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An introduction to exoplanets
An introduction to exoplanets

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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.

An image of the edge of the Earth, with moving arrows above it, each going in circular motions.
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).

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.