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The question How has the number of known TNOs evolved over time? contains a link to this image:

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Number of extrasolar planet discoveries per year through April 2018, with colors indicating method of detection

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In order to be sure, I downloaded the latest catalog (as of October 4, 2021) of the Extrasolar Planets Encyclopaedia, containing 4840 entries, and plotted the number of exoplanets by year of discovery:

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I think I understand why the number of exoplanet detection is declining now: we probably discovered most objects detectable by our current technology. But I can't figured why there was such a gap in 2015. Was the main detection tool broken? Or is it 2016 which is exceptional due to the launch of a new instrument?

Why was there a gap in exoplanet detection in 2015?

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The spike would be due to the release of a "final" catalogue of Exoplanet candidates discovered in the main Kepler mission, which was issued in July 2015. This list was vetted and a catalogue of about 1300 very likely exoplanets was released by NASA in May 2016.

Observations of the Kepler main field ceased much earlier in 2013, but after that the K2 mission observed a number of fields around the ecliptic plane, starting in March 2014. The Kepler K2 data are much less sensitive than the original Kepler data and the planetary candidates have been released in a more drip-by-drip mode.

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  • $\begingroup$ So it is more a spike in 2016 than a gap in 2015, thanks! $\endgroup$
    – Jean-Marie Prival
    Oct 5, 2021 at 8:07
  • $\begingroup$ The linked press release states that Kepler monitored 150,000 stars in four years, and that "in 2018, NASA’s Transiting Exoplanet Survey Satellite will use the same method to monitor 200,000 bright nearby stars and search for planets". So I guess the follow-up question is: why the number of planet discoveries didn't go back up since TESS launch? Do closer stars have less exoplanets than distant stars? Or do the data still awaits validation? $\endgroup$
    – Jean-Marie Prival
    Oct 5, 2021 at 8:10
  • $\begingroup$ @Jean-MariePrival TESS should eventually reveal more exoplanets than Kepler. There have indeed been many TESS-discovered planets, but I guess no big published list yet. $\endgroup$
    – ProfRob
    Oct 5, 2021 at 9:43

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