The evidence from planet K2-18b suggesting it may be teeming with microbial life is tantalising

The building blocks for life are thought have been brought to Earth by asteroids that bombarded our planet long ago.

But the amino acids that form proteins, the ring-shaped molecules that make up DNA and other organic compounds are just chemistry.

It’s in the oceans of the early Earth that they became biology.

Exactly how isn’t known. But life evolved.

And if it did so once, could it have happened again?

The evidence from planet K2-18b is tantalising.

Analysis of how its atmosphere absorbs light strongly suggests the presence of chemicals which on Earth are produced by ocean plankton and other micro-organisms.

The finding isn’t yet certain, and it only indirectly supports the notion that the planet harbours life, but it is the strongest sign yet that we are not alone in the universe.

So far, more than 5,500 planets have been found outside our solar system.

Many are in orbits around stars that would put them in the Goldilocks zone – neither too hot, nor too cold, to have liquid water. In other words, habitable.

But if life evolves by chance, then it’s a numbers game.

Modelling suggests that there could be 300 million planets or moons in our galaxy alone with conditions suitable for life.

That’s 300 million opportunities for biology to get going.

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And if the right conditions are sustained, then there may have been time for intelligent life to evolve, too.

Astronomers scan the skies for radio signals or other signs suggesting alien civilisations. But there’s nothing yet.

Maybe the lifeforms are smart enough to stay quiet. Or maybe we’re looking in the wrong place.

But the odds are we’re not alone.