Ever since people understood that we are living on a planet that is part of a larger solar system, and then galaxy, and then universe, some have wondered if we were “alone”, as in unique, or if there might be intelligent life on some other planet, (assuming we have intelligent life on Earth). Intelligence is relative. More modestly, some wonder simply if there is “life” on some other planet. It may be depressing, but for our solar system, we are the most intelligent life form, and with the possible exception, of Mars, the only place where it seems life can or is likely to exist. And we will know relatively soon if life in any form ever existed on Mars. My childhood dream of an alien civilization living along grand canals is just not the case.
First, what do we mean by life? Here I define life as something (and organism) that is self-sustaining and can replicate. I don’t mean human or even intelligent life, or animal life, or any form of life such as exists on earth. Just life. That covers a wide range of existence. Life started on earth a long time ago, relatively soon after its formation. How it started is not a known and subject to debate on many levels.
What makes a planet capable of sustaining or creating life? What makes earth so special? This is going to sound like Goldilocks and the three Bears. First, it has to be the right distance from the sun, because any closer to the sun and it his too hot, and any further from the sun, it is too cold. It has to be just the right size (as is Earth) to have an atmosphere that is neither carbon dioxide (as in a smaller planet) nor hydrogen (as in a larger planet). Gravity (the size of the planet) is what holds the atmosphere and is key in determining what gasses will form and exist the atmosphere. And the temperature (distance from the sun) makes it possible to have liquid water, which is a very useful ingredient in life. We are looking for some place that if we were to go there that we could survive and adapt and even prosper. And someplace where some form of life would be there to “greet” us. Next article we will consider whether that is even possible or probable.