N_* is the number of stars in our galaxy, the Milky Way, which by some estimates has 200 billion stars. f_p is the fraction of stars with planets (like the Sun), and its value is believed to range between 0.5 and 1. N_e is the number of planets that have environmental conditions favorable to the growth of life as we know it. It is believed that 10 percent of stars meet the criterion.

f_l and f_i are the fraction of planets that have life and intelligent life respectively. Both these values are assumed to be around 0.1. f_c is the fraction of planets that have developed a medium to transmit information into space, and L is the number of years an advanced civilization survives. What if an intelligent civilization destroys itself, say by a nuclear war, and is unable to transmit information about itself into space.

If there are 200 billion stars in the Milky Way and over 100 billion galaxies in the universe then for an extremely small probability of life on a planet, in a given star system, the probability that there is life somewhere beyond Earth is almost 1 which is 100 percent. This is not a difficult conclusion to arrive at, simply because the number of stars is very large.

Origin of Life

About fifteen billion years ago when the universe first began there were clouds rich in hydrogen and helium gases. Hydrogen was the most abundant element present, and under the right conditions of temperature and pressure four hydrogen nuclei (H¹) combined to form a helium nucleus (He⁴). Thermonuclear reactions such as these release enormous quantities of energy, and this energy in the case of stars like the Sun, is transformed into radiation emitted into space. For example, the energy released when one gram of hydrogen is converted into helium is 6 * 10¹⁸ ergs.

As gravitational attraction among gas masses increased, gases began to condense into clouds until it reached the stellar mass. During this evolutionary phase of a star, smaller gas masses formed around it. These would eventually transform into planets. As chemical reactions continued, carbon (C¹²) was formed when three helium nuclei combined. Successive synthesis of elements continued and resulted in the formation of elements of higher atomic weights.

Composite image of the supernova remnant W49B. NASA

This process will continue till iron (isotope Fe⁵⁶) is formed, and any successive reaction with helium nuclei now creates elements that are unstable. A star dies when all of its matter transforms into iron. As the star explodes it will release into space elements like carbon. With a great number of these explosions over billions of years, the universe is enriched with elements floating freely in space, and the abundance of these elements provide the necessary elements for the origin of life.

For a star the size of the Sun or smaller, the collapse is not accompanied by an explosion resulting in a white dwarf. About twenty percent of the mass of the star remains behind as a cloud of gas surrounding the white dwarf. Larger stars explode as they collapse. The bigger the star, greater is the explosion. These supernova explosions release into space most of the star mass as gas.

An element is a pure substance that contains atoms of one type. About 98 percent of the mass of a living organism is composed of six types of elements – carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorous. In nature, many elements exists as several isotopes, which have the same number of protons of the original atom, but with a different number of neutrons in the atomic nucleus. Unstable isotopes, called radioisotopes, give off energy such as alpha, beta, or gamma radiations, and radioactive decay transforms an atom into another atom, usually of a different element. The number of electrons in an atom determines how the atom reacts with another atom. This reaction is called a chemical reaction.

Orbiting electrons in an atom emit spectral lines that can be measured by a light spectrometer. Every element has its own unique set of colored lines which helps identify the element. For example, the spectral lines emitted by carbon are different from those emitted by hydrogen. When sunlight passes through a prism it gets broken into different colors (rainbow); similarly radiation emitted by an atom is separated into its spectral lines by a spectrometer. Spectral analysis helps in determining the composition of stars. Observing solar spectral lines (Frauenhofer Lines) indicates presence of many elements like hydrogen, helium, carbon, oxygen, nitrogen, and potassium, to name a few.

By late 1960s only carbon-hydrogen (CH), carbon-nitrogen (CN), and oxygen-hydrogen (OH) combinations had been found in outer space. In 1968 water (H₂O) and ammonia (NH₃) were detected. Soon thereafter formaldehyde (HCHO), cyanoacetyllene (HCCCN), methyl alcohol (CH₃OH), and methylacetylene (CH₃CH) were discovered. Scientists have confirmed presence of carbon compounds in meteorites, and stellar clouds and dust. Astonishingly, all complex molecules involve carbon chains. Carbon, it is now believed, is essential for the growth of life.

Jean Hiedmann, an astronomer with Paris Observatory, who specializes in the search for advanced forms of life in space, defines the evolutionary process in five steps. At the cosmic stage, characterized by the Big Bang and the origins of the cosmos, chemical synthesis took place together with the formation of stars and planets. The organic stage followed the cosmic stage, and it marks the origin of molecules fundamental to the growth of life. In the prebiotic stage these molecules became more complex. Amino acids and nitrogenous bases developed, and this was succeeded by the primitive biological stage that saw the growth of simple life forms like bacteria. The final stage is the advanced stage, and according to Heidmann this stage foresaw the evolution of life into more complex forms.

Bacteria are unicellular organisms with a simple cell structure. The cell of a bacterium lacks a nucleus. Following the origin of bacteria, eukaryotes evolved. This diversification of early microorganisms into archaeobacteria, the eubacteria, and the eukaryotes happened about 2.3 billion years ago. The earliest fossils unearthed indicate presence of DNA and amino acids in a eukaryote nucleus. An important function of the nucleus is the storage of information necessary for cell division. It is through cellular division, in a multicellular organism, that specialized cells which perform specific functions, are formed. Also found in the eukaryotes is chloroplast that is necessary for the important life process of photosynthesis.

According to Heidmann life appeared in four successive stages. In the first stage simple organic molecules synthesized in space and submarine vents developed a prebiotic chemistry, which was followed by a pre-RNA world where evolution through mutations occurred. In the third stage the RNA appeared, and acting as its own enzyme developed different functions. Lastly, the first microorganism like the progenote appeared which gave rise to the eukaryotes that led to plants and animals.

Initially, it is believed, there was no oxygen in the atmosphere. Before life evolved, earth’s atmosphere was rich in methane, ammonia, and other toxic gases that could not support the growth of life. Photosynthesis carried out by biological activity released oxygen, and one source of photosynthesis was the Cyanobacteria, an aquatic bacteria also known as blue-green algae. Even today colonies of Cyanobacteria can be seen in some coastal regions of Australia. When enough oxygen had been released to the atmosphere it formed the ozone layer that protects mammalian life from harmful ultraviolet radiations of the Sun. As the conditions became more habitable, around 700 million years ago, life evolved as fungi, plants, animals, and protozoans.

In the 1950s Stanley Milley and Harold Urey at the University of Chicago conducted an experiment to simulate conditions on primitive earth. In the experiment Miller and Urey used water, ammonia, methane, and hydrogen. Water was heated to create vapor, and electricity was passed to induce “lightening”. A few days later it was observed that amino acids had been formed. Amino acids when joined by peptide bonds forms proteins that is essential for proper functioning of a living cell. The experiment led credence to the hypothesis that natural processes could produce building blocks necessary for life. If there are conditions in outer space similar to the conditions of the experiment, it could possibly suggest presence of life elsewhere.

DNA Embraces the Planets jonlomberg.com

Some scientists believe that the basic building blocks of life were transported to earth from outer space. The theory that life is distributed in outer space through spores is called Panspermia. Could DNA have traveled from outer space to earth? Francis Crick, one of the discoverers of the DNA molecule structure, believed that seeds of life were carried to earth from elsewhere. Crick called it Directed Panspermia. He argued that these spores of life were deliberately sent to earth by an intelligent civilization since it was very unlikely that spores could have traveled interstellar space.

In a paper he coauthored with Leslie Orgel, Crick wrote: “It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite. As an alternative to these nineteenth-century mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet. We conclude that it is possible that life reached the earth in this way, but that the scientific evidence is inadequate at the present time to say anything about the probability.”

On July 31, 2001 Reuters published a report Scientists Claim Evidence of Life in Outer Space. Chandra Wickermasinghe, a Professor at Cardiff University, was quoted as saying, “There is now unambiguous evidence for the presence of clumps of living cells in air samples from as high 41 kilometers (25 miles), well above the local tropopause (16 kilometers up), above which no air from lower down would normally be transported”. Wickermasinghe and other researchers believe that the presence of earth-like bacteria in space supports the panspermia hypothesis.

Earth lies in the habitable zone where conditions are favorable for the growth of life. A planet close to Sun (or a Sun-like star) will experience temperatures that are exceedingly high, and planets farther away are too cold for life to grow on its surface. Planets larger than Earth will experience greater gravitational force resulting in higher surface gravity. For example, an object on the surface of Jupiter will weigh more than a similar object on Earth. Whether greater surface gravity retards growth of life is a matter of conjecture since life is known to adapt itself in extreme conditions, but evidence suggests it would be extremely difficult if not impossible.

Gliese 876 System A second planet has been discovered orbiting Gliese 876, making it one of the most bizarre systems found to date. The two planets are eternally locked in sync. A lunar landscape is shown at the bottom. Copyright Lynette Cook, all rights reserved

If life on other planets is similar to that on Earth these planets should have a Sun like star to provide warmth, without which liquid water is not possible. One good way to search for life elsewhere is to look for Earth like planets in their own “solar systems”. It is estimated that there are a few hundred million Sun like stars in the Milky Way alone, which could mean billions of planetary systems. The challenge is to detect these stars and the planets revolving around them.

In 1995, 51 Pegasi was discovered in the constellation Pegasus some fifty light years away. This was the first discovery of its kind. Located close to it is the planet 51 Pegasi b, which is closer to its parent star than Mars is to the Sun. Consequently, the surface temperatures are extremely high excluding the possibility of life. Since this monumental discovery other extrasolar planets have been found.

More recently, the European Southern Observatory Telescope at La Silla (Chile) found a planet around the star mu Arae. With this discovery mu Arae now has three planets revolving around it. What makes this discovery spectacular is that the planet is fourteen times the size of Earth and like it made of rocks. Planets of that size are mostly made of gas and ice.

Although no contact has been made with extraterrestrial civilizations it cannot be perceived as evidence that life beyond Earth does not exist. All our explorations so far have been miniscule in comparison with what lies within and outside the Milky Way. Advances in space technology and some recent successes in remotely acquiring data from Mars are encouraging. NASA has announced new goals for space exploration that could see human and robotic missions to the moon, Mars and beyond. When and in what way will the first contact be made is still unknown, but when it happens not only will it be the greatest scientific discovery ever, its effect on human life and “worldview” will be far reaching.