system of another star. Is there life on any of the planets? Is any of it what we would call “intelligent”? How about searching for radio signals? Anyone coming into the solar system would detect the radio signals radiating out from Earth. However, if we detect none, this is not the end of our search.

On our world the use of high-power transmitters for broadcasting is gradually being replaced by low-power networks, fibre links and so on, and using methods of encoding the signals produce signals that just happen to resemble the naturally-occurring emissions produced by cosmic objects.

Maybe squirting radio signals in all directions is just one phase in the evolution of an intelligent species. Eventually it moves towards more efficient systems. In addition, intelligent creatures living in a conductive fluid like water would not have much use for radio.

As we get closer we can examine the individual planets. Do any of them have atmospheres? Once again we cannot assume an atmosphere is a necessary ingredient in the recipe for life, but here on Earth it plays a critical role in protecting us from harmful cosmic radiation and is a prime mover of water vapour and other things around the planet.

All of the known planets in the solar system except Mercury have atmospheres of some kind. These do not automatically indicate life, but they provide a means of searching for it. If we look at our planet’s atmosphere from somewhere out in space we will notice about a fifth of the atmosphere is oxygen. This is a tremendously reactive gas and will combine with minerals to form oxides, and if any hydrogen, methane or other hydrocarbons are present, or maybe ammonia, as we see in Jupiter’s atmosphere, oxygen would combine with them making water and carbon dioxide.

Oxygen is present in our atmosphere because it is being continuously topped up by plant life. Another highly reactive gas is chlorine. This is a poison to us, but could fill the role of oxygen. However, oxygen is a common element so it is probably the most likely candidate.

As we get closer to the worlds we have chosen for closer examination, assuming we have not spotted activity, and are not being shot at, we can look for lines, grids, circles on the ground. These indicate cities and infrastructure. We might see structures like the Great Wall of China, maybe even an occasional huge, right-angled triangle.

———

• Venus lies low in the sunset glow.

• At the same time Jupiter and Saturn are rising in the southeast.

• Jupiter is very bright and dominates the southern sky.

• The moon will be new on the 6th.