[EMAIL PROTECTED] wrote:
We don't really get the opportunity. The anthropic principle applies. We
are here because Earth supports life. That's about all we can say.
We have learnt a lot about planetary evolution. And the retention of an
atmosphere is ultimately a matter of physics. Gravity doesn't have to be
finely tuned to get an atmosphere similar to ours.
Some readers may know that Mars has 0.1 Earth masses and it has minimal
atmosphere. First I note that Mars does have some atmosphere, it's just
very thin. So that proves that it can hold on to one.
A thin one in which we can't survive and there seems little evidence that
anything else has.
I didn't say that there is present life on Mars. But Mars has enough
gravity to retain an atmosphere similar size as Earth's, if only its
magnetic field was stronger.
So the planet has to have a magnetic field, be the right distance from the
sun and have reasonable temperature with not too big a variation, water,
be the right gravity range and a few other things too.
Of these, gravity and water are the less fine-tuned variables. There is
a wide range of perfectly acceptable gravity, and water is very common
in the universe. Earth, Venus and Mars all started out with tons of
water. It was planetary evolution that made two of them go dry (Mars
should still have its water "somewhere").
Distance from the Sun is a more critical factor. Venus shows what
happens when you get the physical composition just right, but the
distance wrong.
There are many reasons why life is probably rare in the universe, but
having to fine-tune gravity isn't one of them. Some that are more
critical include:
- Getting the distance to the sun right.
- A large moon appears to have been very important for Earth. The moon
is the reason why we have a strong magnetic field. In addition, the moon
stabilizes our axial tilt, which keeps our weather more uniform over
millions of years. I don't claim that the latter is critical, but it
sure helps.
Our moon is extremely unlikely, because it is so large. We have by far
the largest moon (relative to the planet) of any planet in the solar
system (Pluto is not considered a planet anymore). Normally a moon
should be at least 10,000 times less massive than the planet. Phobos is
a million times less massive than Mars. But our moon is 1% of Earth's mass.
I don't mean that you absolutely need to have a large moon, but you need
some way to have a good magnetic field and reasonably stable climate.
- Getting the type of star right is very important. Most stars in the
galaxy are in binary or trinary systems. In those systems, stable orbits
are extremely difficult. The only stable orbits are either extremely
close or extremely far from the stars.
In addition, most stars in the universe are smaller and colder than our
Sun. To get enough heat, you have to be closer to the star, but at that
distance the star's gravity causes a tidal lock. So the same side of the
planet always faces the star. This really hurts weather patterns. Oh,
and it prevents a magnetic field.
There are many other things that can go wrong with the star. It could be
a variable star, it could be a giant that burns quickly, etc. In the
end, maybe 10% of the stars in the galaxy are reasonable candidates.
- Our solar system's orbit around the galaxy is quite fortunate. We
don't like in a galactic arm, but in between two galactic arms. We have
an extremely circular orbit, which means that we *stay* where we are,
and don't move toward any of the arms. the galactic arms are blue
because there are a lot of blue giants, which means that there are
supernovas. A nearby supernova would have destroyed all life on Earth.
We are lucky to live in a region where supernovas are rare rather than
common.
Any of these factors is more likely to wreck a planet's chance than not
having the right gravity or not having enough water.
Cheers,
Daniel.
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