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.

2  Atmospheres of planets and their climates

If we can tell what stars are made of by looking for the missing colours, or wavelengths, of light in their spectra, then in theory we can do the same for planets. In both cases, what we can learn directly is restricted to the contents of the outer layers (i.e. the atmospheres of stars and planets). However, there’s one thing which makes applying this technique to planets really difficult. Stars are big and bright, so we can see them even when they’re a long way away, but planets are small and faint. It’s extremely difficult to detect the light coming directly from a planet, let alone split it up into all its various colours.

Instead, astronomers do something ingenious with transiting planets. When a planet transits, it blocks some of the starlight. You learned about this back in Week 4, and you also learned that the amount of light blocked tells you how big the planet is relative to the size of the star.

Remember, this is how the transit depth is calculated:

transit depth equation left hand side equals right hand side open cap r sub p divided by cap r sub star close squared

When you studied transits in Week 4, the calculations assumed that the planet was solid. A lump of rock is completely opaque: no light gets through it. But even a planet like Earth – a rocky planet – is more than just a lump of rock. Except for Mercury, the rocky planets in our Solar System are surrounded by a shell of gas called an atmosphere.

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