The first is the “Rare Earth Hypothesis.” This theory states that each location in the cosmos is unique and that the life potential for each location is also unique.
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The location of the orbit, the rotation of the planet on its axis, the presence of a moon large enough to stabilize the planet’s axial tilt, and many other features, are special to each planet.
The “Rare Earth Hypothesis” acknowledges that there may be microbial life on other planets. However, proponents assert that the evolution of life should take a radically different path than what happened on earth.
They assert that complex life is highly unlikely, and the likelihood of complex intelligent life, which pursues a path of civilization, is even more likely.
In contrast to the “Rare Earth Hypothesis”, there is the “Living Universe” (or “Goldilocks Theory”). This theory asserts that life in the universe is abundant. It occurs everywhere that conditions allow.
Although there are places that are too hot, and places that are too cold; there are also many places where life can take hold and thrive.
Based on our current understanding of biology, life makes no sense without evolution. The changing and adaptation of species over time is essential to describing and explaining how life works.
Therefore, if there is life, even bacterial life, then there is evolution. And, if there is evolution, it can go anywhere, including into intelligent species.
But, is there any credible evidence that life actually exists beyond our earth? Below are three different lines of evidence that are being explored.
The Evidence of Probability
The first evidence of life beyond earth is the sheer size of the universe. The universe is so vast, with so many locations, it’s difficult to believe that we are the only planet where life can be found.
This may not seem very empirical, but with 400 bn stars in our galaxy and 400 bn galaxies, it does seem presumptuous to assert we are all alone.
A more developed version of the evidence of probability is the “Drake Equation.” Here, a list of the essential components necessary for life are used in an equation which generates a number leading to probability.
Dr. Drake, who first presented the equation provided seven essential factors. However, one could argue the case that there are more factors, and that their relationships may be different.
For example, in recent studies, the number of stars in the universe (in different galaxies) has been increased by three times. In addition, the number of essentials of life has been increased. Based on the Drake Equation, this improves the chances that life exists. However, it still remains a probability, not hard observation.
Over the past few decades, scientists have discovered a whole range of species that live in environments thought to be uninhabitable. These species are called extremophiles.
The existence of life in these extreme habitats increases the number and variety of habitats where life can exist. Again, it increases the probability of finding life. However, it is not hard proof.
The Evidence of Inference
The second line of evidence falls closer to inference. The essential elements of life (carbon, hydrogen, oxygen, nitrogen and phosphorous) have been found to be common in the galaxy.
In addition, the combination of these elements into simple compounds, like water, methane and ammonia has also been found to be abundant. Indeed, even complex organic molecules such as amino acids are common in the galaxy.
This means the components which are used to build life are common throughout the galaxy.
Another piece of evidence is the nature of the unpredictable areas where bacteria have been found. We find them in deep crustal rock formations which were deposited on earth before life began.
Such discoveries are contrary to the prevailing paradigm that life began on earth. Instead, it suggests that life rained down from space in the earliest stages of earth’s formation and took hold.
The Evidence of Controversial Discoveries
The third line of evidence is based on discoveries that are more controversial. In the search for earth-like planets in the galaxy, the star Gliese 581 stands out as important.
Of an estimated six planets believed to orbit this star, research teams have determined at least one to be about the size of earth, and to be in orbit within the parent star’s habitable zone.
Another controversial piece of evidence is the discovery of bacterial fossils in meteorites. This particular evidence was so convincing that in 1996 President Clinton announced on national television that NASA had discovered extraterrestrial life.
However, after the excitement passed, the certainty of the evidence was questioned. It should be clarified that these discoveries were not negated. Rather, the observations were found to be inconclusive.
So, what kind of evidence supports a hypothesis that life is widespread in the universe? One line of evidence is that of probability. Another is evidential observation. And a third is controversial observations.
Enumeration of evidence like this might seem pretty convincing. However, in the skeptical mind of science, it is information, but it is not proof. That comes with more study and more searching.