2.2  Selection effects – transits

It turns out that the probability of any particular planet in orbit around any particular star being seen to transit is generally pretty low. It’s nearly 12 per cent for a planet like HD 209458 b, which is very close to its star. However, if any aliens are out there looking, there’s only a 0.46 per cent chance that the Earth would transit the Sun from their perspective.

We must take this into account when we try to understand the numbers of planets found by Kepler. Scientists have done this by looking at the numbers of different kinds of planet Kepler has found, then working out how likely Kepler would be to detect this kind of planet.

It’s not just the likelihood of the planet transiting that matters. Kepler’s ability to detect a transiting planet also depends on the size of the planet, the luminosity of the star, the period of the planet and the distance to the star.

• The bigger the size of the planet relative to the size of the star, the larger the transit depth. Deep transits are easier to measure than shallow ones.
• For a more luminous star, Kepler is able to collect more light. This allows for more precise measurements of a transit. Big, massive main sequence stars are much more luminous than small, low-mass main sequence stars.
• Planets with longer periods have less frequent transits – there is only one transit per orbit. This makes them more difficult to detect.
• For a more distant star, Kepler collects less light and the precision of its measurement goes down.

All of these so-called selection effects combine to allow the Kepler team to work out the likelihood of detecting any given planet. A selection effect is just anything that makes it more likely to discover one type of thing than another type, in this case types of exoplanets. Selection effects need to be understood and accounted for in most areas of astronomy, and indeed in many other fields of research.

Together, all this means that it’s much easier to detect large planets in close orbits around large, bright stars than any other kind of planet. Small planets in wide orbits are the hardest planets to discover using the transit method.