Observatoire de Paris
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Ground of Europa's satellite
The Ground of Europa looks like pieces of a jigsaw puzzle that could slide against each other. A possible explanation for this is that the icy ground covers an ocean of liquid water.
Copyright : NASA / GFSC
  • There are between 100 and 200 billion stars in the galaxy. The age of the galaxy is about 10 billion years. Therefore an estimate of the stellar formation rate is Fét = 10-20 / year (the current estimate yields less than 10). It has been almost constant for the past 5 billion years. This formation rate was the only term of Drake's equation well known until 1995..
  • The probability for a star to host some planets, Ppla, has been well estimated with the recent observations of extrasolar planets. Moreover, only 15% of the stars are isolated (that is, they do not belong to a multiple system, which is unfavourable for the stability of planetary orbits). We have observed that some isolated stars do not host any planets. Finally, we obtain the equation P_pla~=5*% for the stars of the "main sequence" (in a stable phase of their life). It is a major and recent result arising from the discovery of extrasolar planets. Unfortunately, Fét and Ppla are the only well-estimated terms of Drake's equation.
  • The stars can be divided into several groups, depending on their mass. The most massive become very bright but do not live for a long time. The less massive are not very hot or bright, and can additionally have a strong eruptive activity. The solar-type stars represent about 1% of the total of the stars. However, the range of "acceptable" stars (neither too massive, nor those without enough mass) can be extended to about 10%.
    The constraints on the planets (mass and distance from the star) depend on the conditions required for the emergence of life, and thus on the definition of the word "life". We can also restrict the range of acceptable masses : the most massive planets are gazeous giants and do not have any surface (Jupiter, Saturn, Uranus, Neptune), whereas Mercury and the dwarf planets like Pluto do not have any atmosphere (they are not massive enough). In the Solar System, the probability for the mass of a planet to be correct is 1/3.
    The distance of a planet from a star is constrained by what we call the habitable zone, where the presence of liquid water is possible. In the Solar System, the range of acceptable distances of planets from the Sun is 2%. Therefore, we consider at the most a distance of 0.5 - 2.5 UA, that is 4%. Concerning the observations, planets discovered in the habitable zone are very rare, and half of them have highly elliptical orbits, which is unfavourable because it creates high annual temperature variations.
    On the whole, we are still short of data (particularly on the low-mass planets) to be able to estimate precisely the number of habitable planets per star. However, it is possible that life appears on satellites of giant planets : this consideration should be included in the term Npla. In the Solar System, we think in particular about Europa (a satellite of Jupiter that could contain liquid water under an icy surface).
  • The probability for life or intelligence to emerge is much less known. From an optimistic point of view, life and intelligence could certainly appear as soon as the physico-chemical conditions are combined on the surface of the planet (P_vie~=P_int~=1).
    Several indications let astronomers think that life can emerge easily on a planet :
    - The presence of prebiotic molecules is detected in comets and in the interstellar medium. These molecules have formed the first living cells on Earth.
    - The first times on Earth were rough : the Earth, like the other planets, was permanently bombarded by planetesimals, the descendants of which are comets. When this bombardment stopped 3,6 billion years ago, the temperature went down on the surface of the Earth and, almost immediately, the first living cells appeared. The planets Mars and Venus knew the same conditions. It is therefore possible that this process occurred on these planets.
    However, it is also possible that exceptional conditions are required  : for example, Jupiter might have acted as a gravitational shield, preventing many comets from hitting the Earth. Without this shield, the surface of the Earth could have been covered by water, and could have been less favourable for the emergence of life. On the other hand, the Moon (which is relatively massive compared to the Earth) stabilizes the rotation axis of the Earth, and therefore its climate in the long term. The Moon also creates significant tides on Earth, which increases the liquid/solid exchanges.
    Therefore, Pvie and Pint are probably very small, but all of the estimates are possible.
  • Since 1/4 of humanity has spontaneously moved towards technology, we can use the approximate estimate of Pcom = 1/4.
  • Finally, T is totally unknown!
    Our technologic civilization (capable of communication with radio waves) is about 100 years old (although homo erectus appeared one million years ago). Will we still last millions of years, or will we disappear in a few centuries following a natural disaster, the destruction of the ecosystem caused by pollution, or a nuclear war ? A civilization could also shut itself off from the rest of the universe, and stop any attempt at communication.


An optimistic calculation yields Nciv = 20 x 5% x 0.01 x 1 x 1 x 1/4 x 108 = 250 000. A pessimistic one yields Nciv = 10 x 5% x 0.001 x 0.1% x 0.1% x 1/10 x 1000 = 5.10-8 ; In reality, Nciv should be equal to 1 because we are here, but then we'd be alone in the galaxy. There is still a wide range of possibilities. You can try various parameter values with the simulation below.


Click on the icon above to start the simulation.