SOME astrophysicists try to understand the composition of planets that go round other stars — so-called “exoplanets.” They do so by looking at how light from the star is absorbed as the planet orbits around it, which allows them to find out the compounds that make up the planet’s atmosphere.

The technique allows scientists to understand something about these distant planets, though they are separated by many light years.

Some scientists believe that by identifying specific molecules in the atmospheres of these planets, we will be able to observe the signature of life. These signatures would be “biomarkers” — molecules that can’t be produced by simple chemistry, but can only be made by living organisms.

The difficulty in assessing these molecules and the remoteness of the planets make others sceptical that any biomarker would, by itself, ever be enough to categorically say that life exists.