Will we recognize alien life if we find it?

In searching for life on other planets, including on planets orbiting other stars (exoplanets), we are faced with some serious problems.

In the case of exoplanets, we are forced to observe from a great distance. Even at the speed of the Parker Solar Probe, the fastest moving spacecraft we have ever made, it would take us around 6,500 years to reach the nearest star. Looking at exoplanets for signs of life is being done using our telescopes, on the ground and in space. For the planets and moons in our Solar System, we can land spacecraft on them or orbit around them.

In many cases, it isn't out of the question that at some point in the next few years, space-suited feet will be walking on them. The exceptions will be where the conditions are just too extreme, such as on Venus or in orbit around Jupiter. This brings us to the biggest question. What are we looking for?

If we receive signals from distant worlds that are obviously produced by intelligent beings, or when we land on a planet and we see things crawling around, or fossils of possible extinct things that once crawled around, we have an answer. However, the search for alien life is likely to be more complicated than that.

Using what we see around us on our own world will be a help, but we have to be very careful not to adhere too closely to what we see here as an indicator of what extraterrestrial life, or evidence of it, will be like.

Since life appeared here on Earth around four billion years ago, it has gone through a long series of extinctions, where large numbers of species vanished. Those events were caused by environmental disruptions due to tectonic and volcanic activity, and the odd asteroid impact, After things recovered following the extinction, the survivors expanded and diversified to fill the ecological void, only to encounter the next extinction.

An example is some 250 million years ago, at the end of the Permian Period, more than 90% of animal species were wiped out, making room for the age of the dinosaurs to begin. That age ended around 65 million years ago, at the end of the Cretaceous Period, in an extinction where around 75% of species were wiped out.

The survivors of extinctions were not always the smartest. They were often just lucky. That led to humans, and all other living things, sharing this planet, the result of many throws of the genetic dice.

Other worlds will have their own throws of the dice, so there is little chance beings on those worlds will be like those living here, or that any intelligent creatures will look anything like us, or share our ideas and attitudes. Beings living in water would know little about fire and would have real problems with electricity and related technologies.

How then are we to search for extraterrestrial life?

One idea being followed is to look for things that living things introduce into their environments. In the case of our world, the answer is oxygen. This highly reactive gas does not occur naturally, and moreover, will react with minerals, and vanish quickly if not kept topped up.

The oxygen we breathe is a by-product of photosynthesis in plants. Finding oxygen, or other highly reactive gases such as chlorine in the atmosphere of a planet, would be a strong indicator of life being present.

Life as we know it is made up of complicated molecules. As far as we know, only living things make such molecules and as we live and die, those molecules find their way into the environment.

We know, for example, soil samples from the Moon contain no large molecules, whereas a sample of soil from our world would be loaded with them. We are searching for such molecules in the atmospheres of exoplanets, the soils of Mars and hopefully soon, in the under-ice oceans of Europa and Enceladus.

Of course, knowing life exists might not mean we will recognize that life when we see it. Have you ever seen a sea cucumber?

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• After sunset, Venus shines low in the southwest with Saturn close by. Jupiter lies high in the southeast and Mars rises in the northeast.

• The Moon will reach last quarter on Jan 21.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.