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.

3.1  Measuring a transit

In Figure 12 (repeated below) you saw how a transiting planet changes the measured brightness of its star as it blocks some of the star’s light. But what exactly do astronomers need to do to detect a transiting planet?

Described image
Figure _unit5.3.1 Figure 12 (repeated)  Schematic of a transit and the measured light curve

The basic technique for detecting a transiting planet is to observe a star over a period of time and measure how bright it is. If a dip in brightness, such as that shown in Figure 12, is observed, then a planet may have been detected. Planets orbit regularly, so if the dip is caused by a transiting planet, it should also recur regularly. And the time between dips should be of equal length: it’s the time taken by the planet to complete each orbit.

The dip in brightness needs to have the specific U-shape shown in Figure 12 to be caused by a transiting planet. V-shaped dips are more likely to be caused by a pair of stars orbiting each other, just eclipsing each other’s edges. Dips with other shapes could be caused by spotty regions on the surface of a star coming into view, like the sunspots you saw in Figure 4. Unfortunately, there are lots of things that can make a star appear slightly less bright.

You’ve learned that planets are much smaller than stars, so when a planet transits it doesn’t block out much starlight at all. Jupiter would only block out about 1 per cent of the light of the Sun if it were possible to observe its transit from afar. Fortunately, it is not particularly difficult to measure the brightness of a star to within 1 per cent. In fact, there are quite a few amateur astronomers who are now doing this as a hobby.

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