Does extraterrestrial life exist? If it does, how can we find it? From the time of the first telescope observations by Galileo Galilei to the time of the modern space probes, we have a long history of observing the astronomical bodies of the solar system. However we have yet to find proof of extraterrestrial life. There has been a dramatic increase in discoveries of planets outside our solar system in recent years, and naturally we are hoping to discover signs of life somewhere on these many planets.

　Which is really more likely, that we will discover extraterrestrial life within our solar system or beyond it? Optimists (including this writer) who believe that we will find a planet harboring life somewhere outside the solar system base their belief on the theory that life will emerge with near 100% certainty on a planet that has an environment where liquid water is stable. In fact, exploration of the bodies of the solar system is considered an attempt to prove the validity of this theory. Based on a variety of evidence, it is nearly certain that liquid water exists on at least six bodies other than Earth in the solar system (Mars, Europa, Ganymede, Calisto, Enceladus, and Titan), although this water exists only underground. If no life at all exists on these bodies, then it is possible that the emergence of life requires some other condition besides the existence of liquid water. This would force a significant downward revision in our hopes for the existence of life on planets outside the solar system. Conversely if the optimists are correct, then the possibility of finding extraterrestrial life on bodies within the solar system should be extremely high.

　Attempts to find extraterrestrial life within our solar system may appear to be well advanced at first, but in fact we can also say that we have only just now reached the starting line. This is because just like the discovery of extrasolar planets on orbits within the habitable zone, evidence that liquid water exists on bodies within the solar system was found only extremely recently. In other words, we have reached approximately the same stage in the search for life both within and beyond the solar system. So will we arrive at the first discovery of extraterrestrial life within the solar system or beyond it?
　At present, this question is honestly difficult to answer, however if we will first discover life within the solar system, then it will likely be on Mars, where we are continuing to send probes, or on Enceladus, where plumes of water vapor containing organic substances are being ejected from an internal ocean below the surface. On the other hand, for planets outside the solar system, the clues to finding the existence of life are minute amounts of reflected light and thermal radiation. As a result, even if life exists, the biosphere must have advanced to a level where it effects large changes on the atmospheric composition or surface reflectivity if we are to identify the signs of life on that planet. Suppose observers on such a planet had been watching earth continuously ever since its formation 4.5 billion years ago. Life appeared on Earth no later than 3.5 billion years ago, however it would only have been possible to clearly declare that life exists based on remote observations in the most recent few hundred million years, when sufficient oxygen had accumulated in the atmosphere. In order for us to discover such a planet, we must add to our list of planets which have the necessary conditions to possibly support life. This is one of the goals of our project.

　The meaning of the search for signs of life on a planet outside the solar system is different from the search for life within the solar system. Within the solar system, Earth is the only body where life has altered the surface environment. Therefore if we are searching for a planet where life has evolved to the point where there is biosphere of a size that can alter the surface environment – a planet sometimes called a “second Earth,” then we must look beyond our solar system. This means that searching for signs of life outside the solar system means questioning the universe about the causes and conditions that allow an advanced biosphere to evolve. In contrast, extraterrestrial life within the solar system, if it exists, would still be in a primitive form. The search for extraterrestrial life within the solar system means asking about the causes behind the first emergence of life.
　Where did we come from? What exactly are we? Where are we headed? When looking for answer these questions, it is important that we look both within and beyond the solar system.

 

* Habitable zone:

When there is a planet travelling on an orbit at a certain distance from the principal star, this is the range of planetary orbits which produce the atmospheric pressure and temperature conditions that permit the existence of liquid water, an important factor in the existence of life on the surface. In the solar system, liquid water exists on Earth because Earth is within the habitable zone, while Venus is outside the habitable zone and water exists there only as water vapor.

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

Kiyoshi Kuramoto (University of Hokkaido)

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　

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