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An introduction to exoplanets
An introduction to exoplanets

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1  What is a planet?

What is a planet? Most people, if asked, can name at least one planet: the Earth (Figure 1).

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Figure _unit2.1.1 Figure 1  The Earth as seen from the Apollo 17 spacecraft

The word ‘planet’ comes from the ancient Greeks who observed ‘wanderers’ in the night sky – bright star-like objects which gradually moved against the background pattern of stars. We now have the ability to view them as much more than just points of light.

Occasionally, the media showcases stunning photographs of planets, such as the one of Saturn, taken by a spacecraft (Figure 2).

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Figure _unit2.1.2 Figure 2  ‘The Day the Earth Smiled’. This is an annotated image of the planet Saturn and its rings taken by the NASA Cassini spacecraft in 2013. The rings are backlit by sunlight, and the Earth is just visible, appearing between two of the rings in the bottom right corner, you will need to click to view a larger version of the image [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] to see it.

Some of us were lucky enough to catch one of the recent ‘transits’ of the planets Venus and Mercury. In a transit, a planet passes directly between the Earth and Sun, appearing as a dark spot silhouetted against the Sun’s bright surface.

Figure 3 shows the latest transit of Venus across the Sun, which took place in 2012. Venus can be seen as a round black spot towards the top right on the Sun. This image was taken by the NASA Solar Dynamics Observatory with a particular colour filter that is sensitive to the Sun’s outer atmosphere.

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Figure _unit2.1.3 Figure 3  Transit of Venus