Life is defined as the condition that distinguishes animals and plants from inorganic matter, and includes the ability to grow, breathe, reproduce. So when we talk of life on other planets we include little one celled creatures as well as complex and intelligent life. Anything between these extremes would qualify for life on another planet.
To begin with we have to consider what is required for life. Here on earth we know that life requires carbon and water. Carbon, hydrogen, oxygen …..a basis for life here on earth.
Carbon is present in all living things.
Hydrogen and oxygen equals water, essential to life
Do other planets have these elements? Some do….most certainly. Hydrogen and helium were created in the first moments of the Big Bang. All of the remaining elements, including carbon, nitrogen, and oxygen, were created in the nuclear reactions in the stars throughout the universe …. and they are created only in the nuclear reactions of the stars.. As these stars die and explode they scatter their treasure trove of elements throughout the local space. The forming planets are made up of these materials as they first clump together, then begin an accumulation of mass under the influence of an ever increasing gravitational force.
Are these elements in the right conditions to form life? That is a good question. The law of averages would lead you to think yes, given the huge number of planets in the universe……however, belief in the religious sanctity of life here on earth would lead you to think not. Which belief reflects the truth? Can only one or the other be true? Is it possible that life could be everywhere in the universe but that the sanctity of life here on earth was given by God at some point 5,000 years ago…or 50,000 years ago? At one time the Church thought that the Earth was the center of the universe. To think otherwise was heretical. However when proven otherwise, life went on, and the Church didn’t miss a beat. So it may be with life on other planets, the religions of the world may very well not miss a beat if a new species should be discovered on another planet….any more than the multitude of species here on earth….from the insects, to the marine mammals, to the bears and rats and giraffes, to humans. Anyway, that is a digression….important but not the subject of this web site….here we are just looking at the science as we know today, using the rules of physics and biology that God created.
What form could life take on another planet? Another good question. Look at the variety of life here on Earth…..from small, single-cell organisms, to the complexity of human beings; from mammals, to birds, to amphibians, to insects. Our Earth has been home to many extinct species as well. Another world could easily have the same creatures, extinct or not, as well as species unknown to us. What is the most likely? … we should expect the unexpected. And then of course, there is the question of intelligent life, as we call it. At this point the subject rapidly takes on religious hues. So, for the sake of keeping with the scientific character of this discussion, I suggest we simply ignore the issue of intelligent life for the time being. It is surely a prime reason for exploring the Cosmos….that is, to discover intelligent life…but then, on the other hand, when we see it, we’ll know it.
So the obvious question is, Is there other life in the universe ? I like the answer in the movie “Contact” based on the Carl Sagan novel of the same name. At the end of the movie, Ellie (played by Jodie Foster) replied to a group of children… “The universe is a pretty big place. It’s bigger than anything anyone has ever dreamed of before. So if it’s just us, it seems like an awful waste of space….right?”
Habitable Planets
We have, of course, looked closely at planets in our own Solar system for evidence of life. Recently, some moons around Jupiter and Saturn are now known to have conditions suitable for sustaining some form of life, although that life may be of a very simply form. Not too long ago, when we first looked at Mars through a telescope we were sure that there were Martians there….. building canals. Now of course, we know that is not the case but, interestingly enough, with all our science and photos we still do not know if there are smaller, simpler forms of life present on Mars. And, surprisingly enough, there are moons in our solar system that may have conditions suitable for life.
To investigate other solar systems for life we first look closely at the star which serves as the sun for a solar system and then look for planets circling that star. Planets that circle other stars other than our sun are Exoplanets. Finding Exoplanets is very difficult since these stars are so far away and only the largest of the planets can be detected. Smaller planets currently cannot be detected by any method.
So how do we know if other suns have planets circling them, never mind if they are capable of supporting life? Well, surprisingly enough, there are actually observations that one can make to detect planets. Granted these observations are done very carefully and are made through large telescopes. If you look very, very close, you can see the star wobble if it has a planet circling it. Also, one might detect a dimming of the star when a circling planet crosses in front of it, thus blocking out light from the star. Obviously this works for big planets, like Jupiter, close to the star but not so good for small planets far from the star, like Pluto….oh! sorry, it’s not a planet anymore. Currently (2015) there are nearly 5000 Ex0planets detected circling nearby stars. The smallest of these Exoplanets are 3 times the mass of earth. They are categorized as superEarths. The largest Exoplanets detected are 20 times larger than our Jupiter.
Even so, for a habitable planet, we think at the moment that the planet needs to be at the right distance for free water to form. Not so far that it is too cold and the water is frozen, and not so close that it is so hot that the water boils away. This right distance is called the Habitable Zone and means that we have to detect planets at an earth-like distance from the star. This is bit more difficult than just detecting big planets close to the star. At the present time some 1400 Exoplanets have been detected in the Habitable Zone for at least part of their orbit. About 50 Exoplanets have been detected which are entirely within the Habitable Zone of their sun.
In practice the Habitable Zone may be quite large since we realize that some solar system bodies can be beyond the “free-water” distance and very cold but still have liquid water beneath the surface. This liquid water can be due to internally generated heat if the body has a molten interior. Earth is an example of this. Earth is actually beyond the free-water distance, however it’s internal heating due to the molten core and it’s ability to retain solar heat make Earth capable of maintaining a free water presence. Other bodies in our solar system may also maintain sub-surface oceans of water which are heated by internal heat sources. Some form of life may exist in these interior “seas”. Jupiter’s moon Triton may be such an example.
Life on Earth – what is currently thought
Life began on the Earth 3,850 million years ago. Early Earth had lots of carbon and this formed the basis for the rapid development of life…..relatively speaking. The availability of carbon is thought to have allowed many compounds to be formed, among them amino acid, the building blocks of protein, and thus life itself. Bacteria was likely the first form of life to appear. This early bacteria lived in a hostile environment ….little free oxygen, sulfurous oceans, hot, unstable atmosphere. But for billions of years the bacteria converted carbon dioxide to oxygen until the atmosphere could support larger life. Darwin set forth his theory of evolution which was extended to explain how life on earth could have begun and evolved. Beginning with the simplest of single cell life, more complex life forms could have evolved over the billions of years of life on Earth. Obviously this is a matter of deep concern to many people…Creationalism versus Evolution and Intelligent Design versus Evolution. That debate won’t be discussed here, but there are some great web sites. In my opinion it is clear that evolution is God’s plan. When he may have intervened to create man with a soul is known only to Him.
The first human, or genus Homo, appeared 2.5 million years ago….. Man, as we know of him today, Homo sapiens, didn’t appear until 200,000 years ago. Homo neanderthalensis and Homo erectus were separate branches in the Homo genus but both of these died off as recently as 28,000 years ago. Notice that the dinosaurs went extinct 65 million years ago…..and that was a long time before humans walked on this earth!
…..will Homo sapiens be around in another 200,000 years?