We are familiar with the Goldilocks zone, considered necessary for the emergence of life in a planetary system. It is the region where the surface temperature of the planet allows for the existence of liquid water, which is generally considered essential for life as we know it, although in science fiction literature (remember Fred Hoyle's The Black Cloud) there are life forms that might not require this condition.
Planets very close to their star would have a temperature too high for the existence of liquid water; all the water would vaporize, and in some cases escape the planet's gravitational pull. On very distant planets, the temperature would be too low, and all the water would be in a solid state (ice). In both cases, it is thought that the emergence of life would not be possible.
A planet being located in the Goldilocks
temperature zone would be a necessary, but not sufficient, condition for the
emergence of life. If a planet is too small, or is a gas giant, it is thought
that life, as we know it, would be impossible to find. The planet should be
similar in size to Earth, or slightly smaller, or slightly larger (a
super-Earth). This requirement defines a second Goldilocks zone that overlaps with the first, thus decreasing the
probability of finding life on extrasolar planets, of which about 6,000 have
been discovered, along with another 8,000 that await confirmation.
Of the 6,000 confirmed exoplanets, about
70 are located in the Goldilocks temperature zone of their star, meaning a
probability of around 1%. Of these, no more than 30 are also in the second
Goldilocks zone, related to size. Therefore, the combined probability of an
exoplanet being found in both Goldilocks zones would be approximately 0.5%.
A third Goldilocks zone has recently been detected (this time chemical,
unlike the previous two, which are physical), further limiting the probability
of finding extraterrestrial life. It is a balance between the proportions of
oxygen, nitrogen, and phosphorus on the planet's surface. These three elements
are considered necessary for life. Let's see how this balance operates:
·
On
a planet in process of formation, nitrogen and phosphorus can combine either
with oxygen or with iron. The
initial amount of oxygen is critical.
·
If
there is a lot of oxygen,
it will combine with phosphorus, which will remain in the planet's mantle and
the surface, while nitrogen will combine with iron and sink with it into the
planet's core, becoming isolated and unusable for life.
·
If
there is little oxygen,
the opposite happens: phosphorus combines with iron and sinks into the core,
while nitrogen remains near the surface.
·
Since
significant amounts of phosphorus and nitrogen are necessary for the emergence
of life, if either element is lacking, life is thought to be impossible. The
initial amount of oxygen, therefore, would be critical: it should be neither
too high nor too low. This leads us to consider the existence of a new Goldilocks zone.
The researchers who reached this conclusion
simulated the probability of an exoplanet's initial oxygen level falling within
the Goldilocks zone. Their conclusion: this probability is 10%. Therefore, the
probability of an exoplanet simultaneously being in all three Goldilocks zones
would be reduced to 0.05%, meaning that among the 6,000 planets we know of,
there would be at most three with the necessary conditions for life to arise.
To this, we must add other conditions, which are
also considered essential: a) that the planet should have an atmosphere; b)
that it should have a magnetic field that protects life from cosmic rays; c)
that, even if it’s located in the Goldilocks temperature zone, it must not always
present the same face to its star, which would cause the temperature to be too
high on one side of the planet and too low on the other; d) that, in the case
of exoplanets orbiting red dwarfs, whose Goldilocks zone is very close to the
star, the planet's orbit must be almost circular (so that it doesn't leave the
favorable zone), and the star must be very stable, so that it doesn't
occasionally scorch the planet (red dwarfs tend to be unstable).
The combination of all these conditions further
reduces the probability of finding an exoplanet with conditions suitable for
life, which would explain why we haven't found one so far. And extrapolating
this result to the existence of extraterrestrial intelligence, we could find an
explanation for the Fermi paradox.
When Drake proposed his famous formula for
calculating the prior probability of the existence of extraterrestrial
intelligence based on the product of seven terms, he estimated that there
should be around one million detectable civilizations in the Milky Way galaxy.
With the new data, one of the terms in his formula (ne, the number
of habitable planets per solar system) should be divided by 10. Several estimates
of Drake's formula calculate that the number of detectable extraterrestrial
intelligences in our galaxy is likely between 0 (we are alone) and 1 (we are
almost alone).
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| Drake's formula |
Thematic Thread about Life in Other Worlds: Previous Next
Manuel Alfonseca
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