The age-old question: Is there life on other planets?

One of the planets being studied with the James Webb Space Telescope is about the size of the Earth.

However, it lies so close to its star that it only takes four days to complete each orbit. Our planet takes a year to move round the Sun. Mercury, the closest planet to the Sun, takes 88 days.

That distant world is almost certainly a ball of lava. The reason for the interest in such a world is to learn more about how planets evolve. If there is any sort of life there, it will be so utterly bizarre it would be hard to work out how to look for it.

Thousands of worlds have been found so far, so surely, if life colonized our world, there must be other worlds out there with living things on them.

Since we use radio waves for many things, then maybe technically advanced civilizations on other worlds may do the same thing. Even if they are not transmitting “we are here" signals, their radio transmissions for local use will be radiating off into space.

Our uses of radio are so universal that the leakage of those signals out into space makes our world the brightest planetary radio source in the Solar System. Any visiting alien spacecraft should be able to pick up our signals and determine which world to visit - or to avoid.

However, life has been present on our world for over 3.5 billion years, but we have only been using radio for a century or so. Moreover, intelligent marine life on a water world would have little use for radio. We need to come up with other search methods.

Even through the biggest telescopes, most stars just appear as tiny dots. The main way we search for planets is to monitor the brightnesses of as many stars as possible, looking for tiny dimmings as their planets move between the stars and us. A downside of this search method is that it works really well at detecting big planets orbiting close to their stars, but less well for small planets like ours, orbiting further out, where they won't fry.

So our list of planets we have found orbiting other stars almost certainly does not give us a complete picture of the population of planets out there.

Despite this difficulty, we are finding planets where liquid water can exist on their surfaces. That seems to be a key need for life as we know it. But what about life as we don't know it? On Titan, the largest moon of the planet Saturn, temperatures are so low that water is a solid, rock mineral. Instead there are rivers and lakes of liquid methane and other hydrocarbons, sustained by liquid methane rain. Could there be some form of life there? We need some more general ideas as to what is evidence of life.

On our world we have weather, with water circulating from clouds to land, to the sea and back again, moving nutrients around. Also, on the whole, conditions are stable enough to allow living things to survive and proliferate. This would apply to living things everywhere, even creatures living in thousand-degree plasmas in magnetic fields. So moderately dynamic worlds are a good bet.

Another possibility is to search planets for things produced by life processes. On our world, oxygen is present because of plant life, and the only reason there has been oxygen to breathe for all the time life has been present on our world, is that the plants are continually topping up the supply.

Even though we are seeing other worlds only when they pass in front of their stars, by studying the changes in the starlight as they do, we can detect their atmospheres and search for chemical signatures of living things.

We can also get an idea of how stable or dynamic those worlds are. So we have what we need to make a good start in finding out if our living world is alone in the universe.

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• Just after sunset, Venus shines brightly, low in the south-west, with Saturn, much fainter, nearby.

• Jupiter lies in the southern sky, with Mars high in the south-east. The Moon will reach its first quarter on Jan. 28.

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