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An introduction to exoplanets
An introduction to exoplanets

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1  The first transiting planet – HD 209458 b

The idea that an exoplanet might be detectable when it transits its star was first suggested in the 1950s. This was proved in 1999 when HD 209458 b became the first transiting exoplanet to be detected, four years after 51 Pegasi b was discovered by the radial velocity method. HD 209458 b orbits the star HD 209458, which is just a catalogue number.

Like 51 Pegasi b, HD 209458 b is very unlike any planet in our Solar System. Its orbital period is just three and a half days so it orbits very close to its parent star. This means it’s extremely hot – like 51 Pegasi b, HD 209458 b is one of the new class of planets called hot Jupiters. Figure 1 is an artist’s impression of exoplanet HD 209458 b passing in front of its parent star.

An image of an artist’s impression of HD 209458 b.
Figure 1  Artist’s impression of HD 209458 b