Of all the so-called big questions, perhaps none inspires more curiosity than the following: Is there life elsewhere in the universe? The question leads to many other questions, speculations, possibilities and impossibilities. In recent years science has made tremendous progress towards understanding the universe, both in what is out there and how it came to be. The number of stars and galaxies is enormous, and we now know that many stars have planetary systems (nearly 2000 exoplanets have been discovered). Some planets outside our solar system are believed to be earth-like, in size, composition and location in relation to their host star.
Recent discoveries have shown that other locations in the universe may have conditions similar to Earth. Our galaxy, the Sun and the Earth are not unique. That said, the Earth supports life due to a series of coincidences that may be unique. Still an unimaginably large cosmos presents many opportunities for life-giving conditions to align. This means that the possibility for alien life may be greater than once thought. There seems to be 3 ways in which humans could discover extraterrestrials: 1) Searching the universe for life. 2) Sending signals in outer space so aliens could intercept them. 3) Aliens discovering us. Let us examine these possibilities a little further:
Searching the Universe for Life
Numerous unmanned space probes have explored our Solar System. The firsts space probes to visit other planets were launched in the sixties, even before the first lunar landing. By the seventies probes were reaching the outer planets, and in 2015, New Horizons made its historic Pluto flyby (the farthest planet when I was in school). Several rovers have landed on Mars, transmitting images and analyzing soil samples (the first successful mission was in 1976). Presently, the rovers Opportunity and Curiosity are still operating on Mars.
Scientists have discovered much about the composition of the planets and their moons, including evidence for liquid water. A few moons of Jupiter and Saturn are believed to contain oceans of liquid water beneath their icy surfaces. And in October 2015, NASA made the announcement that liquid water flows on Mars. A number of conditions are necessary for life to exist, but liquid water is a must for all known life on Earth (the starting line in the search for life). At least if life exists somewhere without water, it would be to foreign for us to imagine.
If there is alien life in our Solar System, it would be simple life and probably microbial; but what about intelligent life? How far do we have to look? The Solar System is merely our cosmic neighborhood.
In 1995, the first exoplanet was discovered and many more followed. The existence of the planets is inferred by studying minor changes in starlight, which are caused by the presence of planets; however, the distances involved are immense. The closet star system is Alpha Centauri (a 3 star system), which is 4.25 light years away. By comparison it takes about 8 minutes for the Sun’s light to reach the earth. The Milky Way alone is 100,000 to 120,000 light years in diameter, and contains over 200 billion stars. Beyond our home galaxy, there are over 100 billion galaxies in the observable universe.
By numbers alone, it seems that the opportunities for extraterrestrial life are endless. But the odds against discovering alien life seem equally as great. At this time indirect evidence is all we have. For example: exoplanets that may be located in habitable zones or distant regions that have chemical compositions similar to our Solar System. Maybe all we will find is information or signals which have to be decoded, and conclusive evidence may never be found.
Sending Signals in Outer Space
Humans have been inadvertently sending signals to the universe since the first radio and television broadcasts. By now the signals have reached thousands of star systems. However, they travel as electromagnetic waves and will go undetected unless someone has an appropriate receiver at the other end. Even if the signals have crossed advanced civilizations, what are the chances that they have built earth-like technology? There is also the evolutionary timeline to consider. Could some civilizations be too early in their development, or could others have long gone extinct?
Attempts were made to purposefully send messages to outer space. In 2008, the Beatles song “Across the Universe” was broadcasted towards Polaris (the North Star). But even traveling at light speed the signal will take over 300 years to reach its destination. And if we get a reply, it will take another 300 years.
The space probes Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 have completed their missions exploring the planets, and have left the Solar System (speeding away indefinitely). They all contain time capsules, with information about humans and our location in the universe. Incidentally, the well-known image of the pale blue dot was taken by Voyager 1 as it left our Solar System; a snapshot of the earth from 6 billion miles away.
Sending space probes into interstellar space solves one problem, but creates another. On the one hand they are concrete objects (not like radio waves), on the other hand they travel much slower than radio waves. For example, the nearest star system is 4 light years away (that’s 4 years for a radio signal). By comparison, it will take 70,000 years for the space probes to travel the same distance. Either way, the odds appear slim that our messages will ever be noticed.
Aliens Discovering Us
It is possible that aliens have already discovered the Earth; they may even have tried to communicate with us. Some people believe that aliens have visited the Earth, but for a logically minded person the stories are far-fetched. From a scientific perspective, there is no evidence to support such claims. Everything scientists know about space travel makes alien visitations practically impossible. The distances are simply too great; it would take hundreds of generations to make the voyage (unless aliens have lifespans of a 1,000 years or use teleportation and wormholes, though we shouldn’t believe everything we see in Star Trek).
The most likely form of alien contact would be indirect, such as something moving at light speed, like an electromagnetic wave. An alien space probe sent many years ago would be a possibility, however, it would be a tremendous stroke of luck to pass anywhere near the Earth. Then again, it depends on how many probes are out there. The odds of being found or finding something is proportional to how many are looking. Therefore, we don’t know if humans are the only species looking to the stars for life.
A Numbers Game
By studying the light spectrum of distant galaxies, astronomers have discovered that the chemistry of the universe is similar throughout. In addition, at the largest of scales the universe has evolved basically the same everywhere. The Earth has intelligent life because of a series of fortunate events (fortunate for us); it could also have occurred elsewhere. Or maybe a very different form of life evolved due to totally different circumstances.
For example, take the Earth’s distance from the Sun as one of many specific variables. The Earth is about 93 million miles from the sun, just the right distance to allow for liquid water. To appreciate how precise the location is, the change from summer to winter is caused by a 23.5 degree tilt of the earth’s axis. As the Earth orbits the sun it either tilts towards (in summer) or away (in winter) from the sun. That’s it. Of course the northern and southern hemispheres have their seasons in reverse relation to each other.
So is the existence of life simply a numbers game? Given the unimaginable size of the universe, is it inevitable that conditions will be just right somewhere else? With the number of planets that likely exist, even if the odds for life were a billion to one, there would still be life on a billion planets. If I had to make a call, I think the odds are good that there is life elsewhere in the universe. However, the odds are slim that we will ever discover it. The distances involved present challenges that may be too much to overcome.
References: Big Picture Science: Life in Space, April 20, 2015.
Big Picture Science: How to Talk to Aliens, January 12, 2015.
Universe Today: 10 Facts About the Milky Way, by Matt Williams, http://www.universetoday.com/22285/facts-about-the-milky-way/ December 3, 2014.
Universe Today: What is the Closest Star, by Fraser Cain, http://www.universetoday.com/102920/what-is-the-closest-star/ June 14, 2013.