In Search of Another Earth: An Extremely Rare Planet

Abstract

Although nascent or rudimentary life may originate and survive under extreme environment that is much different than the environment we experience in our daily life, complex life like eukaryotic multigenerational, multicellular organisms require numerous favorable conditions to survive and to develop into large animals. We cannot search for rudimentary or even developed life in all places of the vast galaxy. Further, the only way for us to find out life in outer space is by analyzing the light we receive from the celestial objects, and hence, we can only search for signatures of life in outer space. Fortunately, the presence and evolution of life influence the environment in the same extent the environment affects the survival and the evolution of life. Therefore, the first step towards finding out life outside the Earth is to find out another Earth. Another planet which is very similar to the Earth should have almost all the conditions that favor life to originate and to evolve. But what are those conditions? We are so accustomed to live under this planetary environment that we often do not realize them. We are even not aware of many of the conditions that indirectly preserve our life. If some day, the temperature rises high, we feel terribly hot and we realize that a few more degrees increase in the temperature may cause severe health problems and sometimes even death. On the other hand, we need sufficient protection to survive in the extreme cold of Antarctica. But these are direct effect. There are several indirect phenomena that provide protection to life in long term. Unless we find out a planet having all these favorable conditions, we cannot say that the planet is like our Earth, and so we cannot say with certainty that life exists there. It is worth mentioning here that a rocky planet that has appropriate temperature for water to exist in liquid state is called a habitable planet. But it is not necessary that all habitable planets may harbor life. Let us pose this problem in a different way—let us assume that somewhere in our galaxy there exists another Earth wherein intelligent creatures live and they too are curious to know if there is life beyond their own planet. Suppose that they have similar astronomical and biological knowledge that we have but their civilization is much older and hence more advanced than our civilization and so they have far better technological means to search for life outside their planet. How can such intelligent creatures find out our planet, the Earth? How will they realize that the Earth is not only habitable but also harbors life? Of course they cannot investigate each of the 200 billion stars that our galaxy contains because that will take millions of years even if they have highly advanced technology and sophisticated instruments such as ultrafast computers and extremely powerful telescopes. So the extraterrestrial astronomers will use their knowledge on the galaxy, stars and their properties, etc. and adopt a few clever steps to reduce the time and effort. Their ultimate goal is to find out our Earth and to realize that life exists here on the Earth. In other words, their aim would be at the least to understand that the planet Earth is capable of sustaining life. These steps are related to the conditions needed for complex life to survive and to evolve.

It took less than an hour to make the atoms, a few hundred million years to make the stars and planets, but five billion years to make man.

—George Gamow

(In The Creation of the Universe, 1952)