Even before we had the Hubble telescope and NASA’s Kepler spacecraft, both of which are used, in part, to discover new planets, there was a strong belief among scientists and science fiction authors that there must be other Earth-like planets in the universe, with alien species similar to us. For example, famous rocket scientist Wernher von Braun stated, “Our sun is one of 100 billion stars in our galaxy. Our galaxy is one of billions of galaxies populating the universe. It would be the height of presumption to think that we are the only living things in that enormous immensity.” Popular science fiction author Isaac Asimov attempted to come up with a plausible number of habitable planets among the estimated billions of stars in the just the Milky Way galaxy, His calculation focused on civilizations of alien life at or around our own current level of biological evolution. Asimov’s estimate came to 500,000. With today’s technology, it’s fair to say both von Braun and Asimov were not only right, but might actually have been conservative.

On November 4, 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-like planets within just the Milky Way Galaxy. Before we proceed, we’ll address a fundamental question. What makes a planet Earth-like? When we use the term “Earth-like,” we mean the planet resembles the Earth in three crucial ways:

1)   It has to be in an orbit around a star that enables the planet to retain liquid water on one or more portions of its surface. Cosmologists call this type of orbit the “habitable zone.” Liquid water, as opposed to ice or vapor, is crucial to all life on Earth. There might be other forms of life significantly different from what we experience on Earth. However, for our definition of an Earth-like planet, we are confining ourselves to the type of life that we experience on Earth.

2)   Its surface temperature must not be too hot or too cold. If it is too hot, the water boils off. If it is too cold, the water turns to ice.

3) Lastly, the planet must be large enough for its gravity to hold an atmosphere. Otherwise, the water will eventually evaporate into space.

If a planet is Earth-like, will it have life on it? The odds are it will. Hard to believe? It will become more believable if we examine how life spreads around in the universe. To understand this phenomenon, we will start with our own planet, which we know had life on it when the dinosaurs became extinct 65 million years ago.

From the fossil record, the extinction of the dinosaurs most likely occurred when an asteroid, approximately 10 km in diameter (about six miles wide), and weighing more than a trillion tons, hit Earth. The impact killed all surface life in its vicinity, and covered the Earth with super-heated ash clouds. Eventually, those clouds spelled doom for most life on the Earth’s surface. However, this sounds like the end of life, not the beginning. It was the end of life for numerous species on Earth, like the dinosaurs. However, the asteroid impact did one other incredible thing. It ejected billions of tons of earth and water into space. Locked within the earth and water—was life. The asteroid’s impact launched life-bearing material into space. Consider this a form of cosmic seeding, similar to the way winds on Earth carry seeds to other locations to spread life.

Where did all this life-bearing earth and water go? A scientific paper from Tetsuya Hara and colleagues, Kyoto Sangyo University in Japan, (Transfer of Life-Bearing Meteorites from Earth to Other Planets, Journal of Cosmology, 2010, Vol 7, 1731-1742), provide an insightful answer to our question. Their estimate is that the ejected material spread throughout a significant portion of the galaxy. Of course, a substantial amount of material is going to end up on the Moon, Mars, and other planets close to us. However, the surprising part is that they calculate that a significant portion of the material landed on the Jovian moon Europa, the Saturnian moon Enceladus, and even Earth-like exoplanets. It is even possible that a portion of the ejected material landed on a comet, which in turn took it for a cosmic ride throughout the galaxy. If any life forms within the material survived the relatively short journey to any of the moons and planets in our own solar system, the survivors would have had over 64 million years to germinate and evolve.

Would the life forms survive an interstellar journey? No one knows. Here, though, are incredible facts about seeds. The United States National Center for Genetic Resources Preservation has stored seeds, dry and frozen, for over forty years. They claim that the seeds are still viable, and will germinate under the right conditions. The temperature in space, absent a heat source like a star, is extremely cold. Let me be clear on this point. Space itself has no temperature. Objects in space have a temperature due to their proximity to an energy source. The cosmic microwave background, the farthest-away entity we can see in space, is about 3 degrees Kelvin. The Kelvin temperature scale is often used in science, since 0 degrees Kelvin represents the total absence of heat energy. The Kelvin temperature scale can be converted to the more familiar Fahrenheit temperature scale, as illustrated in the following. An isolated thermometer, light years from the cosmic microwave background, would likely cool to a couple of degrees above Kelvin. Water freezes at 273 degrees Kelvin, which, for reference, is equivalent to 32 degrees Fahrenheit. Once the material escapes our solar system, expect it to become cold to the point of freezing. If the material landed on a comet, the life forms could have gone into hibernation, at whatever temperature exists on the comet. If an object in space passes close to radiation (such as sunlight), its temperature can soar hundreds of degrees Kelvin. Water boils at 373 degrees Kelvin, which is equivalent to 212 degrees Fahrenheit. We have no idea how long life-bearing material could survive in such conditions. However, our study of life in Earth’s most extreme environments demonstrates that life, like Pompeii worms that live at temperatures 176 degrees Fahrenheit, is highly adaptable. We know that forms of life, lichens, found in Earth’s most extreme environments, are capable of surviving on Mars. This was experimentally proven by using the Mars Simulation Laboratory (MSL) maintained by the German Aerospace Center. It is even possible that the Earth itself was seeded via interstellar material from another planet. Our galaxy is ten billion years old. Dr. Hara and colleagues estimate that if life formed on a planet in our galaxy when it was extremely young, an asteroid’s impact on such a planet could have seeded the Earth about 4.6 billion years ago.

Given the vast number of potential Earth-like planets, why haven”t we detected alien life? The most convincing two reasons to my mind are:

  • First, the Earth-like planets are typically a long distance from Earth. The closest ones are ten to fifteen light years from Earth. The furthest are thousands of light years from Earth. The point is that even the closest ones are hard to study for signs of alien life. To illustrate this, let’s consider why haven’t we detected at least radio signals? The fact is radio waves defuse quickly with distance. For example, if we sent radio signals to a planet about ten to fifteen light years from Earth, the radio signal reaching the planet would be a billion, billion, billion times smaller than the original signal generated on Earth. Would they even be able to detect it and distinguish it from the background noise? If the aliens were extremely advanced, would they even be using conventional radio communications? The answer to both questions is unknown and problematic. This example does illustrate, however, that the distance between Earth-like planets makes the discovery of alien life an extremely difficult proposition.
  • Second, of the 40 billion Earth-like planets within just the Milky Way Galaxy only a fraction may support alien life and an even smaller fraction support advanced alien life. However, even with those odds, there must literally be thousands of advanced aliens inhabiting some of the Earth-like planets. So why don’t they communicate? One reason to consider is a highly advanced alien species may not deem Earth worthy of their efforts to communicate. Ask yourself this question. Do we attempt to communicate with ants and share our knowledge of nuclear technology? No! The question itself seems absurd, but that is exactly how we may appear to a highly advanced alien species. Let’s consider a scenario where they are technologically inferior to us. In this scenario, they would have no way to communicate. There are other possible scenarios, including a deliberate policy to not communicate, since such communication may lead to dire consequences for all concerned. Perhaps advanced aliens prefer to maintain a low profile to avoid detection by other advanced aliens or they may harbor concerns that they would significantly disrupt the natural evolution of a lesser advanced species.

Of course, there may be numerous other reasons we don’t encounter advanced aliens, all of which a simple internet search will uncover. Some argue advanced aliens have already contacted Earth, but governments in the know have kept it a secret. Others scenarios suggest highly technology advanced civilizations eventually destroy themselves. Look at our Earth’s point in evolution. Technologically advanced countries have developed various types of weapons of mass destruction. Many philosophers suggest that humanity has a 50% probability of falling victim to its own technological advances before the end of this century.

To directly address the subject question of this article, here is my view. It is highly unlikely we are alone in the universe. Said more positively, it is highly likely advanced alien civilizations exist on some of the Earth-like planets. We have not detected them because of our technology limitations. Those that are capable of communicating with us have chosen not to do so for one of several reasons. They do not consider us worthy of communication or they are concerned such communication is not in the best interest of either species. Lastly, they may be communicating, but only with the governments of selected advanced countries, which have kept such communication a secret.