3 The Milky Way variety pack
The results from Kepler and other projects have enabled astronomers to estimate the total number of planets in the Galaxy. They also allow us to estimate how many planets of each type there are. You saw in Section 2.3 how these estimates are made. A similar type of logic can be applied to samples of stars studied to look for radial velocity (RV) variations owing to planets. For RV measurements we don’t know the size of the planets discovered, but we do know about their mass.
Because Kepler has discovered many more planets than those discovered through work undertaken on any other project, it has the largest sample size to work with. Large sample sizes lead to more reliable statistics. So, we will mostly focus on the results from Kepler.
An example of Kepler’s predictions for the number of planets in different size categories are shown in Figure 3. These results are for planets with orbital periods less than 85 days, roughly that of Mercury orbiting the Sun. The numbers on the vertical axis show how many planets of a particular size there are for each star. So, for example, for each star there are 0.06 planets with orbital periods less than 85 days and radii between 2.8 RE and 4 RE, or equivalently, 6 such planets for every 100 stars. The vertical ‘error bars’ show the experimental uncertainty in the figures.
Kepler’s results have shown that small planets are much more common than giant planets.
The likelihood that small, Earth-sized planets are common should be good news for scientists who are interested in the possibility of finding another planet like Earth, or even one that’s inhabited – the more planets that are out there, the more likely it will be that we’ll find one. But finding Earth-sized planets is difficult, and space is really, really big.