 |
| Allen Telescope Array in the SETI project |
In its initial formulation, the weak anthropic principle says that, although the appearance of intelligent life on a planet may be very unlikely, the Earth must meet all the conditions, since we exist. We know that the Milky Way contains about 1011 stars. At least one (the sun) has a planet populated by intelligent life. It looks like the probability of this happening should be equal to or greater than 10-11. Note that the weak anthropic principle does not say what the value of that probability might be.
 |
| Frank Drake |
One of the first calculations of the
probability of the existence of intelligent beings in a galaxy was made in 1961
by Frank Drake. With his famous
formula, he estimated that in our galaxy there should be about 10
civilizations with which it would be possible to communicate. But the
uncertainty of the parameters of the formula is so great, that a later estimate
calculated that this number may actually be any value between 0 and 182
million.
Astronomers, geologists, and biologists
have studied the properties that a planet should have for the emergence of life
to be possible.
·
The planet should be at such a distance
from its star that its surface temperature is
neither too hot nor too cold. The zone compatible with life is
very narrow and is called the Goldilocks zone,
after the tale of the three bears. Mars, for instance, revolves around the sun at
a too far distance, so it is too cold; Venus, is too hot. The composition of
the atmosphere also influences the temperature, as in Venus and Earth.
·
It is believed that life can only appear if water is liquid, which
limits the temperature range between 0ºC and 100ºC, although a more practical
range would be between -3ºC and 45ºC. However, after life appears, some
organisms can adapt to regions with higher or lower temperatures: on Earth
there are living beings between -70ºC and 112ºC (see this
post).
·
The orbit of the planet should be
almost circular, for if it were very elliptical, the
planet could leave the Goldilocks area during some part of the year.
·
The planet should not be too big
or too small. A protoplanet with a mass ten times that of the
Earth would become, by gravity, a gas giant. And if it were too small, it would
have almost no atmosphere.
·
The planet should have appreciable
amounts of liquid water on its surface. The amazing
properties of water make it suitable as a basis for life, as I explained in my
book La Vida en otros Mundos (Life
on Other Worlds). It is highly unlikely that life would arise in
liquid methane or ammonia, as sometimes has been said.
·
The planet must be surrounded by an atmosphere of density not very different from ours.
Too dense an atmosphere will lead to a runaway greenhouse effect, as on Venus.
A thin atmosphere, as on Mars, will not protect life against radiations that
can destroy it. On the other hand, the existence of oxygen is not important: on
Earth, it is a consequence of life, not a condition for it to appear.
·
The planet must have a strong magnetic field (an iron core) so that
radiations are deflected before they reach the surface, where they would be
harmful to life.
· It seems convenient that the axis of rotation of the planet has a certain inclination
with respect to the perpendicular to its orbit, so there be seasons. Otherwise,
large areas of the planet could be unaffordable for life, because a perpetual
summer or winter would push them out of the proper temperature range.
·
The orbits of the largest
planets in the system shouldn’t be too elliptical. In
many extrasolar planetary systems, there is at least one giant planet with a
very elliptical orbit, which would prevent stable planets in the Goldilocks
zone, or push them away from that zone. Our planetary system is very stable
thanks to Jupiter, whose great mass (a thousand times less than the sun, but over
300 times greater than that of the Earth) and its almost circular orbit make it
the guardian of the solar system, preventing our being reached by many comets
and asteroids, which would cause catastrophic impacts.
·
The planetary system must have been stable since its formation, so
that the orbits of the planets do not get in and out of the Goldilocks zone.
This restricts life to the spiral arms of the galaxy, since in the center the
interactions of stars can cause alterations in planetary orbits.
·
The presence of the moon
may have made easier the evolution of life. See Isaac Asimov's article The Triumph of the Moon in the collection
The Tragedy of the Moon. In
proportion to its planet, the moon is the largest satellite in the solar
system: its mass, only 81 times less than that of the Earth, stabilizes
our rotation speed; the tides it causes made easier the passage of life
from the sea to the mainland; the moon is also a screen against
catastrophic meteor impacts.
Two additional properties with possible
anthropic importance have recently been added:
- From
the composition of white dwarf stars, it appears that the composition of rocky
planets surrounding them before they became red giants was quite different
from the Earth. See this
recent article. Perhaps the composition of our solar system is not
typical.
- Simulations
of the composition of exoplanets seem to indicate that the geological
conditions for the appearance of life could be less frequent than
previously thought. See this
article.
Each of these properties can have a
different impact on the appearance and evolution of life. We know that the Earth
combines them all, for there is intelligent life. Does this mean that life won’t
have appeared on a planet that does not meet all? Perhaps not, but at the speed
of evolution, it is likely that it will have just microscopic life, or life that
has not come out of water.
Remember that, on Earth, there has been
life for 75% of its history; multicellular life for 16%; terrestrial
multicellular life for less than 10%; and intelligent life for less than 0.02%.
Thematic thread on Life in other Worlds: Preceding Next
Manuel Alfonseca
No comments:
Post a Comment