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Muslim God
March 23, 2019
Angels and Demons
March 23, 2019

Aliens Are They Around us?

The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, chiefly in works of science fiction. Over the years, science fiction communicated scientific ideas, imagined a wide range of possibilities, and influenced public interest in and perspectives of extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try for contact with intelligent extraterrestrial life. Others—citing the tendency of technologically advanced human societies to enslave or wipe out less advanced societies—argue that it may be dangerous to actively call attention to Earth.[4][5]

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,[6] as well as well-regarded thinkers such as Winston Churchill,[7][8] it would be improbable for life not to exist somewhere other than Earth.[9][10] This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth.[11] 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.[12][13] 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 a process called panspermia.[14][15] In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.[16] 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.[16]

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 —in terms of composition—of many billions.[17] As of 2013, only a few planets have been discovered in these zones.[18] Nonetheless, on 4 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,[19][20] 11 billion of which may be orbiting Sun-like stars.[21] The nearest such planet may be 12 light-years away, according to the scientists.[19][20] Astrobiologists have also considered a "follow the energy" view of potential habitats.[22][23]


A study published in 2017 suggests that due to how complexity evolved in species on Earth, the level of predictability for alien evolution elsewhere would make them look similar to life on our planet. One of the study authors, Sam Levin, notes "Like humans, we predict that they are made-up of a hierarchy of entities, which all cooperate to produce an alien. At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation, and keep the organism functioning. We can even offer some examples of what these mechanisms will be."[24] There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches.[25]

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.[27][28] 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.[29]

About 29 chemical elements play an active positive role in living organisms on Earth.[30] About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. 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.[31] The unique characteristics of carbon make it unlikely that it could be replaced, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets.[32]

Planetary habitability in the Solar System

Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can exist, particularly those with possible subsurface oceans.[33] 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. Important insights on the limits of microbial life can be gleaned from studies of microbes on modern Earth, as well as their ubiquity and ancestral characteristics."[34]

Mars may have niche subsurface environments where microbial life might exist.[35][36][37] A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms.[38][39][40]

The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth could have been seeded from elsewhere (exogenesis). The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras.[41] In the 19th century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834),[42] Kelvin (1871),[43] Hermann von Helmholtz (1879)[44] and, somewhat later, by Svante Arrhenius (1903).[45] Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) are important proponents of the hypothesis who further contended that life forms continue to enter Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.[46]

Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization,[47] but considering an early "RNA world" Crick noted later that life may have originated on Earth.[48]


Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of a liquid water ocean beneath its ice surface.[38][40] Hydrothermal vents on the bottom of the ocean, if they exist, may warm the ice and could be capable of supporting multicellular microorganisms.[86] It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.[87]

The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior.[88][89]

On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa.[90] The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists.[90] The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2025.[91][92] Europa's subsurface ocean is considered the best target for the discovery of life.[38][40]


Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth.[95][96][97] Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there if present, could be consuming hydrogen, acetylene and ethane, and producing methane.[98][99][100]


Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects.[93][94] The Cassini–Huygens probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The temperature and density of the plumes indicate a warmer, watery source beneath the surface.[57]


Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter.[74] The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely.[75] In contrast, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa,[38][39][76] Ganymede,[77][78][79][80][81] and Callisto.[82][83][84] The EJSM/Laplace mission is planned to determine the habitability of these environments.


Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere.[66][67] Frost on the surface may also have been detected in the form of bright spots.[68][69][70] The presence of water on Ceres has led to speculation that life may be possible there.[71][72][73]


In the early 20th century, Venus was often thought to be similar to Earth in terms of habitability, but observations since the beginning of the Space Age have revealed that Venus's surface is inhospitable to Earth-like life. However, between an altitude of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it has been speculated that thermoacidophilic extremophile microorganisms might exist in the acidic upper layers of the Venusian atmosphere.[49][50][51][52] Furthermore, Venus likely had liquid water on its surface for at least a few million years after its formation.[53][54][55]


Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil.[56] The origin of the potential biosignature of methane observed in Mars' atmosphere is unexplained, although hypotheses not involving life have also been proposed.[57]

There is evidence that Mars had a warmer and wetter past: dried-up river beds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. Nevertheless, present conditions on Mars' subsurface may support life.[58][59] Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013 strongly suggests an ancient freshwater lake that could have been a hospitable environment for microbial life.[60][61]

Aliens Could Detect Life on Earth. Here's How.

As the universe's only known harbour for life, Earth is arguably one strange rock. But light-years from our solar system, other intelligent beings on a similar planetary oasis might be gazing in our direction and seeing us as a sign that they're not alone in the universe.

To date, astronomers have confirmed the existence of nearly 4,000 planets beyond our solar system, including some that just might have the conditions necessary to support life as we know it. As our technology improves, we should be able to learn more about these worlds and their chances of hosting plants, animals, and maybe even civilisations.

That means if aliens are out there, they could just as easily discover us.

First, they'd need to find Earth from afar, either by watching our sun wobble as the planet's gravity tugs against it, or by seeing the sun dim as Earth blocks a tiny fraction of sunlight during its orbit. Nine known alien worlds can see Earth transit across our sun, just as we've seen thousands of alien planets dim their host stars.

Once spotted, our planet would likely intrigue E.T. Our sun is relatively stable, not prone to disastrous flares that'd rip our atmosphere to shreds. What's more, we fall squarely within our sun's habitable zone, the area around a star where liquid water can persist on a planet's surface.

Faraway scientists might then attempt to spot our atmosphere, to see whether life's thumb is on the chemical scales. But what would they be looking for? And could they really infer life's presence across trillions of miles?

Life Is Pretty Gassy

If you're afraid of alerting aliens to your presence, that ship sailed more than two billion years ago. Our planet's abundant oxygen is a major hint that something here is alive.

Oxygen is a highly reactive element, glomming on to other atoms and molecules with such alacrity that it's tough to find it in abundance by itself—unless something is breaking down oxygen-rich compounds and pumping out loads of O2. On Earth, you can thank photosynthetic plants for being generous oxygen factories.

But oxygen alone wouldn't be enough for E.T. to prove our existence. “We have discovered several ways in which O2 can accumulate in the absence of life,” says Stephanie Olson, an astrobiologist at the University of California, Riverside. “High levels of O2, or the processes culminating in high levels of O2, may actually preclude the emergence of life on some planets.”

In addition to oxygen molecules, alien astronomers would look at our atmosphere's levels of nitrogen, carbon dioxide, nitrous oxide, and methane. Only life could keep Earth chemically off-kilter enough for all these gases to persist at once.

Terra Cognita?

Beyond those chemical clues, alien astronomers with truly massive telescopes might even be able to map Earth's surface from afar, down to major urban areas.In a paper published on the arXiv in 2017, astronomers Svetlana Berdyugina and Jeff Kuhn outlined how astronomers could actually map the surfaces of alien planets from light-years away. To pull off such extreme cartography, they'd need a telescope at least 130 feet wide, custom-built for only one task: seeing the faint glow of light reflected off an alien planet. Variations in this reflected light over time would connote regional differences in the planet's terrain.

As a proof-of-concept for this telescope, called the ExoLife Finder, or ELF, Berdyugina and Kuhn simulated how nearby aliens using the telescope would see Earth. From 25 trillion miles away, E.T. could not only map Earth's continents, but they'd also be able to see signs of intelligent life.

“The ELF telescope has the sensitivity to see a Los Angeles basin,” says Kuhn. “We don't see the lights, but we see the heat signature.” Their group, the Planets Foundation, is now building a single-mirror telescope in Hawaii to test the underlying tech. If all goes to plan, they say it's possible to build ELF within a decade.

“It would be like Star Trek, the reality show,” says Berdyugina. “We could virtually visit these planets.”

Advanced Intelligence

If intelligent life is nearby, the most straightforward way to find earthlings would be to listen for us. For the last century, human civilisation has been broadcasting its existence to the cosmos via our leaky radio transmissions. Occasionally, we've broadcast messages to E.T. intentionally, and sent golden records into the void on the off-chance that aliens stumble across our interplanetary spacecraft.

Aliens wouldn't be able to detect our radio presence, much less our golden records, unless they're within 590 trillion miles of Earth. If intelligent life is any farther away than that, our earliest radio signals simply haven't reached them yet.

But in several centuries' time, intelligent aliens may see other signs of our tech-savviness. In a study recently accepted to The Astrophysical Journal, for example, astrophysicist Hector Socas-Navarro says we could find intelligent life by looking for artificial satellites orbiting alien worlds. And that means they could find us using similar methods.

As a satellite-fringed world drifts in front of its home star, its satellites would block some starlight in front of and behind the transiting planet. This metallic belt would look unnatural when compared to known planetary rings.

Currently, Earth's satellite network isn't anywhere near dense enough to be detected, nor will it be anytime soon. But our footprint in space is growing exponentially: If we keep launching satellites at our present pace, Socas-Navarro says in his study, nearby aliens armed with telescopes as powerful as the ones we have today could spot our satellites by 2200.

Winds of Change

Of course, Earth is more than 4.5 billion years old, and life has changed a lot over the eons. What if alien astronomers had looked our way even a billion years ago? In a 2018 paper in Science Advances, Olson and her colleagues simulated how Earth's atmosphere has changed over time. Even three billion years ago, aliens may have been able to infer life by sniffing out methane and carbon dioxide in the early atmosphere. But our modern atmosphere—a literal beacon for life—arrived only about 500 million years ago.

“For more than a billion years of Earth history, an alien astronomer may have even been sufficiently misled to conclude that Earth was sterile—despite the fact that life was flourishing in our ocean at the time,” says Olson.

Still, if the aliens were advanced and committed enough, even an early Earth would yield compelling clues for life, says study coauthor Joshua Krissansen-Totton of the University of Washington.

“The presence of life on Earth has been fairly obvious for the last 4 billion years to anyone who could build a big telescope,” he says in an email. “If there was anything nasty out there, then they would have extinguished life on Earth long ago. I think we are safe inviting them over to visit and exchange notes on the cosmos.”

If aliens are anything like us, perhaps the news that they aren't alone in the cosmos wouldn't be their equivalent of earth-shattering. In a study published in Frontiers in Psychology in February, researchers found that people, at least, would take the discovery of alien life in stride.

“People will be able to accommodate even high-impact scientific discoveries without their worldviews collapsing,” theologian Ted Peters said at the time.

But like us, aliens may fear the prospect of hostile, intelligent extraterrestrials—in this case, humans—arriving unannounced on their cosmic doorstep.

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