In Search Of Habitable Planets : A Special article on Space Science
Since man first understood the existence of the star and planets, he has always wondered and dreamt of life beyond earth. Our advancement in technology has helped us a lot in the intense search of extraterrestrial life and it all starts with finding habitable planets. In fact the definition of “habitable” is constantly changing. Yet there is a number of “basic ingredients” required for any planet to hold at least the smallest forms of life, however, they are strict and uncommon among planets in the Universe. While we turn our powerful eyes to the countless stars and planets, we keep these criteria in mind.
The most fundamental criteria can be summarized in three categories: a source of energy, complex chemistry, and atmospheric protection from harmful radiations. Using these criteria finding out habitable planets is the first step of humanity in finding life beyond earth.
Source of energyThis is primarily essential. Here for us SUN is the source of energy that comes in the form of heat and light. There are two reasons why energy is essential. One is that, with the presence of sufficient heat, liquid water is able to exist. Scientists have long believed that wherever in the galaxy one finds water, one finds life. Of course, this need not always is the case. Liquid methane has been suggested as a possible replacement for water because it is an organic molecule. The second reason why energy is essential is that it is necessary for chemical reactions to occur. Every chemical reaction requires activation energy (the minimum energy required for a chemical reaction to occur); the energy required is supplied from the parent star. Without chemical reactions, life would be impossible. The energy have some limits also, if the planet is closer to the star then all the molecules may degrade. So the planet should be in a position only to have sufficient amount of energy, not more not less.
Complex chemistryA complex chemistry is required for life to form and exist, an abundant amount of molecules such as hydrogen, carbon, nitrogen, oxygen and halogens must be present. This is not the case with many planets, so it is at this basic criterion that the search for habitable planets narrows. For example, phosphorus is necessary to produce a molecule known as ATP, which provides the energy for most chemical reactions to occur in most living organisms on Earth. As long as there are these fundamental atoms that make up organisms, it is possible for life to exist on a planet.
Presence of atmosphereThe third criterion that scientists look for in a planet is the presence of an atmosphere. Every star bombards radiations on the planets around it; these radiations may cause mutation to DNA of a living organism. If the planet can form an atmosphere around it, it can filter the harmful radiations and protect the life forms that exist. The gravitation of the planet helps in forming the atmosphere; it holds the gasses to the surface. This atmosphere also helps to maintain the temperature at night times of the planet. However, it is possible for life forms to be protected by some other means. For example, on Saturn’s moon Titan, life may exist beneath the frozen surface of the moon, because the underground layer is at least protected from radiation.
Astrobiologists keep these criteria in mind, but it is difficult to determine how well a planet fits these. When scientists do find a planet, they assess it with two indices: one is the Earth Similarity Index and the second is the Planetary Habitability Index. The Earth Similarity Index is simply based on how large the planet is compared to Earth and how it may be similar (in terms of position in the solar system, number of moons, etc.). The Planetary Habitability Index uses the criteria mentioned already, as well as attempting to answer the question: is photosynthesis possible on this planet? By being specific to these two indices, Astrobiologists can narrow down from the trillions of planets out there to a select few.
We have found something interesting in our solar system, Saturn’s moon “Titan” has a habitability index of 0.64, Mars have a habitability index of only 0.56, and also we have found planets outside the solar system with similar habitability indices as Titan and Mars. The fact is Earth has the value of 0.96 which is much more than the found values of others. Yet scientists remain optimistic and open-minded, willing to accept that liquid methane could support life instead of water or that a planet could be its own source of heat (like the thermal vents at the bottom of Earth’s ocean). Perhaps one day we will find some extraterrestrial microbes, or not. Either way, it all begins with determining the possible planetary candidates.
Let’s hope that we’ll one day find some Friends outside, who are now just in our imaginations.