A warm, wet world

Bizarre exoplanets — planets orbiting other stars — are interesting in that they help us understand how planetary systems form and how they work.

However, what we seek more than anything else are planets like ours: planets that could have life at least something like ours.

The sheer size of the universe compared with our speck of a world makes it logical that we are not alone, and a terrible waste of space and resources if we are.

One of the better candidates for an exoplanet bearing life as we know it has just been discovered. It has been named, not exactly creatively, as K2-18b.

It lies about 111 light years away; its surface temperature lies somewhere between 10 and 40 degrees Celsius and its atmosphere contains lots of water vapour.

It is little more than twice the diameter of the Earth, and has around eight times its mass.

On its surface we would weigh a bit less than double what we do on the Earth. It would not be healthy for us but we could probably tolerate it for a short while.

Of course, we did not evolve for life on such a world. If there are oceans on that world, things swimming in them would be almost weightless, just as they are here on Earth.

Planet K2-18b is orbiting a dim, red dwarf star. These stars are really miserly in the amount of energy they radiate.

The range of distances from a star where planets can have surfaces warm enough for liquid water to be present is known as the Goldilocks Zone. For dim stars, this zone is narrow and lies close to the star.

Planet K2-18b lies in the zone and is indeed close to its star, taking only 33 days to orbit it once. Our Earth's trip around the Sun takes a year.

Considering how far away this star is, and that nobody has actually seen the planet as even a dot in a telescope, how can we have deduced all these things?

The main way we detect planets beyond our solar system is to look for minute dimmings in the brightness of distant stars as their planets pass in front of them. Obviously this only works for systems where we happen to be looking in the plane in which the planets are orbiting their star. 

By measuring how long it takes to transit across the disc of its sun and the time between two successive dimmings, we get estimates of how far it is from its sun and how long it takes to complete each orbit.

In addition, from the time taken for the light to dim and then the time it takes for the starlight to rise again to its undimmed condition, we can deduce the size of the planet.

We can go further. Stars are very hot, so when we look at them we see bodies made of atoms, not molecules. Some cool stars have molecules in their atmospheres, but none that have any biological significance.

If the planet has an atmosphere, as it passes in front of its star, some of the light reaching us has passed through the planet's atmosphere, and in the process has impressed on it the signatures of what the atmosphere is made of.

In most cases, we can be sure that if we see signatures of molecules, those molecules are present in the planet's atmosphere.

We get a double check by analyzing the light of the star when the planet is not passing in front of it. The atmosphere of this new planet has a strong signature of water vapour, so life is a possibility.

With the new space telescopes coming on line in the next few years, it will be possible to dig deeper. Life on a planet changes its atmosphere; for example our planet's atmosphere is rich in oxygen.

This gas is so reactive it would vanish quickly if it were not continually topped up by our plant life.

It could be that alien creatures might not be fans of oxygen, but if we detect any unusual chemicals that would vanish if not continually produced, we could be well on our way to proof we are not alone.

• Jupiter and Saturn lie low in the southern sky after dark.
• The moon will be new on the 30th.

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