I have placed this is IMHO because that is mostly what I would like to see. I am not an astro-physist or archeologist, just an amateur of both. I also am a huge fan of the space program in all of it’s forms. I say this up-front to avoid any misunderstandings derived from the nature of this post.
I have taken some time to try to organize some of the various information that has been shaping my opinion for some time now. I have not prepared a paper like this in decades so please be kind on format.
Having said this, I am curious what the minds of the Dope think about the likihood of discovering life outside of our planet? Please find my humble attempt to present my thoughts here.
All throughout human history there has been speculation on the possibility of life on other planets. This has never been more so than it is today with the search for life ranging from Mars to the moons orbiting the great gas giants. Recent advances have led to the identification of planets orbiting stars other than our Sun. It is this possibility of extraterrestrial life I would like to consider.
First, I would like to make some arbitrary limits on the scope and definitions of our considerations. There are many levels of life to consider. For the purpose of this discussion I would like to broadly group them as Primitive Life Forms (single cell up to say jellyfish), Complex Life Forms (up to say mammals including dolphin and whales) and Intelligent Life Forms (this is to be tool-users and city builders).
Secondly, I would like to limit the sphere of consideration to 15 parsecs or 48.9 light years. Excluding such things as warp drive or worm holes, I think this is a distance at the margin of human travel with the foreseeable future as this would take nearly a century of in-flight time allowing for a build up of speed to even a fraction of light and then the necessary braking.
The most recent advances in dating put the age of the Earth at 4.55 billion years. [Link](http://www.answers.com/age%20of%20the%20earth)
It is also generally accepted that the Earth’s Moon is the result of a collision with a mars-size object called Theia Link
and sometimes referred to as “The Big Whack” and it went something like this:
“According to the hypothesis, 4.533 billion years (4.533 Ga) ago, 34 million years after the Earth formed, a Mars-sized planetesimal hit the Earth at an oblique angle, destroying the impactor and ejecting most of the impactor and a significant portion of the Earth’s felsic-rich mantle into space.”
link
This accident of planetary trajectories left us with one of the largest moons in the solar system (when compared with the host planets mass) and a brand new surface of molten rock. It has been argued that the presence of this large moon and it’s proximity to the Earth have had an important, if not crucial, role in the development and subsequent evolution of life on this planet.
“The fact that there are tidal zones on the beaches of the world had an enormous effect on the way that life got started and evolved on earth. There are also tidal effects, less visible, but equally important, on the earth’s atmosphere, which influence the weather and climate. To have a large tidal effect on a planet, a moon must be not only large, but also reasonably close.”
Link
In addition to the tidal effects that it provides, there is a stabilizing effect on the rotation of the Earth that has created stable seasons .
“The Moon has had dramatic effects on our planet and the life that inhabits it, researchers believe. The Moon stabilizes Earth’s rotation, for example, preventing otherwise dramatic movements of the poles that would fuel climate swings that some scientists figure might have doomed any chance for life to form, let alone evolve”
Link
Additionally, the Earths magnetic field protects life from solar winds that otherwise would strip the atmosphere away and expose life to lethal doses of solar radiation. It is entirely likely that a significant proportion of the iron at the Earth’s core may have been contributed by Theia.
“It is likely that some of the early Martian atmosphere has been lost to space because Mars, like Venus has no substantial intrinsic magnetic field to protect the atmosphere from solar wind scavenging.”
It is my purpose here to suggest that for a planet to be of the right size and composition may not be enough to see the beginnings of life. It must also inhabit the “Goldilocks” zone of a solar orbit that is cool enough to keep water from boiling away into space but warm enough to allow liquid water for life. It must have an iron core sufficient to create a magnetic field that can protect life from cosmic rays. It must have enough mass to hold an atmosphere or an ocean rich enough to provide oxygen to life. Additionally, the presence of a moon-sized object in close orbit my be essential as well.
Having considered the planetary requirements for the origin of life, we must consider the type of life. Even after four billion years, life was still of the Primitive Life Form (PLF) type I referred to earlier. This very long and broadly defined era was known as the Precambian.
“Nearly 4 thousand million years passed after the Earth’s inception before the first animals left their traces. This stretch of time is called the Precambrian. To speak of ‘the Precambrian’ as a single unified time period is misleading, for it makes up roughly seven-eighths of the Earth’s history. During the Precambrian, the most important events in biological history took place. Consider that the Earth formed, life arose, the first tectonic plates arose and began to move, eukaryotic cells evolved, the atmosphere became enriched in oxygen – and just before the end of the Precambrian, complex multicellular organisms, including the first animals, evolved.
Link
“Four thousand million years” of a lifeless planet. But once it begins, it spreads throughout the seas and continues to evolve in complexity. And then life really begins to evolve in earnest and a wide range of creatures come into being including land-dwellers very much like the dinosaurs we know and love. Even with this complexity and extremely long period of evolution, what we recognize as intelligence never shows up as a survival mechanism. Then at the end of the Permian era we find “The Great Dying” of the Permian-Triassic extinction. Very nearly all life on earth is destroyed.
“…not to be confused with the better-known Cretaceous-Tertiary extinction that signaled the end
of the dinosaurs 65 million years ago. Whatever happened during the Permian-Triassic period
was much worse: No class of life was spared from the devastation. Trees, plants, lizards,
proto-mammals, insects, fish, mollusks, and microbes – all were nearly wiped out. Roughly 9
in 10 marine species and 7 in 10 land species vanished. Life on our planet almost came to an
End.”
Link
The slate is very nearly wiped clean. But, remarkably, life goes on and makes a spectacular come back. Life continues to evolve and flourish for millions of years with one of the most successful branches of life we call the Dinosaurs. They are masters of this world and exist in many, many forms and develop many survival strategies. None of which is higher intelligence. In all that time and all those forms, no self-awareness or tool-making develops as far as extensive study can determine. If not for the fresh start arranged by yet another fortuitous major impact 65 million years ago, it seems unlikely that the small, frail proto-mammals could have developed the size to support a larger brain and the higher thinking it affords.
Lately, astronomers have been able to identify planets orbiting around distant stars. There are about 40 such planets identified so far. However, most of these planets have a greater mass than Jupiter with only three with a mass somewhat less than Saturn. The size of these planets has come as a surprise to many but equally interesting is the fact of that planet’s distance from it’s sun and revolution period are much less than earth. Often times these giants are completing a revolution in two to five days as opposed to 365 for Earth.
Site
Conjecture on these finds led some to think that near-sun giants that have formed by sweeping the inner system area of most or all of the leftover materials following the birth of a star. As likely as this seems, if this is typical what implications does this have for earth-sized planets developing inside the “Goldilocks” region in nearby systems and subsequently, what are the implications for life on other worlds in other star systems? Granted, there have been many discussions on life’s ability to thrive in the most hostile of environs like polar caps and deep sea thermal vents. However, I wonder if life could have begun there or has it stared in more accommodating surroundings and these creatures have adapted to occupy a niche? Is life beginning in such places as likely as migration and adaptation?
The Earth has been broadcasting radio signals to the stars for around 70 years now. Since these signals travel through space at the speed of light, in essence we have created a radio signal sphere 140 light years in diameter and radiating out every day. A planet within the 50 L.Y. we discussed at the beginning would have been able to hear these signals for twenty years now. I stands to reason that a planet populated with an ILF may well have discovered radio signals and even if they were only slightly ahead of us in technology we should be inundated with many channels of various communications. It is equally likely that their broadcast lifespan would be far greater than ours and therefore their “radio wave sphere” should be many times larger than our own and even to the point of overlapping other spheres. Yet, in the years since 1960, S.E.T.I. has been searching vast areas of the sky only to find silence.
Link
In summary, my question is this. Given the following criteria:
A collision with an impactor of approximately half the mass of the proto-earth on a sufficient angle to cause a large moon in low orbit to form without destroying both worlds. In the event of mutual destruction, it is likely that a single planet would have formed much latter and would not have had a large moon.
Not one, but two major impacts creating so much damage to the environment that it almost sterilizes the planet the first time and wipes out the dominant species the second time. In both cases, artificially altering the natural progress of natural selection to allow the rise of creatures that may well not have been able to come into existence.
In both of the extremely long periods of life developing on Earth preceding the current epoch, the survival method of intelligence never arose or a rudimentary form was unable to survive long enough to flourish.
What are the possibilities of life that we might recognize as an Intelligent Life Form (ILF) might develop on a planet that is within a distance to be practical to humans. I would love to hear of life being discovered to have come into being on a world independently of the life we know. I suspect that there is a possibility of PLF within our own system. Perhaps a moon of Jupiter or Saturn will yield such a monumental discovery, perhaps even some form of Complex Life Form, but my hopes for communication with an intelligent race is tempered by my feeling that our existence here is the result of events that have proven beneficial to the development of a frail, slow, hairless, clawless creature that has survived by out smarting the other creatures with which it shares a world. Based on the two extremely long epochs that preceded our own, it would appear that the development of intelligence is a fluke, not a norm.
I fear that if the future holds a galaxy of planets populated with Intelligent Life Forms, it will be because we have made it so.
“In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.” – Carl Sagan
Thoughts? Are we alone? Is it up to humanity to populate this part of the galaxy if we wish to see the type of interplanetary future we have dreamt about?
A sphere of space with radius 50 l.y. has a volume of 125,000 ly3. I don’t know the average distance between stars, let’s say 6 l.y, which means there is a sphere of 9 ly3 around each star. This gives us just under 14,000 star systems in this region of space. I know a few years ago people thought multiple star systems could not support planet formation, but I seem to remember reading something saying this was no longer believed. Let’s say a tenth can, that gives us 1,400 planetary systems. Let’s say a fifth of these havereasonably sized planets in a habital zone - this gives us 280.