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Number of candidates and confirmed exoplanets

Number of exoplanets

 Automatic translation  Automatic translation Updated September 03, 2019

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.

 Kepler-62 exoplanets in the habitable zone

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 .

Planetary systems Candidates Confirmed Date
3 003 4 026 4 044 Sep 03, 2019

The circumstellar habitable zone or ecosphere is a theoretical sphere surrounding a star where the temperature on the surface of planets in orbit, allow the appearance of liquid water. Liquid water is vital, because of its role in biochemical reactions. It has properties that are a catalyst for the production of chemicals necessary for life.
Frank Drake calculated the distance of this area in its equation of 1961. This equation takes into account the size, the brightness of the star and the luminosity of the sun.
A star that has 25% of the luminosity of the Sun will have a habitable zone centered at about 0.50 AU. A star that has twice the brightness of the sun, will have a habitable zone centered at 1.41 AU.
A habitable zone (ZH) does not mean that it contains within it life, but there is a possibility depending on many other factors, that a terrestrial planet hosts life.
For example, Earth is home life, but the Moon, located in the same area, is sterile.
The planet candidate in the development of life, must be massive enough to prevent water from escaping. In addition, the space area is not a fixed area, it changes according to the temperature of the star, such as during their evolution the stars becoming brighter and more hot, the habitable area s logically away from the star. A planet will therefore remain as long as possible in this area, to develop all the molecules necessary for life form.
To determine the habitability of a planet, we must not look all conditions met our Earth, or no other planet in the universe might be harboring life, each planet is unique. Only the condition of having liquid water seems necessary, it is considered an essential element for a viable ecosystem because it helps tremendously transporting materials necessary for biochemical activity.


Water is a perfect item to dissolve the materials it stores very well the chemical elements.
So life may be present outside the habitable zone, just that there is liquid water on the surface or depth of a terrestrial planet, a source of energy and chemicals whose life needs to build its materials.
Astrobiologists believe that life forms may exist on other objects in our solar system like Europa, a Galilean moon of Jupiter at 500 million km from Earth. Europe is a covered ice ball the size of our Moon and has a sea of tens of km deep, liquid held by the energy of Jupiter's tidal forces. The friction caused by this stretching causes sufficient heat to maintain liquid water below the frozen surface.
The search for Exoplanets began in 1990.

July 20155 5833 7041 879

Size Breakdown 
Stars with Planets1 167
Multi-planet Systems471
Gas Giant465
Hot Jupiter1 098
Super Earth210
 circumstellar habitable zone or ecosphere

Image : The ecosphere or habitable zone of a solar system based on its brightness and its mass. Life can still be present outside the habitable zone, just that there's liquid water, a source of energy and chemicals whose life needs to build its materials.
Find the liquid water is finding the life...


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