Search for planets in the habitability zone

Let us suppose that all the conditions are met and on some planet life is possible or is already present. How can we discover that when it is still quite difficult to discover and conclusively confirm a planet in the habitable zone around its star. At the enormous distances at which even the nearest stars with exoplanets are situated, it is not possible to directly observe signs of life. There is no way to take pictures, similar to those of Earth made from the artificial satellites. We can not send spacecrafts either. The closest star system to us, Alpha Centauri, is at a distance of over 4 light years and the present day human space probes will have to travel for over 100 000 years to reach it!

Of course, the end goal in the study of exoplanets is to find unambiguous evidence for the existence of intelligent alien life. Whether it will happen sooner, or later, or not at all depends on two unknowns: how widespread is life in the Galaxy and how lucky we get. But unless we get lucky, the search for signs of life could take decades. Discovering another inhabited planet among the billions of stars is like searching for a particular sand grain on the beach….with naked eye and from the balcony of your hotel room. So the first step is to build even larger and powerful telescopes and keep our fingers crossed. Or hope someone comes to visit. As to the first unknown – the incidence of life in the Galaxy – an answer is given by the Drake equation.

Drake equation and the probability for alien life and sentience

Drake equation allows us to make an estimate of the probability for alien life and sentience. It estimates the number N of advanced civilizations in the Galaxy.

Here R* is the rate with which stars form in the Milky way (i.e. the number of stars that are being born in one year), fp is the part of the stars with planet systems, ne is the number of habitable planets in the planet system, fl is the probability for life to actually develop on a habitable planet, fi is the probability the emerged life to develop in its evolution sentient species, fc is the probability the sentient species to develop an advanced civilization able to search for other intelligent life, L is the lifetime of the civilization (or at least the time during which the civilization searches for and sends signals in space).

In 1960, when Drake first writes his equation, all included in it parameters were unknown. The only thing we could say with confidence was that N equals at least 1, we do know that our own civilization exists, after all… Due to the ambiguities in the formula, it is likely that every person on Earth have their own opinion about the final result. The estimates of the value of N so far are quite speculative and vary from “one” (the Earth) to one billion depending on the optimism of the researchers. These days there is an indisputable progress being made and among the almost 4000 discovered so far exoplanets, we know of at least 55 terrestrial type planets and superearths that are in the Goldilocks zone. This strengthens the hopes that alien life is widely spread out in the Galaxy. Otherwise, as Carl Sagan said: “ is going to be an awful waste of space”.

One way or the other, we still do not have any credible evidence for the existence of either alien life or alien sentience. What we do know is that “ the lack of evidence in not an evidence for the lack of life” and the astronomers have grounds to keep searching.