2.4  But what is it really like?

There is a lot of speculation about conditions on Proxima b. Strictly speaking, all we really know for certain is its orbital period, its minimum mass and how much starlight it receives.

As soon as the 2016 discovery of Proxima b was announced, astronomers started observations designed to search for transits. The probability of transit is increased by the closeness of the planet to its star, but it would still be very lucky indeed if the orbit happens to line up in this way. Because the star is so small, there is still only a 1.5 per cent chance that the orbit will be aligned such that we see transits. Returning to our hide-and-seek analogy from Week 5, the situation with Proxima b is like hiding behind a slender tree: your alignment needs to be exact.

It would have been exciting if Proxima b did transit. We would then be able to measure the size of this potentially habitable planet and hence its average density and likely composition. If a transit is discovered, all the telescopes we have will be trained on Proxima Centauri to measure the transit depth at all possible wavelengths. This will allow astronomers to discover whether Proxima b has an atmosphere and, if there is an atmosphere, what atoms and molecules it contains. At the time of writing (February 2019), it seems that the orbit of Proxima b is not favourably aligned for it to transit its host star: astronomers would probably have detected the transit by now.

However, there are other nearby stars.