Cliff Smith wrote:

Here are some of my thoughts on the existence of alien life.

In assessing the probability of other advanced civilisations existing
elsewhere in the galaxy, it's important to maintain both a sense of
perspective and a firm grasp of the laws of physics and chemistry, at least
as far as they are currently understood.
So far, 115 planetary bodies with a mass less than 18x that of Jupiter have
been discovered orbiting Sun-like stars relatively (in astronomical terms)
close to our own. The search is ongoing, and detection techniques are being
constantly refined.
It is consistent with current theories regarding planetary formation to
suggest that smaller, denser planets might accompany these gas giants so far
detected. Giant planets can be detected because of the perturbations they
cause in the movement of their respective stars. Smaller Earth-like planets
don't cause enough movement to be detectable with our current instruments,
but still may be present.
As is well understood, the chemical elements that make up our environment
were produced from lighter elements by the fusion processes within stars,
processes that are apparently consistent in most stars, therefore it is
likely that any solid planets orbiting other stars would be made up of much
the same elements that make up the planets in our own solar system.

There are very few of those chemical elements with the range of complex
bonding characteristics necessary to support the types of molecules required
for life as we know it to exist. It is possible that silicon-based
biochemical molecules could exist at higher temperatures, and even that
ammonia could form the basis of an alternative biochemistry, but carbon is
more stable at a wider range of temperatures, and therefore much more
likely. Therefore it is not unreasonable to assume that if life exists
elsewhere, the majority of it will be carbon-based. Speculations about
ethereal beings, machine intelligences and super-intelligent shades of the
colour blue are unhelpful and are best left in the realms of science
fiction. If we want to make contact with aliens, we need to be able to
recognise and communicate with them, so it will be helpful if we at least
have a basic biochemistry in common.

Another assumption is that our solar system is in no way out of the
ordinary. We occupy no special place either in space or in time. There are
stars far older than our own, as well as stars younger than our own. There
are over 100,000,000,000 (one hundred thousand million) stars in our galaxy,
so there are naturally many millions of stars close to the age of our sun.
If planetary formation is as common as it now appears to be, there are very
likely millions of planetary systems at approximately the same level of
stellar evolution as our own, and potentially capable of supporting life.

The only real unknown is the likelihood of life arising in the first place.
Since we only have one experimental subject, our survey is far from
complete. We have as yet not completely ruled out the existence of some form
of life on Venus, Mars or the moons of Jupiter and Saturn in our own solar
system, so any speculation on the existence of life in other solar systems
is based on a very limited set of data. However, it is at least possible
that there could be a great many life-bearing planets in the galaxy, and
therefore it is also possible that many of them could be at least as
advanced as our own if not more so. One could argue that in an infinite
universe, anything that is not impossible is inevitable.

However, having established that life on other worlds is possible at least
in principle, we must turn to the likelihood of our ever being able to
communicate with, let alone visit, these hypothetical aliens. If life on
other worlds must obey the same rules of chemistry as us in order to exist,
then it must also obey the same laws of physics. For nearly a century, it
has been an established axion of relativity that information cannot be
propagated faster than the speed of light, which is as far as we know a
constant throughout the universe. We cannot travel faster than light, and
neither can the aliens. While we might be statistically certain that there
are other civilisations in distant solar systems, we will almost certainly
never be able to visit them and they will never be able to visit us.

So there; anyone got anything to add?

The main problem is that the giant planets found to date would make it impossible for Earth-like planets to exist at Earth-like distances from the stars. It is beginning to look like our solar system really is unusual in its layout of giant planets far away from the star and small rocky planets close in. The planets being discovered show just the opposite pattern.

You also forgot to mention the importance of having a very large moon to stabilize the long-term climate of an Earth-like planet. Such large moons are rare (at least in our solar system).

--
The Evil Michael Davis(tm)
http://mdavis19.tripod.com
http://skepticult.org Member #264-70198-536
Member #33 1/3 of The "I Have Been Killfiled By Tommy" Club

"There's a sucker born every minute" - David Hannum (often erroneously attributed to P. T. Barnum)