Why Scientists Struggle to Find Signs of Life on Other Planets

Universe Today carried a story about how statisticians and astronomers cannot agree on the likelihood of finding life elsewhere in the universe. The SETI (Search for Extra-Terrestial Intelligence) approach is to scan the heavens for non-natural patterns in the electromagnetic strata that could be indications of intelligent manipulation.

According to various anecdotes from SETI scientists, such non-natural patterns have been discovered on several occasions. To date, all but one have proven to have an Earthly origin. The one odd example (the so-called SETI WOW! Signal) was briefly detected in 1977 by Dr. Jerry R. Ehman. It has defied explanation and has not made its presence known again.

According to our best estimates, several hundred extra-Solar planets have been identified within a small nearby section of our Milky Way galaxy. Most of these planets are too large to be Earth-like and the orbit stars that are as unlike Sol as one can imagine. We lack empirical evidence of true Earth-like planets (ranging from 0.5 to 2.0 Earth masses, orbiting middle-aged warm stars at a distance to support both an oxygenated atmosphere and a substantial amount of liquid water, and geologically active) beyond the Solar System. Of the three Earth-like planets within the Solar System (Venus, Earth, and Mars), only one (Earth) is known to harbor life, much less intelligent life.

However, because our knowledge of the Solar System is vastly more detailed than our knowledge of other stars’ planetary systems, we cannot make fair comparisons or build reliable models about what has happened in our galactic neighborhood. Our sun is no longer traveling near its sister suns, the stars that were born from Sol’s own stellar nursery. Those stars are now scattered around the galaxy, moving at their own speeds. Hence, we cannot expect many stars in our present vicinity to seem much like Sol at all.

Scientists believe that life arose on Earth about 3.5 billion years ago. Current theory suggests that Earth life required 3 billion years to evolve into complex species that formed an ecosystem similar to the one we live in today. Over the past 540 million years, however, that ecosystem has been shocked by 15-16 minor extinction events and 5 major extinction events.

Each extinction event contributed to the path that ultimately led to the rise of the human race. Hominids probably appeared no longer than 10 million years ago. Modern humans probably appeared no longer than 400,000 years ago. In other words, life has devoted relatively little time to developing and shaping an intelligent species like the modern human race.

By comparison the most intelligent cetaceans (whales and dolphins) began appearing about 20 millions years ago, some 30 million years after their ancestors began moving from land into the water. Some whales and dolphins are extremely intelligent although they have not formed complex tool-making behaviors such as humans. Hominids broke off from the family that gave rise to the apes, and today’s apes have displayed human-like intelligence in a number of ways but their intelligence is not deemed equivalent to our own.

We know, therefore, that intelligent life can evolve into multiple forms. What we don’t know is how much time is really necessary for intelligent life capable of creating a star-faring civilization (or at least one capable of approaching that level of complexity) to evolve from more primitive forms of life. For example, though we have found no indication of civilization from among dinosaurs, is it possible there were intelligent dinosaurs capable of building complex social structures? Recent research does indicate that some dinosaurs cared for their young and traveled in large groups.

Also, human civilization has only been around for something like 10-15,000 years. It has taken only that long for hominids to progress to the point where they can launch probes into space and scan the universe with sophisticated technology.

Assuming that Earth is a typical planet in a presumptive “life-forming class of planets”, it follows that we need only identify Sol-like stars of a similar age with planets in their habital zones. Some estimates suggest that as many as 1-2 percent of the stars in the Milky Way are like Sol. But how many of those stars are the same age as Sol?

Earlier this year NASA estimated that the Milky Way may have 50 billion planets, of which 500 million could exist in the “Goldilocks Zone” — the habitable region of a planetary system. That may mean up to 5 million Goldilocks planets exist around Sol-like stars.

Those 5 million planets vary in age, however. Some of them are much younger than Earth and some of them are much older than Earth. An interesting question is: How many Sol-like stars in the Milky Way galaxy are approximately the same age as Sol? There may be at most only 1-200,000 such stars, in which case only a few thousand of them may have Earth-like planets that exist in their habitable zones.

If we look for signs of life only from stars matching Sol’s profile (approximately 4.5 billion years old) we may miss everything completely, because if Earth’s evolutionary progress is typical then we have not yet reached a sufficiently advanced stage to be able to prove our existence to distant observers. We would either have to make contact with them directly or create such a disturbance in the galaxy around us that our environmental impact would “resonate” in some measurable way.

If we look for signs of life from Sol-like stars that are younger than our own, our chances of finding such life are even more slim.

Hence, our best chances of finding intelligent life on other stars is to look at Sol-like stars that are older than the Solar System, but probably not much more than 500 million years older. That extra 500 million years could be sufficient to give rise to complex life forms with hominid-like intelligence. Assuming their planets pass through multiple extinction events and warming/cooling cycles the same way Earth has, there should be ample opportunities for nature to shock those planets’ lifeforms into creating civilizations.

A planet that is merely 500 million years older than Earth will have had 1 billion years in which to experiment with life forms that may require no more than 10-20 million years to become intelligent enough to build a space-faring civilization. A space-faring civilization need only evolve to the point where it can create self-sustaining biospheres capable of venturing into interstellar space with a high probability of surviving long journeys in order to spread from one star to another.

Civilizations leave traces of themselves in the environment. They do so by substantially changing their environment through industry, war, and population. “Industry” encompasses everything from draining swamps to building dams to founding high-tech cities. Human industry has existed (and directly impacted our environment) for thousands of years.

It is by environmental changes that we are most likely to detect civilizations from afar. It is by direct contact that we are most likely to detect unEarthly intelligences. If we can hone our observational capabilities sufficiently, we’ll be able to not only detect Goldilocks planets, we’ll be able to sift them through by cataloging their environmental signatures and filtering out the “natural” signatures.

If we can hone our space-faring capabilities sufficiently, we’ll be able to send out human explorers (or machine proxies) capable of interacting with unEarthly intelligences across vast distances of space and time.

We just have to find the right signals to look for, and develop the right technologies to look for them. The problem is that we’re using inefficient methods to search for extra-terrestial life because we have not yet developed an economic incentive to find it. We devote such a small fraction of our resources to studying the universe and looking for signs of life out among the stars that we are essentially taking the long, scenic route to the destination (of determining whether there is discoverable life within our sphere of existence).

If we were faced with another extinction-level event today, what would we do to survive? Would we attempt to propel ourselves into space? Would we attempt to alter our environment so as to avert the extinction event? Would we reach out to other stars with pleas for help (not knowing whether anyone was out there to receive our pleas)?

We could, if we were willing to make the sacrifices, devote more resources to studying the universe. At the present time, however, we lack sufficient motivation to undertake such a dramatic exercise in human exploration.