The meteorite named ALH84001 impacted the Earth in present-day Antarctica more than 15 million years ago. An extensive study of this specimen reveals that the components contain structures, which appear more or less like bacteria or other similar microscopic organisms. Such potential bacteria-like microbes must have fossilized in the meteorite during its formation. This further strengthens the argument that millions of years ago, life did exist on Mars!
Extraterrestrial life has always been the subject of human expressions in various forms like movies, paintings, or popular stories. This topic has been a subject of intense debate, not only among the common people, but also among the intellectuals around the world. However, one fact clearly stands tall; there is no proof till date that can conclusively state or authenticate the presence of a life form other than that on Earth. Even though there is no evidence, the possibility of a civilization existing somewhere out there may be worth considering. Crop circles, another phenomenon that has benumbed humans, is thought to have been influenced by the aliens.
5 Controversial Theories of Life on Other Planets
Drake Equation Theory
In 1961, US astronomer Frank Drake had developed an equation that suggested the presence of many more life-bearing planets, and the possibility of an alien life form that can communicate. The equation includes several factors like the age and formation of stars, number of Earth-like planets, probability of communication, etc. Initially, this method garnered much attention, as the statistics produced were quite plausible, but later on, as numerous space systems were studied, the calculation started becoming more complex, ultimately producing results that vary a lot.✦
The different components of the equation given above can be explained in a very simple, yet effective format. Out of all the components, the mean rate of star formation R*
has been calculated approximately till now, which indicates that about four new stars are born at least in our galaxy, the Milky Way, every year. The other factors can only be assumed or guessed, depending on the level of research.✦
In a typical galaxy, only a quarter of all the stars are included in solar systems. These formations have at least two planets that can be potentially inhabitable. The components of development of life, evolution of intelligence, and the ability to be technologically superior might develop at a chance of 50% on such planets. If at all a civilization or a race evolves, it might attempt communication in around 4,000,000 years. Thus, putting all these factors in the formula, the equation becomes:
NT = 4*0.25*2*0.5*0.5*0.5*4,000,000
Thus, the number of potential civilizations that are still undiscovered is about a million, which may attempt contact in the near future. But, on a more practical scenario, the existence of different planets that can harbor such a number of forms is very rare and remote.✦
The complexity arises from the fact that there are countless factors and parameters that must operate before a life form actually emerges and evolves. If all the variables are summed up, they can stated as: combination of various inorganic molecules that remained in a residual form or a part of the primordial soup (a term that explains the state of matter after the Big Bang), formation of organic molecules that from the inorganic ones, the existence of a star like the Sun that will provide energy for biochemical reactions to take place, change in conditions that are suitable for unicellular life, extreme obstacles that have to be overcome in order that multicellular life becomes successful, etc. As living proof, all the organisms, whether unicellular or multicellullar, primitive or advanced, have passed through these various stages described above.
The term 'panspermia' has a Greek origin that means 'seeds everywhere'. According to this theory, life was 'seeded' on Earth from outer space, i.e., life was brought to our planet from a foreign space body. Such bodies could have been comets, meteors, or even asteroids that caused an impact on our planet. Mostly, the organisms were in the form of microbes, as multicellular life did not exist millions of years ago. Thus, if life did not first originate on Earth, did it flourish on other planets, before the Earth was even born? If abiogenesis (the process of appearance of life) is possible on Earth, can it take place even on other planets?✦
The answers to such questions can be attempted with the help of this theory that was proposed in 1973 in its modern form by two people: a physicist called Francis Crick and a chemist called Leslie Orgel. The theory in its crude form was first mentioned around 450 BCE by a Greek philosopher called Anaxagoras. There are three subtypes of the panspermia concept that explain the origin of life.Directed Panspermia
According to this concept, highly advanced civilizations that are more developed than us do exist on planets that are close to our world. These civilizations would have intentionally or accidentally spread the biochemical structure to our planet, during its early stages of formation. Though more or less improbable and a bit fictitious, this concept is nevertheless possible.Ballistic Panspermia
The spreading of life forms from one planetary body to another that are present in the same solar system is called ballistic panspermia. This concept is widely used to explain and debate that life on Earth in fact originated due to seeding from Mars, which was possibly inhabited by alien life forms millions of years ago.Lithopanspermia
One of the most difficult processes, lithopanspermia refers to the seeding of biochemical forms on another planet that is present in a different solar system, as compared to the system from where the spreading medium belongs to. The DNA molecules have to survive the harsh conditions of outer space before reaching the particular celestial body.
Although the panspermia concept has garnered much attention in recent years, it does not explain a few points.
- A major one is that if life existed on Mars before Earth, where is it now? If it got extinct, then suitable fossil appearances should have been found till now by the three robotic rovers on the red planet.
- Also, if a more advanced civilization spread the biomolecules to our planet, why didn't they contact us, i.e., humans, again after our planet underwent significant changes regarding evolution and development of higher life forms?
Such questions might be answered if we ever find aliens on another planet.
Biochemical Evolution Theory
The most important factor in determining the presence of life is the environment. If we assume that the conditions such as those on planet Earth can be created anywhere in the Universe or already exist in it, then the idea of life on that planet is almost a certainty. Based on the survival mechanism of human beings, we may be able to predict the possibility of a whole new set of beings similar to us.✦
Human beings need air to breathe, water to drink, and food for survival. The availability of these factors is a must for each one of us to sustain on Earth. Hence, there is a lot of curiosity among experts with regard to the planet Mars, Mercury, Jupiter, the moons of Jupiter and Saturn like Europa and Titan, respectively, in our solar system. These have been found to have traces of water on their surface. Therefore, there is every chance that life will be able to thrive on these bodies.✦
The interaction between inorganic compounds gives rise to organic molecules like proteins, amino acids, carbohydrates, lipids, etc. For such processes to take place, the availability of liquid water is essential. On our planet, the appearance of unicellular forms and the subsequent multicellular organisms too, first took place in the oceans. Moons like Titan, Europa, Enceladus, etc., are being said to consist of oceans underneath their solid surfaces. These bodies particularly might contain methane (CH4
), which plays a crucial role in the development of primitive life forms. Thus, the chances of finding extraterrestrial microbes on such moons are quite high, though the occurrence of complex life forms in our own solar system is not detected as of today.✦
The 1976 Viking probes sent to planet Mars had detected chemical activity, which was quite similar to the daily activity of living organisms on Earth. NASA had conducted detailed analysis of the samples collected by the probes and closed the speculation, citing lack of concrete results. In 2001, another updated theory based on a complex set of equations suggested the presence of many planets that could sustain life. Jupiter's moon Europa radiates infrared waves, which is typical of an organism such as bacteria. Hence, it is subject to a very intense scrutiny and observation by NASA, but substantial evidence still eludes us. However, all the above incidents have encouraged a lot of interest and a renewed effort by researchers all over the world to settle the claims of life on other planets once and for all.
Theory of Extremophiles
The organisms (mostly microbes) that can survive the most harsh conditions ever present on our planet, as well as in the outer space are called extremophiles. These life forms are seen on our planet, where extreme conditions are present. Extremophiles are known to exist below the Antarctic ice sheets, near volcanic fumaroles and vents, underwater volcanoes and hot spots, the Earth's exosphere, etc. Some examples of such organisms are: Tardigrades, Chernobyl Fungus, Pompeii Worm, etc. Few multicellular and bioluminescent organisms also exist only at great depths in the ocean, where sunlight cannot penetrate.✦
Some planets and even moons also exhibit such harsh conditions, though exact replication is not present. According to expert study and astrobiological research, if the Earth can harbor such organisms under inhospitable parameters, such microbes might also be present on other planets, depending on the intensity of the specific aspect, where the microbes might be found. Such life forms also have to survive against the bombardment of cosmic rays, gamma rays, solar wind, etc. In 2013, a bacterium named Tersicoccus phoenicis
was discovered in two regions of the world, one each in South America and the USA. This microbe was only found in the docking chambers of the spacecrafts and nowhere else. Till present, even after a year, extensive studies have been carried out in various regions on our planet, and this organism has not been found in any environment. Thus, it can be assumed that Tersicoccus phoenicis
was carried by the spacecraft on entry in our atmosphere from the outer space onto our planet. This microbe may possibly thrive in the exosphere region, or even beyond in the space vacuum.✦
Another experiment that is famous for proving this theory is the one where few scientists created surface and subsurface conditions that are similar those assumed to be present on the Europa moon of Jupiter. This moon was particularly chosen as it is the best candidate for the occurrence of marine or oceanic conditions below its surface. In the study, microbes like N. magadii
and D. radiodurans
were subjected to intense UV radiation. The results showed that most of them died, but few actually survived the onslaught of the UV rays. This indicates that Europa is a prime candidate for the thriving of microorganisms in its oceans, apart from our planet.✦
The theory of extremophiles is the least credible one among all, as very little data has been obtained or is available with scientists. This subfield of astrobiology needs sufficient attention, if we continue our search of alien life on other celestial bodies.
Habitable Planet Theory
Another theory assumes that there is a unique structure (as that of the Earth) that has got the right components to enable life to flourish. The discoveries until now have failed to conclusively prove the existence of conditions exactly similar to those on Earth. If we consider part of the Universe that we know, as a sample of the larger expanse, there might be no life at all. This is again just a probability based on the knowledge, which we have been able to garner till today.✦
The Earth is placed in a habitable zone in our solar system, and this zone encompasses the space consisting of at least half of Venus and the whole of Earth and Mars. It extends a bit beyond the Martian orbit. The recently discovered planet called Gliese 581g is a potential example of a planet that exists in the habitable zone of its solar system. There are six planets including 581g that orbit a red star called Gliese 581. This body is more than 20 light years away from the Earth. Other examples of planets that exist within the habitable zones of their system are Kepler-186f and HD 40307g. These exist about 40 and 500 light years away from Earth, respectively. Every exoplanet discovered is first classified on the basis of its surface temperature, presence or absence of atmosphere, size, distance from its sun, composition, etc. Based on these factors, it is categorized to be under a non-habitable or a habitable zone.✦
The presence of life beyond the solar system is a very contentious matter. There is a section of people who believe that on account of the expanse of the Universe, there must be a planet like Earth that supports life
. The issue is a matter of research and speculation. The argument that there could be a planet like Earth, that exists in the realms of the Universe is a very likely event. Considering the sheer expanse of the Universe, we may be tempted to believe in the theory that there is a parallel civilization flourishing somewhere out in the cosmos. The only constraint facing us and perhaps the aliens is that both sides are unaware of a communication mechanism. We, humans, have come a long way since our evolution on this planet. But our achievements have not yet been able to make us aware of such an extraterrestrial being (if at all it exists). Similarly, if we again assume that a civilization does exist somewhere out there, even they have a long way to go and actually communicate with us.
The SETI (Search for Extraterrestrial Intelligence) Center located in Mountain View, California is dedicated to finding life in the Universe. This organization works in two broad areas: (i) research and development and (ii) projects. The first part includes development of new search techniques and equipment that would facilitate the search―for example, large telescopes and communication devices. The projects are the actual work directed with a well-planned strategy to search for the evidence of life. A lot of people claim to have seen an alien life form. The being is 'immortalized' in our memories by its depiction in various Hollywood movies, which leads us to firmly believe that there is life on other planets.
They say, "reality is more important than dreams", and all our assumptions and theories are still distant dreams created by our mind. The fact remains that so far we have not been able to detect life on other planets, but at the same time, there is no denying the possibility that it might actually exist!