So we've established that life seems to be precious, rare and difficult to manufacture, and that conditions for life to exist appears to go beyond simply being the right distance away from a star and being the right size.
The problem, though, is that since we don't really know how this whole process of life gets started we can't accurately say whether not a planet will actually contain life or not. So how can we even begin to answer the question about life existing elsewhere in the Galaxy?
Perhaps we need to come at things from a different perspective. Leaving all the biology and chemistry stuff alone for a moment (thank goodness) let's instead focus on habitability of planets. Maybe this can give us a clue about how regularly life pops up in our Galaxy.
Usually in such discussions the primary tool discussed is the Drake Equation. The object of this equation is to estimate the number of civilizations in our Galaxy with the capability of communication at any given time.
The Drake Equation arrives at this number by multiplying together fractions that represent values ranging from:
- The average rate of star formation per year in our galaxy.
- The fraction of those stars that have planets.
- The average number of planets that can potentially support life (per star that has planets).
- The fraction of planets that can support life that actually go on to develop life at some point.
- The fraction of planets that develop life that eventually develop intelligent life.
- The fraction of civilizations that develop communication technology capable of reaching Earth.
- The length of time for which such civilizations can attempt communications.
It's a fun little calculation to do. The problem is that, while some of the parameters are getting closer to being determined by scientific observation, most of them we really have little or no basis for.
We may very soon have a good grasp on the likelihood of a given star having an Earth-like planet around it. But we are far from understanding what the likelihood of life actually evolving there, and therefore just as unsure what percentage will develop intelligent life.
Current estimates of the Drake Equation spit out numbers ranging from far less than 1 (meaning we are assuredly along in the galaxy) to tens of thousands (meaning life would be rather common in the galaxy and we should expect contact at any moment). But, again, these values are based, for the most part, on very little data.
In fact the only Earth-like planet that we know of for which we can measure whether life exists is, well, Earth. So on that basis intelligent life seems to evolve 100% of the time on Earth-like worlds. But basing such an estimate on a single data point is galactically stupid.
So what is a better estimate then? Truthfully, we have no clue. That is one primary reason why we get such a broad range of solutions to the Drake Equation.
The other parameter that makes this entire exercise worthless is the last one; estimating the time scale over which a civilization can send out communications into deep space. As has become apparent over the last century, evolved beings eventually will discover ways to annihilate themselves (such as nuclear weapons) and face devastating diseases (AIDS, resistant cold and flu viruses).
Their ability to overcome these obstacles and resist self-annihilation are key to a civilizations survival. In theory, a civilization could persist for billions of years, but that would probably require some level of luck. Conversely, the inhabitants of some world may not make it past the growing pains of technological revolution, and bring their existence to an end after a few thousand years.
And what might a reasonable average be for this, the most tenuous of parameters? Again, we have no idea.
So, where does this leave us? With a very simple, yet unsatisfying conclusion. Could life exist elsewhere in our galaxy? Absolutely. Are we certain of it? Not even close.
Unfortunately, until we actually make contact with a people not of this world, or at least begin to fully understand how life came to exist on this tiny blue rock, that question will be answered with uncertainty and estimation.
In closing though, I put to you one final thought; a phenomenon known as the Fermi Paradox posits: If life is so common that it should spring up on any world where the conditions are right, and this life eventually evolves into beings capable of technological development, why then have we never been contacted by any such race of beings? We've certainly been listening.
This brings to a close the series on the question of life existing elsewhere in our Galaxy. However, I will be expanding on this topic later, looking at different un-answered questions on the search for extraterrestrial life, in the coming weeks and throughout the year. So come back for more relating to this exciting topic.
Artist's Impression of a gas giant orbiting a distant star. Courtesy of NASA/JPL-Caltech