Nov 02 2012

How Common Is Life in the Universe


There almost certainly is life elsewhere in the universe. There is no reason to think that conditions and events that led to life on earth are unique, and the universe is a ridiculously vast place. So the odds strongly favor that there must be many occurrences of life out there. There are interesting sub-questions, however – how common is life, how common is complex life, and how common are technological civilizations? Is the universe teaming with bacteria and fungus and little else, or are the stars buzzing with spacefaring races of every description?

The problem with trying to answer this question is that we have an N of 1 – Earth is the only example of life in the universe that we have. We may find examples of life elsewhere in our own solar system (Mars, Europa, Titan, and Enceladus are the current prime candidates), but that will only give us a tiny bit more data. Life elsewhere in our own solar system (especially Mars) may have been seeded from Earth or vice versa, and so we may find life on Mars but still only have evidence for a single life origin in our solar system.

We may also find multiple independent life origins in our solar system, and that would be extremely cool, but would still only answer one of the three questions above. That would tell us that the origin of some kind of life is likely common, and can occur under a variety of conditions, but would not tell us how common complex life or civilizations are.

We are still left with the problem that we only have one example of a solar system (our own) with life, and we do not yet have enough data to tell us how typical our solar system is (and also how typical the Earth as a planet is). In order to evolve complex life which then has enough time to develop into a technological civilization, stability over a long period of time is necessary. On earth it took about 4 billion years. Even if that is slower than average, it seems very likely that billions of years of stability are necessary.

The question then becomes – what are all the factors that led to sufficient conditions and stability on Earth that allowed for the evolution of complex life and us? Astronomers have identified many such factors, and we are starting to gather information about them. One such factor is the arrangement of planets in our solar system. Small rocky worlds are relatively close to the sun, around the Goldilocks zone where liquid water on the surface is possible. Three planets are potentially in that zone, although Mars was too small to hold onto its atmosphere long enough, and Venus suffered from a runaway greenhouse effect making it too hot.

Large gas giants (Jovian planets) are farther out. These large planets (especially Jupiter) act like sentinels, shielding the inner solar system from comets and other debris that could potentially impact those worlds and cause mass extinctions. Life can obviously tolerate some impacts, but too many would likely hamper life’s evolution.

NASA scientists have recently identified another feature of our solar system that may contribute to this stability – the asteroid belt. They write:

“To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt,” Livio explained. “If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve.”

The location of the gas giants seems to be important. In some systems they migrate very close to their star – so-called “hot Jupiters.” In such systems inner rocky Goldilocks zone planets could not survive with a stable orbit. Martin and Livio are now saying that, not only do we need a system without a hot Jupiter, complex life requires a Jovian planet just outside the snow line that has migrate a little, but not too much. The snow line is the distance from a star at which volatile substances can exist without evaporating, so you can have icy worlds with frozen water and other compounds. Jupiter is right outside our snow line, and the asteroid belt is just inside. Jupiter is responsible for the asteroid belt forming as it did, and not forming into another planet.

The weakest link in their argument is that if a system has no asteroid belt (because it has no Jovian world just outside the snow line) then the lack of such a belt would reduce asteroid impacts on a potential life-bearing world. Such impacts deliver water and organic materials to the surface of the planet, and also a steady stream of impacts (but not too many) provides an occasional kick in the pants to ecosystems, furthering their evolution. These are not unreasonable assumptions, but they are just assumptions and are not empirically verified to the point that we can use them as solid premises.

Other systems may have similar rates of asteroid or cometary impacts without such an asteroid belt. It’s also possible that the range of such impacts compatible with life is large. Another factor is the Moon. The Moon is another factor that may have contributed to the Earth’s stability, and one role that it plays is to further shield the earth from potential impacts.

There are many variables that could affect the long-term conditions on a potentially life-bearing planet. I think it’s too early to say how they will all shake out in a typical system. The ultimate goal is to come up with an estimate of the percentage of systems that could harbor complex life. We are not yet close to this. We may also need to expand our concept of potentially life-bearing worlds. What about moons of Jovian planets, for example?

While we are gathering more and more information about the composition of other solar systems, our data is still hugely biased by our search methods. Larger planets closer to their stars are easier to find than smaller farther out worlds.

More importantly, we currently have no data on the existence of life of any kind outside our own solar system, so we can only guess at how the variables of solar system composition we are finding affect the probability of complex life forming. We have no prospects of directly finding such life anytime soon. We may see oxygen in the atmosphere of distant worlds, and that might strongly imply the presence of life, but we won’t know for sure. Unless SETI finds and starts downloading the Encyclopedia Galactica we have no hope of surveying our galaxy for life anytime soon with any extrapolation of current technology. A truly game-changing technology (and underlying physics) would need to be discovered.

But I may be wrong. Future scientists often prove to be more clever than futurists imagine.

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