Probabilities in the Galaxy
A Distribution Model for habitable Planets
Copyright © Klaus Piontzik Claude Bärtels

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10 – The Drake-Seager-Equation

10.1 - The Equation from Sara-Seager

Sara Seager Sara Seager [1] was a Canadian-American astrophysicist (*1971 – †2011). She introduced a modified approach to the Drake equation.
This approach is called the Drake-Seager equation.
In contrast to the Drake equation, their approach does not work with the star formation rate, but with a fixed set of stars., namly systems from the spectralclass M.

Seager's approach is limited to the so-called M-stars, also called red dwarfs, and the future James Webb Space Telescope (JWST), [2] and the planned TESS space probe (Transiting Exoplanet Survey Satellite). [3]


The James Webb Space Telescope is virtually the successor to the Hubble telescope and is scheduled to be launched in 2018. The TESS satellite is to go on the journey in june 2018 and look for exoplanets using the transit method. (Sarah Seager uses other indices than defined in Definition 2.7.2)

The Drake-Seager equation is:

10.1.1 Equation N = N* · fQ · fHZ · fO · fL · fS

The following values for the probability factors are taken from a document that Sara Seager has placed on the Internet.[4]

N* stands for the number of M-stars (Red Dwarfs), which can be examined with the coming telescope JWST.(30,000 - 50,000)

fQ stands for the proportion of quiet M-stars. The amount of stars, which repeatedly throw large amounts of gamma rays into space is 20 %.

fHZ is the proportion of those systems that have a planet in the habitable zone. (cirka 15 %)

fO quantifies the number of planets that are visible for the JWST visibly past their star (1% of the potentially observable planets pass before their star, 10% of which are close enough to the earth for observation) (0.01 x 0.1= 0.001)

fL represents the share of planets with life. The factor is set to one here because one assumes that life could be created on every habitable planet.

fS s a measurable biosignature, in the atmosphere. (50 %)

Substituting the values in the equation 10.1.1 yields

N = (30,000 bis 50,000) · 0.8 · 0.15 · 0.001 · 1 · 0.5
N = 1.8 – 3 technological civilizations

According to Sara Seager, N = 2. This result shows that intelligent life is also possible in the case of red dwarfs, as the central star.


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