In reference to the Kepler 22b news: The Kepler team had to wait for three passe

Question

In reference to the Kepler 22b news:

The Kepler team had to wait for three passes of the planet before upping its status from "candidate" to "confirmed".

This is possible because the planet has a orbital period ('year') of 289 days, and so it has transitioned more than once (as above) since the Kepler mission started.

However I've seem some confirmed exoplanets that have orbital periods in 10s and 100s of years, and therefore could only have been detected once by Kepler.

As far as I'm aware all methods of discovering exoplanets require the planet to pass in from of their parent star (i.e. we can't use a telescope to 'zoom in' and see them directly)?

So how are exoplanets confirmed, when we haven't been searching long enough to cross in front of their parent star? How long does the transition even last?

Explanation / Answer

Though the definition of "confirmed" is not a firm one, and is really an operational matter of peer review and consensus among researchers, the most widely accepted method of confirmation is high-resolution radial velocity analysis: the observation of shifts in the spectrum of the parent star as it is alternately "tugged" towards and away from an observing telescope by a candidate planet.

The observations used for such analysis require large and sophisticated optical telescopes with sophisticated spectrographic equipment, so that only a relatively small number of the hundreds of exoplanet candidates detected by transit surveys such as the Kepler Mission can be confirmed in this way. Additional confirmation methods include observation of additional transits, and (in rare cases) direct imaging. As the questions correctly notes, observation of multiple transits is constrained by their frequency, so that planets with very long orbital periods require a (potentially quite long) time to confirm in this way. In practice, these multiple lines of evidence (along with careful modeling, knowledge of stellar history and variability, etc.) are taken together to build a case for the "confirmation" of an exoplanet.

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