In a previous post I talked about the probability of the existence of extraterrestrial intelligence and mentioned the difficulty of its calculation, as we don’t know of any planet where they exist, apart from Earth, and to calculate the probability of an event one must know the number of favorable cases and the number of possible cases. For extraterrestrial life, we don’t know either.
In another post I detailed the conditions that should be necessary if life similar to ours were to be possible on a planet similar to Earth. These conditions are many, which reduces the probability that we will find life on some extrasolar planet located in our vicinity. In fact, among the almost 10,000 planets detected so far (of which just over half have been confirmed), 65 are at a distance from their star that could be favorable for life (the Goldilocks zone), but only three of them orbit around stars similar to the Sun (of the stellar class G).
Various measures
of the probability that an extrasolar planet is favorable to life have been
developed, known by many different acronyms: ESI (Earth-similarity
index), SPH
(standard primary habitability), and at least three others, none of which
are complete.
One of the pieces
of information that is often missing when calculating the plausibility of life
existing on an extrasolar planet is the age of the star. This is important,
because a star that meets all the requirements for the existence of life, but
is very young (a few hundred million years old) won’t be old enough for life to
appear on any of its planets. And things get even more complicated if we are
trying to assess the plausibility of the existence of extraterrestrial
intelligence.
Let’s look at what
is known about the history of life on Earth. Remember that the age of the solar
system is about 4.65 billion years.
|
Life levels |
Appeared million years ago |
% Earth age |
|
3500 |
75,3 |
|
2000 |
43,0 |
|
1000 |
21,5 |
|
1 |
0,02 |
I know it is risky
to take as a basis the data of the only case we know, but we have no others.
Let’s see what this table tells us:
- Prokaryotes (bacteria and archaea)
have existed on Earth for three-quarters of its history. It is therefore
to be assumed that on any extrasolar planet that meets the minimum
conditions for the apparition of life, life at the bacterial level could
have appeared on that planet if the star is at least 1,000 million years
old.
- Assuming that life has appeared on a certain
extrasolar planet, it could have reached a level similar to that of unicellular eukaryotes if the conditions
necessary for their existence have been maintained for at least 2,500
million years.
- Multicellular life may have appeared on that planet, if the age of
the star in question is greater than 3,500 million years.
- Finally, we shouldn’t expect intelligent life to have appeared on a
star whose age is less than 4,500 million years.
 |
| Enrico Fermi |
The problem is, it’s
not easy to calculate the age of a star. Various methods have been proposed.
One of the most promising is gyrochronology, which
is based on the observation that the speed of rotation of a star depends on its
age: younger stars rotate around their axis faster than older ones. Thus, the
equator of the Sun, which is about 4.65 billion years old, rotates on its axis
in 25-28 days; while the stars in a star cluster about 1 billion years old
appear to rotate in 10-18 days.
Given the great
variety of planetary systems discovered so far (just over 4,000), of which only
56 have planets in the Goldilocks zone, the probability of finding
extraterrestrial civilizations close to us is very small. If we do find life,
it is most likely to be life at the bacterial level. And we must not forget Fermi’s
paradox.
Life in other worlds: Previous Next
Manuel Alfonseca
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