Astronomy - Extraterrestrial Life

Extraterrestrial Life


Definition of Life

Encyclopaedia Britannica

Life, living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction.


From a physics perspective, living beings are thermodynamic systems with an organized molecular structure that can reproduce itself and evolve as survival.

Extra-terrestrial life also called alien life, is life that occurs outside of Earth and that probably did not originate from Earth.

These hypothetical life forms may range from simple single cell organism to beings with civilizations far more advanced than humanity.

Since the mid-20th century, there has been an ongoing search for signs of extra-terrestrial life.

This encompasses a search for current and historic extra-terrestrial life, and a narrower search for extra-terrestrial intelligent life.

Depending on the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send communication signals.

Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe.

This hypothesis relies on the vast size and consistent physical laws of the observable universe.

According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, as well as well-regarded thinkers such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth.

Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.

The Fermi Paradox

The Fermi paradox or Fermi's paradox, named after physicist Enrico Fermi, is the apparent contradiction between the lack of evidence and high probability estimates for the existence of extra-terrestrial civilizations.

The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old.

Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread by meteoroids, for example between habitable planets.

In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.

According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.

Since the 1950s, scientists have proposed that "habitable zones" around stars are the most likely places to find life.

Numerous discoveries in such zones since 2007 have generated numerical estimates of Earth-like planets.

On 4th November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.

Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth.

More generally, life based on ammonia (rather than water) has been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.

About 29 chemical elements play an active positive role in living organisms on Earth.

About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.

These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.

Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms.

According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life.

In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components, building blocks for life as we know it, may be formed extra terrestrially in outer space.

In October 2011, scientists reported that cosmic dust contains complex organic matter that could be created naturally, and rapidly, by stars. One of the scientists suggested that these compounds may have been related to the development of life on Earth.

In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system.

The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA.

This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets.

Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites. Some claim to have identified evidence that microbial life has existed on Mars.

An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. Lack of corroborating evidence from other experiments on the same samples, suggests that a non-biological reaction is a more likely.

Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.

If there is an advanced extra-terrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans.

The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.