Number of candidates and confirmed exoplanets

Number of exoplanets

What is the difference between a candidate exoplanet and an confirmed exoplanet? The discovery of exoplanets began in 1990 and since, thousands of exoplanets are referenced.
An candidate exoplanet is a likely planet discovered by the different instruments, but the information must yet to be verified. Since the beginning of the Kepler mission in 2009, the list of "candidate exoplanets" became very long. Kepler has made ​​thousands of discoveries probable exoplanets, but that each planet is considered "confirmed" its existence must be verified using other instruments, a process that takes time. As well the Kepler team and other researchers around the world sift through the huge collection of data from the Kepler mission. Among this collection, it is possible that some candidates turn out to be "false positives."
A confirmed exoplanet is thus a planet validated by multiple observations and several different instruments so that astronomers have a high degree of confidence. Sometimes, new data cause the delisting of a planet confirmed but it is a fairly rare phenomenon. Against certain findings that define exoplanets in the beginning, may later prove to be the representation of other cosmic phenomena. However, it is very likely that the vast majority of candidates identified by the Kepler telescope, are true exoplanets, especially those located in multi-planet systems.
To what extent can we be confident?
Exoplanets are so difficult to spot that astronomers must bear a special attention to all sources of errors that might creep in their observations. They also need to calculate the probability of inaccuracy of their observations.

Usually, the level of confidence in a particular result is expressed in a figure of probability. For example, a group of astronomers could find a possible exoplanet and calculate the probability of error of this discovery to 5%. In other words, they are only 95% sure that their discovery is correct. This level of accuracy (known as "two-sigma") is generally not sufficient for a planet is considered "confirmed." Generally, a new discovery must have a minimum confidence level of 99.9999% to be considered a discovery "confirmed". In summary, scientists do not tolerate only one error in a million. This level of confidence is referred to as "five-sigma". However confidence levels are based on all known sources of error. An unknown error by definition is not included in this level of confidence and thus it is possible to find a flaw in their method of calculation or their instrument. This would make their previous level of confidence inaccurate.
Since scientists know detect exoplanets, they are looking for worlds similar to our Earth because the ultimate goal is of course to find in the Universe, favorable conditions for the emergence of life, to resolve the agonizing question that disturbs humanity has since forever, "Are we alone in the universe?".
Each discovery brings us one step closer to that goal, it is now a matter of time before we know if our galaxy is full of planets like Earth, or if we are a rarity.

Image: Kepler-62 system, this diagram compares the planets of the inner solar system to Kepler-62, a five-planet system, located about 1,200 light years from Earth discovered in 2013. Scientists do not yet know if the planets located in the habitable zone of this system have a predominance rocky, gaseous or liquid fully composition. But it is possible that the atmosphere in these worlds there is a life. Credit image: NASA Ames / JPL-Caltech .