5.2  Prevent the stars from twinkling

As you’ve seen in Section 2, hot Jupiter transits can be measured from the Earth with inexpensive equipment. However, there is a problem: the air we breathe interferes. Earth’s atmosphere tends to get in the way of light coming from space, especially if conditions are humid and there is a lot of water in the atmosphere. You can see this in action when you look at a bright star – the star often appears to twinkle. That twinkle is caused by atmospheric currents that cause the air you’re looking through to move.

The animation in Figure 3 shows how the motion of air currents in Earth’s atmosphere can cause stars to appear to twinkle – to apparently flicker or very slightly change their positions. As you might imagine, this effect makes it difficult to get a really precise measurement from the ground of how much light is coming from the star.

Figure _unit6.5.1 Figure 3  Why stars twinkle

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This is an image of the edge of the Earth, with moving arrows above it, each going in circular motions. There is a set of straight arrows coming from the top left corner, towards the Earth. This is labelled Starlight. There is also text above the arrows: moving pockets of air at different temperatures change the refractive index.

Figure 3  Why stars twinkle

Telescopes in space, however, don’t have this problem. Compare the light curve of HD 209458 b from the ground with the one taken by the Hubble Space Telescope (Figure 4).

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Figure _unit6.5.2 Figure 4  Light curves of HD 209458 b from space and from the ground

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This is a graph showing light curves of HD 209458 b from space and from the ground.

Figure 4  Light curves of HD 209458 b from space and from the ground

The right-hand panel shows the transit of HD 209458 b as observed from the ground, and the left-hand panel shows the transit observed using the Hubble Space Telescope. Observing from space allows much more precise measurements.