An introduction to exoplanets
An introduction to exoplanets

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

An introduction to exoplanets

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.

Described image
Figure 3  Planet population by size derived from Kepler discoveries

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.

EXO_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371